1. business and industrial
2. energy
3. renewable energy
4. hydroelectric energy

CONTROLLING OXYGEN, IRON AND MANGANESE IN WATER-SUPPLY
RESERVOIRS USING HYPOLIMNETIC OXYGENATION
Paul A. Gantzer, P.E.
Dissertation submitted to the Faculty of the
Virginia Polytechnic Institute and State University
in partial fulfillment of the requirements for the degree of
Doctor of Philosophy
in
Civil Engineering
Dr. John C. Little, Chair and Advisor
Dr. Daniel Gallagher
Dr. Marc Edwards
Dr. Tom Grizzard
Mr. Robert Benninger, P.E.
Mr. Mark Mobley, P.E.
26 March, 2008
Blacksburg, Virginia
Keywords: bubble-plume, diffuser, hypolimnion, manganese, oxygenation, spreading
Copyright © 2008, Paul Anthony Gantzer
CONTROLLING OXYGEN, IRON AND MANGANESE IN WATER-SUPPLY
RESERVOIRS USING HYPOLIMNETIC OXYGENATION
Paul A. Gantzer, P.E.
ABSTRACT
Hypolimnetic oxygenation systems, such as linear bubble-plume diffusers, are used to improve
raw water quality. Linear bubble-plume diffusers were installed in Spring Hollow Reservoir
(SHR) and Carvins Cove Reservoir (CCR). Diffusers induce mixing that aids distribution of
oxygen throughout the hypolimnion. The induced mixing also creates an undesirable effect by
increasing hypolimnetic oxygen demand (HOD).
Nevertheless, oxygenation systems are
commonly used and long-term oxygenation is hypothesized to actually decrease HOD.
Increased oxygen concentrations in combination with the induced mixing affect the location of
the oxic/anoxic boundary relative to the sediment water interface. If the oxic/anoxic boundary is
pushed beneath the sediment/water interface, the concentrations of soluble iron and manganese
in the bulk water are reduced.
This work was performed to further validate a recently published bubble-plume model that
predicts oxygen addition rates and the elevation in the reservoir where the majority of the oxygen
is added. Also, the first field observations of a theoretically expected secondary plume are
presented. Model predicted addition rates were compared to observed accumulation rates to
evaluate HOD over a wide range of applied gas flow rates. Observations in both reservoirs
showed evidence of horizontal spreading that correlated well with plume-model predictions and
of vertical spreading below diffuser elevations, showing oxygen penetration into the sediment.
Experimental observations of a theoretically expected secondary plume structure also correlated
well with model predictions. Plume-induced mixing was shown to be a function of applied gas
flow rates, and was observed to increase HOD. HOD was also observed to be independent of
bulk hypolimnion oxygen concentration, indicating that the increase in oxygen concentration is
not the cause of the increased HOD. Long-term oxygenation resulted in an overall decrease in
background HOD as well as a decrease in induced HOD during diffuser operation. Elevated
oxygen concentrations and mixing, which occur naturally during destratification and artificially
during oxygenation, were observed to coincide with low dissolved metal concentrations in CCR.
Movement of the oxic/anoxic boundary out of the sediment, which is also common during
stratified periods, appears to facilitate transport of reduced Mn to the overlying waters.
Hypolimnetic oxygenation increased oxygen concentrations throughout the hypolimnion,
including down to the SWI, and induced mixing, although not to the extent observed during
destratification. Subsequently, elevated Mn concentrations were observed to be restricted to the
benthic waters located immediately over the sediments, while bulk (hypolimnion) water Mn
concentrations remained low.
The good agreement between the model and the experimental data show that the model can be
used as a predictive tool when designing and operating bubble-plume diffusers. Linear bubbleplume diffusers provide sufficient horizontal and vertical spreading to enable oxygen to reach the
sediments. Hypolimnetic oxygenation, despite the increased HOD, is a viable method to manage
the negative consequences of hypolimnetic anoxia in water-supply reservoirs.
iii
ACKNOWLEDGEMENTS
I would like to acknowledge the Western Virginia Water Authority (2002 – 2007) and the
Education Department (GAANN Fellowship) (2007 – 2008) for financial support.
I would very much like to thank my academic and research advisor, Dr. John C. Little, for the
hours he has provided me; both in debate and review. I have cherished my rapport with Dr.
Little and appreciated his grace when listening to me propose increasingly elaborate sampling
plans year after year. I would like to offer my gratitude and thanks to Mr. Bob Benninger for
providing guidance and sponsorship in addition to the resources and engineering design work at
the Western Virginia Water Authority. In addition to supporting my Ph.D. research, this also
enabled me to successfully complete the requirements and pass the Professional Engineering
exam.
I wish to express my appreciation to my advisory committee members, Dr. Dan Gallagher, Dr.
Marc Edwards, Dr. Tom Grizzard, Mr. Robert Benninger, P.E., and Mr. Mark Mobley, P.E. for
their help, encouragement and understanding during my time at Virginia Tech.
I also thank Vickie Singleton, Lindsay Olinde, Elizabeth Rumsey, Kevin Elam, Jeff Booth and
the entire staff at Spring Hollow and Carvins Cove for their help, support, and patience with my
excessive sampling and data collection schedules. Without their considerable help, I would not
have successfully been able to collect or analyze the hundreds of samples or thousands of
profiles. I would also like to thank Lee Bryant for being an incredible friend. Lee provided
invaluable feedback and editing of my writing, as well as endless support throughout our work
together.
Finally, I would like to thank my Mother (Mrs. Mitzi Gantzer-Kueter) for her support and
encouragement, and (in memory) my Father (Charles M. Gantzer) for his traits, ideals, and
characteristics that have so strongly influenced me. Lastly, I thank my wife (Charlotte Gantzer)
for her commitment and unwavering belief in me.
iv
TABLE OF CONTENTS
LIST OF FIGURES................................................................................................................................................. viii
LIST OF TABLES.......................................................................................................................................................x
CHAPTER 1: EXECUTIVE SUMMARY.................................................................................................................1
1.1 INTRODUCTION ...............................................................................................................................................1
1.2 LITERATURE REVIEW ......................................................................................................................................2
1.3 RESEARCH SIGNIFICANCE .............................................................................................................................4
1.4 OBJECTIVES AND SCOPE OF RESEARCH.....................................................................................................6
1.5 CONCLUSIONS..................................................................................................................................................7
1.6 REFERENCES ....................................................................................................................................................9
CHAPTER 2: HYPOLIMNETIC OXYGENATION USING LINEAR BUBBLE-PLUME DIFFUSERS IN
TWO WATER-SUPPLY RESERVOIRS................................................................................................................15
2.1 INTRODUCTION .............................................................................................................................................16
2.2 MATERIALS AND METHODS .........................................................................................................................18
2.2.1 Study sites ................................................................................................................................................18
2.2.2 Water column profiles..............................................................................................................................18
2.2.3 Bubble-plume model................................................................................................................................18
2.3 RESULTS AND DISCUSSION..........................................................................................................................19
2.3.1 Bubble-plume diffuser operation .............................................................................................................19
2.3.2 Primary bubble-plume .............................................................................................................................20
2.3.3 Secondary bubble-plume..........................................................................................................................21
2.3.4 Horizontal spreading................................................................................................................................22
2.3.5 Vertical spreading ....................................................................................................................................24
2.4 CONCLUSION..................................................................................................................................................24
2.5 ACKNOWLEDGEMENTS ................................................................................................................................25
2.6 REFERENCES ..................................................................................................................................................25
2.7 FIGURES AND ILLUSTRATIONS ...................................................................................................................29
2.8 TABLES.............................................................................................................................................................40
v
CHAPTER 3: EFFECT OF HYPOLIMNETIC OXYGENATION ON OXYGEN DEPLETION RATES IN
TWO WATER-SUPPLY RESERVOIRS................................................................................................................41
3.1 INTRODUCTION .............................................................................................................................................42
3.2 MATERIALS AND METHODS .........................................................................................................................44
3.2.1 Study sites ................................................................................................................................................44
3.2.2 Data collection .........................................................................................................................................44
3.2.3 Data analysis ............................................................................................................................................45
3.3 RESULTS AND DISCUSSION..........................................................................................................................50
3.3.1 Hypolimnetic anoxia................................................................................................................................50
3.3.2 HOD and gas flow rates ...........................................................................................................................52
3.3.3 Hypolimnetic warming ............................................................................................................................53
3.3.4 Effect of bulk oxygen concentration on HOD .........................................................................................53
3.3.5 Effective depth .........................................................................................................................................55
3.3.6 Long term reduction of Background HOD ..............................................................................................55
3.3.7 Long term reduction of induced HOD .....................................................................................................56
3.4 CONCLUSION..................................................................................................................................................57
3.5 REFERENCES ..................................................................................................................................................57
3.6 FIGURES AND ILLUSTRATIONS ...................................................................................................................61
3.7 TABLES.............................................................................................................................................................70
CHAPTER 4: CONTROLLING IRON AND MANGANESE IN A WATER SUPPLY RESERVOIR USING
HYPOLIMNETIC OXYGENATION......................................................................................................................73
4.1 INTRODUCTION .............................................................................................................................................74
4.2 MATERIALS AND METHODS .........................................................................................................................75
4.2.1 Carvins Cove Reservoir ...........................................................................................................................75
4.2.2 Data collection and analysis.....................................................................................................................75
4.2.3 Water column profiles..............................................................................................................................76
4.2.4 Diffuser operation ....................................................................................................................................77
4.3 RESULTS AND DISCUSSION..........................................................................................................................77
4.3.1 Historical observations.............................................................................................................................77
4.3.2 Comparison between stratified and destratified periods ..........................................................................77
4.3.3 Diffuser operations in 2006......................................................................................................................80
4.3.4 Diffuser operations in 2007......................................................................................................................83
4.4 CONCLUSION..................................................................................................................................................85
vi
4.5 REFERENCES ..................................................................................................................................................86
4.6 FIGURES AND ILLUSTRATIONS ...................................................................................................................89
4.7 TABLES.............................................................................................................................................................97
vii
LIST OF FIGURES
CHAPTER 2
Figure 2.1 Schematic of water column regions with plume overlay. .......................................... 29
Figure 2.2 Topographic map, bottom profile, sample locations, and diffuser position in Carvins
Cove Reservoir.............................................................................................................................. 30
Figure 2.3 Topographic map, bottom profile, sample locations, and diffuser position in Spring
Hollow Reservoir. ......................................................................................................................... 31
Figure 2.4 Temperature profiles (top) and dissolved oxygen profiles (bottom) in Spring Hollow
Reservoir during 2005 oxygenation collected in front of intake tower. ....................................... 32
Figure 2.5 Temperature profiles (top) and dissolved oxygen profiles (bottom) in Carvins Cove
Reservoir during 2005 oxygenation collected in front of intake tower. ....................................... 33
Figure 2.6 2004 plume image in Spring Hollow Reservoir. DO is shown as shaded contours,
temperature is shown as line contours, and plume model predictions (aligned vertically and
horizontally) overlaid.................................................................................................................... 34
Figure 2.7 2006 plume image in Carvins Cove Reservoir. DO is shown as shaded contours,
temperature is shown as line contours, and plume model predictions (aligned vertically and
horizontally) are overlaid.............................................................................................................. 35
Figure 2.8 Oxygen concentration in SHR during 2007 diffuser operation.................................. 36
Figure 2.9 Oxygen concentration in CCR during 2006 diffuser operation.................................. 37
Figure 2.10 Oxygen concentration in CCR during 2007 diffuser operation................................ 38
Figure 2.11 Oxygen concentration for sediment cores collected during 2006 diffuser operations
in CCR. ......................................................................................................................................... 39
CHAPTER 3
Figure 3.1 Bathymetry, bottom profile, diffuser locations, and sampling stations in Carvins Cove
Reservoir. ...................................................................................................................................... 61
Figure 3.2 Bathymetry, bottom profile, diffuser location, and sampling stations in Spring
Hollow Reservoir. ......................................................................................................................... 62
Figure 3.3 Oxygen isopleths in SHR during 2001 summer stratification.................................... 63
viii
Figure 3.4 Hypolimnetic oxygen demand in SHR (2003-2007) and CCR (2006-2007) as a
function of applied gas flow rate. ................................................................................................. 64
Figure 3.5 Hypolimnetic warming in SHR as a function of elevation and applied gas flow rate.
....................................................................................................................................................... 65
Figure 3.6 Total warming rates per unit hypolimnion volume in CCR and SHR as a function of
applied gas flow rate. .................................................................................................................... 66
Figure 3.7 Volume-averaged DO in the metalimnion and hypolimnion in SHR (2004-2007). .. 67
Figure 3.8 Background hypolimnion and metalimnion oxygen demand observed in SHR
between 2003 and 2007. ............................................................................................................... 68
Figure 3.9 Net oxygen loss (HODconc) in SHR and CCR over time with applied gas flow rate (in
NCMH) shown on data points. ..................................................................................................... 69
CHAPTER 4
Figure 4.1 Bathymetry, bottom profile, diffuser locations, and sampling stations in Carvins Cove
Reservoir. ...................................................................................................................................... 89
Figure 4.2 Soluble Fe in Carvins Cove Reservoir (2000-2008). ................................................. 90
Figure 4.3 Soluble Mn in Carvins Cove Reservoir (2000-2008)................................................. 91
Figure 4.4 Soluble Mn in Carvins Cove Reservoir during 2006 and 2007.................................. 92
Figure 4.5 Hypolimnion DO, bottom DO, and applied gas flow rate (top) and average soluble
and particulate hypolimnion Mn concentrations and metalimnion DO (bottom)......................... 93
Figure 4.6 Total Mn concentration as well as total solids accumulated in sediment traps,
superimposed on DO contours...................................................................................................... 94
Figure 4.7 Total Mn mass as a function of elevation................................................................... 95
Figure 4.8 Soluble Mn collected at 338.3 m elevation at both CC and C3. ................................ 96
ix
LIST OF TABLES
CHAPTER 2
Table 2.1 Summary of parameters in Spring Hollow and Carvins Cove Reservoirs. ................. 40
CHAPTER 3
Table 3.1 Summary of reservoir and oxygenation characteristics for SHR and CCR................. 70
Table 3.2 Summary of initial DO and depletion rates for hypolimnion and metalimnion in SHR.
....................................................................................................................................................... 71
Table 3.3 Summary of sediment trap data collected in CCR between September and October
2006............................................................................................................................................... 72
CHAPTER 4
Table 4.1 Summary of reservoir and oxygenation system characteristics................................... 97
x
CHAPTER 1: EXECUTIVE SUMMARY
1.1 INTRODUCTION
Lakes and reservoirs in temperate climates undergo seasonal stratification, creating
distinct layers, with a warmer, less dense epilimion, a transitionary metalimnion, and a colder
denser hypolimnion at the top, middle, and bottom, respectively (Wetzel, 1975). The well-mixed
epilimnion typically remains close to saturation with oxygen from exposure to the atmosphere,
whereas the vertical density gradient limits oxygen transfer from the surface to the lower
hypolimnetic water (Mackenthun and Stefan, 1998). The hypolimnion, being isolated from an
oxygen source, can become anoxic if oxygen consuming processes exceed available oxygen at
the beginning of the stratified period. This leads to reductive dissolution of metals (such as iron
and manganese) and release of these and other undesirable dissolved compounds from the
sediment to the hypolimnetic water (Schaller et al., 1997; Wann et al., 1997). Water quality
deteriorates as the anoxic period lengthens (Townsend, 1999; Matthews and Effler, 2006).
Several techniques have been employed to address hypolimnetic anoxia, including air-lift
aerators, hypolimnetic oxygenation and artificial destratification. A linear bubble-plume
diffuser, hereafter referred to as “diffuser”, is an oxygenation device that adds oxygen bubbles
directly to the deep waters of the hypolimnion. Hypolimnetic oxygenation, hereafter referred to
as “oxygenation”, can minimize the negative impact on water quality by preventing anoxic
conditions in deep lakes. Although oxygenation is becoming more widely used to address
hypolimnetic anoxia, oxygenation systems have been observed to increase: (1) hypolimnetic
warming, (2) turbulence and (3) oxygen demand.
The scope of this research is limited to diffuser operations in Spring Hollow Reservoir
(SHR) and Carvins Cove Reservoir (CCR), which are two water-supply reservoirs operated by
the Western Virginia Water Authority in southwestern Virginia. This research is focused on
plume characteristics and subsequent spreading of oxygen within the hypolimnion, full-scale
evaluation of increased oxygen demand resulting from diffuser operation, and the effect on
soluble metals concentrations from diffuser operation.
This dissertation is organized into four chapters. Chapter 1 is the Executive Summary
which includes Introduction, Literature Review, Research Significance, Objectives and Scope of
1
Research, and Conclusions. Chapters 2-4 are written as independent, stand-alone manuscripts,
which are in approximate publication format.
1.2 LITERATURE REVIEW
Over the years, several studies have been conducted regarding hypolimnetic oxygen
depletion (HOD) and its effect on lake and reservoir water quality (McQueen and Lean, 1986;
Favre, 1991; Gibbons, 1994; Jaeger, 1994; Soltero et al., 1994; Gemza, 1995; Nordin et al.,
1995; Beutel and Horne, 1999). Two major contributors to HOD are (1) water column oxygen
demand (WOD) from organic detritus undergoing aerobic decomposition in the presence of
oxygen and (2) sediment oxygen demand (SOD) from incompletely oxidized detritus that
deposits on the bottom, and is incorporated in the sediments (Higashino and Stefan, 2005;
Matthews and Effler, 2006; Beutel et al., 2007). HOD in excess of available oxygen in the
hypolimnion at the beginning of the stratified period can completely deplete the hypolimnion
oxygen, causing anoxia (Moore, 2003; Matthews and Effler, 2006; Beutel et al., 2007).
Oxygenation systems provide a solution to hypolimnetic anoxia and increase DO levels
in hydropower reservoirs (Mobley, 1997), recreational lakes (Soltero et al., 1994) and watersupply reservoirs (Jung et al., 1999). Comprehensive reviews of oxygenation systems are
presented by Beutel and Horne (1999) and Singleton and Little (2006). Linear bubble-plume
diffusers release oxygen bubbles directly into the hypolimnion. When designed appropriately,
the resulting plume induces mixing that distributes the oxygenated water within the hypolimnion
(Miller et al., 2001; Socolofsky and Adams, 2003; McGinnis et al., 2004; and Singleton et al.,
2007). Plume formation resulting from diffuser operations has been modeled for circular (Wüest
et al., 1992) and linear bubble-plumes (McGinnis and Little, 2002). The recently published
linear bubble-plume model (Singleton et al., 2007) has successfully predicted plume
characteristics such as depth of maximum plume rise (DMPR) and elevation of equal density
(ED). This full-scale model validation focused on the primary plume, formed immediately
above the diffuser. Secondary plumes, which can form above a primary plume if bubbles remain
in the water column, have been modeled to predict plume behavior for accidental oil-well blow
outs in the deep ocean (Socolofsky and Adams, 2003), but up to this point have only been
theorized to occur in lakes and reservoirs.
2
Hypolimnetic anoxia and corresponding water quality deficiencies have led to extensive
research regarding oxygenation apparatus and bubble transfer characteristics (Speece et al.,
1973; Ashley, 1985; Wüest et al., 1992; Little, 1995; Burris and Little, 1998; McGinnis and
Little, 2002), but little work has been performed on ecological demands. Positive results have
been obtained from hypolimnetic oxygenation, however, some negative effects have been
observed, such as hypolimnetic warming (Bernhardt, 1967; Fast et al., 1975; Hess, 1975; Wirth
et al., 1975; Soltero et al., 1994; Thomas et al., 1994), increased oxygen demand during and
immediately following oxygenation (Smith et al., 1975; Lorenzen and Fast, 1977; Ashley, 1981;
Ashley et al., 1987; McQueen and Lean, 1984; Stefan et al. 1987; Soltero et al., 1994; Thomas et
al., 1994; and Moore et al., 1996)., and increased turbulence(Stefan et al., 1987; Soltero et al.,
1994). Increased HOD is hypothesized to result from increased SOD (Moore, 2003; Beutel et
al., 2007) and WOD (Ashley, 1983). Factors affecting SOD, such as turbulence (Stefan et al.,
1987; Soltero et al., 1994), have been extensively studied in laboratory incubated sediment
samples (Moore et al., 1996; Mackenthun and Stefan, 1998; Moore, 2003; Beutel et al., 2007);
however, small-scale laboratory results do not accurately predict the naturally occurring response
at the sediment/water interface (SWI) throughout a reservoir (Moore et al., 1996). Despite the
frequent occurrence of increased oxygen demand (hereafter referred to as “induced-HOD”)
during oxygenation, this method of lake and reservoir management is still widely used.
Based on the theory that induced-HOD originates from factors affecting SOD, and that
SOD partially results from the annual deposition of incompletely oxidized detritus, it can be
deduced that induced-HOD from diffuser operation is meeting historical SOD from previous
years organic (detritus) deposition. Adding oxygen should enable annual WOD to be more
completely oxidized and historical SOD to be more completely satisfied over several years of
oxygenation, thus decreasing HOD over time. Indeed, decreased HOD over several years of
oxygenation has been hypothesized (McQueen and Lean, 1984; Matinvesi, 1996; Moore et al.,
1996; Matthews and Effler, 2006), however; no long-term studies of HOD reduction have been
published to date.
Dissolved metals such as iron (Fe) and manganese (Mn) present in water-supply
reservoirs are commonly transported from the sediments to the water column (Hoffmann and
Eisenreich, 1981; Balzer, 1982; Zaw and Chiswell, 1999) during anoxic conditions (Yagi, 1996;
Wann et al., 1997). Manganese is highly reactive at the redox boundary (Hoffman and
3
Eisenreich, 1981; Wann et al., 1997) and is easily reduced in the absence of oxygen (Nealson, et
al., 1988), thus causing Mn to be especially sensitive to the location of the oxic/anoxic boundary.
The oxic/anoxic boundary was observed to (1) limit soluble Mn transport to the water column
when located below the SWI (within the sediments) (Balzer, 1982; Katsev et al., 2007) and (2)
promote soluble Mn transport to the water column when located above the SWI (Hoffman and
Eisenreich, 1981; Balzer, 1982; Yagi, 1996; Schaller et al., 1997; Wann et al., 1997; Schoemann
et al., 1998). Manganese oxidation was also observed to be dependent on the biogeochemical
cycle (Nealson et al., 1988) and despite unfavorable thermodynamics, remains in the soluble,
reduced form (Balzer, 1982). Dissolved metals can be effectively removed in the water
treatment process; however, recent studies have shown that the formation of anoxic conditions in
the settled sludge results in re-dissolution of Mn (Budd et al., 2007). Source-water control of Mn
is addressed with hypolimnetic anoxia management strategies; however, results are inconclusive
(Zaw and Chiswell, 1999) or vague (Burns, 1998; Ismail et al., 2002).
1.3 RESEARCH SIGNIFICANCE
The literature provides a strong background for oxygen demand, anoxia, the formation
and transport of dissolved metals and methods such as oxygenation to control or limit the
negative effects of low DO on water quality. The literature, however, does not provide sufficient
depth in areas related to oxygen distribution and spreading, secondary plume formation, the
various oxygen demands and metals behavior during oxygenation.
The significance of this research is therefore as follows:
a. Linear bubble-plumes and oxygen spreading
Mathematical models are commonly used to predict events occurring in nature. A linear
bubble-plume model, recently validated by Singleton et al. (2007), was used to predict
oxygen addition and distribution elevations during diffuser operations as well as a
theoretically expected secondary plume structure. There is very little applied use of the
plume model beyond validation studies in the literature. This research further
4
demonstrates the applicability and reliability of the plume model for predicting bubbleplume behavior in two different water-supply reservoirs.
b. Induced and long-term effects on hypolimnetic oxygen demand
Currently, induced-HOD is a parameter that is estimated during the design of
oxygenation systems. Some applications apply a safety factor (Stefan et al., 1987;
Soltero et al., 1994), whereas others project SOD rates from samples undergoing
perturbation in the laboratory (Beutel et al., 2007; Moore, 2006). However, it is well
known that laboratory samples do not represent overall SOD because oxygenation
devices only directly influence a portion of the sediment surface in a reservoir (Moore et
al., 1996). This research uses HOD in a holistic fashion (including the components such
as SOD, WOD and metals oxidation) to explain induced oxygen demand. Experience
shows that water column profiling is more universally practiced than the collection of
sediment samples in the water-treatment industry. Establishing a relationship between
oxygen demand and diffuser operation (expressed by the applied gas flow rate) is an
approach that is well suited for reservoir management. This research also provides
evidence of the effect of long-term oxygenation on HOD, which has not been published
to date.
c. Effects on metal concentrations during hypolimnetic oxygenation
If the source water could be managed in such a way as to minimize the availability of Fe
and Mn, re-dissolution during the water treatment process would be avoided. Manganese
is a very complex element, identified as a nuisance contaminant by the water-treatment
industry (Stauffer, 1986). Despite its easy removal in the treatment process, it can still
become problematic, for example, when anoxic conditions occur in settling basins (Budd
et al., 2007). This research demonstrates effective source-water management of Mn in
the bulk hypolimnion using hypolimnetic oxygenation.
5
1.4 OBJECTIVES AND SCOPE OF RESEARCH
The objective of this research is to develop a better understanding of (1) linear bubbleplumes and oxygen spreading; (2) the induced and long-term effects on HOD and (3) the effects
on soluble metal concentrations during hypolimnetic oxygenation. The results will be used to
improve the design and operation of linear bubble-plume diffusers as a long-term source-water
management strategy for water-supply reservoirs similar to SHR and CCR.
The scope of the research includes the following:
a. Linear bubble-plumes and oxygen spreading
Chapter 2, titled “Hypolimnetic Oxygenation using Linear Bubble-Plume Diffusers in
Two Water-Supply Reservoirs” compares water column profiles (collected laterally
across and longitudinally along the reservoirs) with predictions from a recently published
linear bubble-plume model (Singleton et al., 2007). The comparison will be used to
provide (1) further evidence in support of the model predictions for elevation in the water
column most influenced by the plume, (2) the first experimental observations of a
theoretically expected secondary plume structure and (3) experimental evidence of the
vertical and horizontal spreading of oxygenated water during diffuser operation,
including penetration of oxygen below the SWI.
b. Induced and long-term effects on hypolimnetic oxygen demand
Chapter 3, titled “Effect of Hypolimnetic Oxygenation on Oxygen Depletion Rates in
Two Water-Supply Reservoirs” will (1) establish a correlation of plume-induced mixing
and applied gas flow rate by analyzing hypolimnetic warming and (2) compare oxygen
accumulation rates calculated from water column profiles with predicted oxygen addition
rates from a linear bubble-plume model to determine HOD. HOD is then analyzed with
respect to plume-induced mixing (applied gas flow rate) and bulk hypolimnion oxygen
concentrations to show the effect on induced-HOD from different applied gas flow rates.
Lastly, the long-term effects on HOD from continued oxygenation will be evaluated.
c. Effects on metal concentrations during hypolimnetic oxygenation
6
Chapter 4, titled “Controlling Iron and Manganese in a Water Supply Reservoir Using
Hypolimnetic Oxygenation” will monitor (1) Mn concentrations in both soluble and
insoluble forms throughout the water column at different locations, (2) oxygen
concentrations and plume-induced mixing (turbulence) in the lower hypolimnion during
diffuser operations and (3) the location of the oxic/anoxic boundary above or below the
SWI and its effect on promoting or inhibiting Mn transport.
1.5 CONCLUSIONS
The results of this research show that bubble-plume diffusers are a successful method of
hypolimnetic oxygenation for water supply reservoirs such as SHR and CCR. Despite the
accelerated hypolimnion warming and the induced demand, oxygenation has been shown to be
an effective source-water management strategy. Specific conclusions are presented below.
Chapter 2: Hypolimnetic Oxygenation using Linear Bubble-Plume Diffusers in Two WaterSupply Reservoirs
A recently validated bubble-plume model was used to predict plume behavior in SHR
and CCR during diffuser operations at different applied gas flow rates. The plume model
predictions for DMPR and ED correlated well with water column data showing the elevation
range of oxygen addition. The model was also successful in predicting a secondary plume in
CCR, which correlated well to transect water column profiles. Horizontal spreading beyond the
diffuser was observed in both reservoirs, and vertical spreading of oxygen was also observed
below the diffuser elevation.
The recently validated bubble-plume model was successful in predicting the elevation of
oxygen addition and has proven to be a valuable tool for predicting the elevation range of oxygen
spreading and the formation of the secondary plume.
Chapter 3: Effect of Hypolimnetic Oxygenation on Oxygen Depletion Rates in Two WaterSupply Reservoirs
HOD was evaluated in two full-scale applications of hypolimnetic oxygenation with
linear bubble-plume diffusers. Diffusers in SHR and CCR were operated over a wide range of
7
applied gas flow rates to monitor hypolimnetic warming, mixing and oxygen demands in an
effort to gain a better understanding of induced-HOD. A bubble-plume model was used to
predict oxygen addition and was compared to observed accumulation rates calculated from water
column profiles. The difference between model predictions and observed accumulations
provided an estimate of overall HOD during diffuser operation. Water column profiles for
temperature showed accelerated hypolimnetic warming and correlated well with applied gas flow
rates, implying that accelerated warming was synonymous with mixing. HOD was also observed
to be correlated to applied gas flow rates and the following relationship was established:
HODmass for SHR and CCR were 6 and 22 times applied gas flow rate (NCMH) plus the
background HOD of ~50 and ~420 (kg day-1), respectively. An evaluation of increased oxygen
concentration resulting from diffuser operation showed that HOD was independent of DO
concentration, implying that induced-HOD was caused by plume-induced mixing. Prolonged
diffuser operation showed an overall decreased HOD and a decreased induced-HOD during
diffuser operation. This suggests that more organics are being oxidized during stratification, thus
less “carry-over” oxygen demand is present during subsequent years. Despite the increased
oxygen demand and warming during diffuser operation, bubble-plume diffusers can be used to
successfully manage anoxia during summer stratification and can also decrease oxygen demand
over the long term. These results support hypolimnetic oxygenation as a viable method to
manage source water in water-supply reservoirs such as SHR and CCR.
Chapter 4: Controlling Iron and Manganese in a Water Supply Reservoir Using Hypolimnetic
Oxygenation
The location of the oxic/anoxic boundary (either above or below the SWI) has been
observed to facilitate or inhibit transport of soluble metals such as Mn to the bulk hypolimnion.
Elevated oxygen concentrations coupled with turbulence, common during periods of
destratification, appear to be necessary to move the oxic/anoxic boundary below the SWI. Low
concentrations of soluble Mn were observed during periods of destratification in CCR and
appear to be a result of (1) dilution from the completely-mixed water column and (2) decreased
transport from the sediment.
Diffuser operations in CCR were observed to increase the oxygen concentrations in the
hypolimnion as well as down to the SWI, and were also observed to induce mixing (turbulence),
8
although to a lesser degree than observed during destratification. Manganese concentrations in
the bulk hypolimnion water were observed to be significantly lower during periods of
oxygenation than in prior years. This suggests that conditions (oxygen and mixing) promoted
during diffuser operation minimized metals transport to the bulk hypolimnion water.
This research identified the sediments to be a major source of soluble Mn to the water
column. It has also shown that DO maintenance throughout the water column was an important
component to minimizing Mn transport to the bulk hypolimnion water. While not completely
simulating the conditions during destratification (this is actually undesirable because it would
destratifiy the entire reservoir), the oxygenation system was nevertheless able to suppress
substantial transfer of Mn to the bulk water, indicating that source-water control of dissolved
metals can be accomplished with hypolimnetic oxygenation.
1.6 REFERENCES
Ashley, K. I. (1981) “Effects of Hypolimnetic Aeration on Functional Components of the Lake
Ecosystem.” Masters of Science Thesis, University of British Columbia.
Ashley, K. I. (1983) “Hypolimnetic Aeration of a Naturally Eutrophic Lake: Physical and
Chemical Effects.” Canadian Journal of Fisheries and Aquatic Sciences, 40 (9), pp.13431359.
Ashley, K. I. (1985) “Hypolimnetic Aeration: Practical Design and Application.” Water
Research, 19 (6), pp. 735-740.
Ashley, K. I., Hay, S., and Scholten, G. H. (1987) “Hypolimnetic Aeration: Field Test of the
Empirical Sizing Method.” Water Research, 21 (2), pp. 223-227.
Balzer, Wolfgang (1982). “On the Distribution of Iron and Manganese at the Sediment/Water
Interface: Thermodynamic Versus Kinetic Control.” Geochimica et Cosmochimica Acta,
46, pp. 1153-1161.
Bernhardt, H. (1967) “Aeration of Wahnbach Reservoir Without Changing the Temperature
Profile.” Journal of American Water Works Association, 59, pp. 943-963.
Beutel, M. W. and Horne, A. J. (1999). “A Review of the Effects of Hypolimnetic Oxygenation
on Lake and Reservoir Water Quality.” Lake and Reservoir Management, 15 (4), pp.285297.
9
Beutel, Marc, Hannoun, Imad, Pasek, Jeff, and Kavanagh, Kristen Bowman (2007). “Evaluation
of Hypolimnetic Oxygen Demand in a Large Eutrophic Raw wAter Reservoir, San
Vicente Reservoir, Calif.” Journal of Environmental Engineering, 133 (2), pp. 130-138.
Budd, George, Knocke, William, Hanchak, John, Rice, Craig, and Billman, Mellisa (2007).
“Manganese Control Issues with Changing Treatment Sequences – Need for
Understanding Manganese and Related Profile in a Plant.” VWEA & VA-AWWA JOINT
ANNUAL MEETING WATER JAM 2007, September 16-20, pp. all.
Burns, Frank L (1998). “Case Study: Automatic Reservoir Aeration to Control Manganese in
Raw Water Maryborough Town Water Supply Queensland, Australia.” Water Science
and Technology, 37 (2), pp. 301-308.
Burris, V. L. and Little, J. C.(1998) “Bubble Dynamics and Oxygen Transfer in a Hypolimnetic
Aerator.” Water Scienc & Technology, 37 (2), pp. 293-300.
Fast, A. W., Dorr, V. A., and Rosen, R. J. (1975) “A Submerged Hypolimnetic Aerator.” Water
Resources Research, 11 (2), pp. 287-293.
Favre, R. P. (1991) “Hypolimnetic Aeration in Three Swiss Lakes.” 2nd International Symposium
Gas Transfer Water Surface ASCE, NewYork, pp. 660-669.
Gemza, A.(1995) “Some Practical Aspects of Whole Lake Mixing and Hypolimnetic
Oxygenation. Ecological impacts of aeration on lakes and reservoirs in southern
Ontario.” Lake and Reservoir Management, 11 (141), abstract.
Gibbons, H. L., Bogus, B., and Williams, G. (1994) “World’s Largest Attempt at Hypolimnetic
Aeration.” Lake and Reservoir Management, 9 (76), abstract.
Hess, L. (1975) “The Effect of the First Year of Artificial Hypolimnion Aeration on Oxygen ,
Temperature, Depth Distribution of Rainbow Trout (Salmo gairdneri Richardson) in
Spruce Knob Lake.” Proceedings of the West Virginia Academy of Science. Morgantown,
West Virginia, pp. 176-183.
Higashino, Makoto and Stefan, Heinz G. (2005). “Sedimentary Microbial Oxygen Demand for
Laminar Flow Over a Sediment Bed of Finite Length.” Water Research, 39, pp. 31533166.
Hoffmann, Michael R. and Eisenreich, Steven J. (1981). “Development of a ComputerGenerated Equilibrium Model for the Variation of iron and Manganese in the
Hypolimnion of Lake Mendota.” American Chemical Society, 15 (3), pp. 339-344.
10
Ismail, R., Kassim, M.A., Inman, M., Baharim, N.H., and Azman, S. (2002). “Removal of Iron
and Manganese by Artificial Destratification in a Tropical Climate (Upper Layang
Reservoir, Malaysia).” Water Science and Technology, 46 (9), pp. 179-183.
Jaeger, D. (1994) “Effects of Hypolimnetic Water Aeration and Iron-phosphate Precipitation on
the Trophic Level of Lake Krupunder.” Hydrobiologia, pp. 433-444.
Jung, R., Sanders, J. O., Lai, H. H., and Lai, Jr. (1999) “Improving Water Quality Through Lake
Oxygenation at Camanche Reservoir.” North American Lake and Management Society
Annual Symposium, December 1, 1999.
Katsev, Sergei, Chaillour, Gwenaelle, and Sundby, Bjorn (2007). “Effects of Progressive
Oxygen Depletion on Sediment Diagenesis and Fluxes: A Model for the Lower ST.
Lawrence River Estuary.” Limnology and Oceanography, 52 (6), pp. 2555-2568.
Little, J. C. (1995) “Hypolimnetic Aerators: Predicting Oxygen Transfer and Hydrodynamics.”
Water Research, 29 (11), pp. 2475-2482.
Lorenzen, M. W. and Fast, A. W. (1977) “A Guide to Aeration / Circulation Techniques for Lake
Management.” Ecol. Res. Ser. EPA-600/3-77-004. US. Environmental Protection
Agency.
Mackenthun, Alan A. and Stefan, Heinz G. (1998). “Effect of Flow Velocity on Sediment
Oxygen Demand: Experiments.” Journal of Environmental Engineering, 124 (3), pp.
222-230.
Matinvesi, Jukka (1996). “The Change of Sediment Composition During Recovery of Two
Finnish Lakes Induced by Waste Water Purification and Lake Oxygenation.”
Hydobiologia, 335, pp. 193-202.
Matthews, David A. and Effler, Steven W. (2006). “Assessment of Long-term Trends in the
Oxygen Resources of a Recovering Urban Lake, Onondaga Lake, New York.” Lake and
Reservoir Management, 22 (1), pp. 19-32.
McGinnis, Daniel F. and Little, John C. (2002) “Predicing Diffused-Bubble Oxygen Transfer
Rate Using the Discrete-Bubble Model.” Water Research, 36 (18), pp. 4627-4635.
McGinnis, D.F., Lorke, A., Wüest, A., and Little, J.C. (2004) ”Interaction Between a Bubble
Plume and the Near Field in a Stratified Lake.” Water Resources Research, 40 (10),
citation number W10206.
11
McQueen, D. J. and Lean, D. R. S. (1984) “Hypolimnetic Aeration: Changes in Bacterial
Populations and Oxygen Demand.” Archiv Fur Hydrobiologie, 99 (4), pp. 498-514.
McQueen, D. J. and Lean, D.R.S. (1986) “Hypolimnetic Aeration: An Overview.” Water
Pollution Research Journal Canada, 21, pp. 205-217.
Miller, Theron G., Mackay, W. C., Walty, David T. (2001) “Under Ice Water Movements
Induced by Mechanical Surface Aeration and Air Injection.” Journal of Lake and
Reservoir Management, 17 (4), pp. 263-287.
Mobley, Mark H. (1997). “TVA Reservoir Aeration Diffuser System” Proceedings of the 1997
International Conference on Hydropower, August 5-8, pp 2010-2019.
Moore, B. C., Chen, P. H., Funk, W. H., and Yonge, D. (1996) “A Model for Predicting Lake
Sediment Oxygen Demand Following Hypolimnetic Aeration.” Water Resources Bulletin
American Water Resources Association, 32 (4), August 1996.
Moore, Barry (2003). “Downflow Bubble Contact Aeration Technology (Speece Cone) for
Sediment Oxygenation.” Remediation of contaminated sediments – 2003 proceedings of
the Second International Conference on Remediation of Contaminated Sediments,
September 30, pp. all.
Nealson, Kenneth H., Tebo, Bradley M., and Rosson, Reinhardt A. (1988). “Occurrence and
Mechanisms of Microbial Oxidation of Manganese.” Advances in Applied Microbiology,
33, pp. 279-318.
Nordin, R., Ashley, K., and Law, P. (1995) “Hypolimnetic Aeration of St. Mary Lake, British
Columbia, Canada.” Lake and Reservoir Management, 11 (176), abstract.
Schaller, Tobias, Moor, H. Christopher, and Wehrli, Bernhard (1997). “Sedimentary Profiles of
Fe, Mn, V, Cr, As, and Mo as Indicators of Benthic Redox Conditions in Baldeggersee.”
Aquatic Sciences, 59, pp. 345-361.
Schoeman, V., W. de Baar, H.J., M. De Jong, J.T., and Lancelot, C. (1998). “Effects of
Phytoplankton Blooms on the Cycling of Manganese and Iron in Coastal Waters.”
Limnology and Oceanography, 43 (7), pp. 1427-1441.
Singleton, Vickie L. and Little, John C. (2006). “Designing Hypolimnetic Aeration and
Oxygenation Systems – A Review.” Environmental Science and Technology, 40, pp.
7512-7520.
12
Singleton, V.L., Gantzer, P., Little, J.C. (2007). “Linear Bubble Plume Model for Hypolimnetic
Oxygenation: Full-scale Validation and Sensitivity Analysis.:” Water Resource Research,
43, W02405, doi:10.1029/2005WR004836.
Smith, S. A., Knauer, D. R., and Wirth, T. L. (1975) “Aeration as a Lake Management
Technique.” Technical Bulletin No. 87, Wisconsin Dept. Natural Resources, pp. 39.
Socolofsky, Scott A. and Adams, E. Eric (2003). “Liquid Volume Fluxes in Stratified Multiphase
Plumes.” Journal of Hydraulic Engineering, 129 (11), doi: 10.1061/(ASCE)07339429(2003) 12-:11(905), pp. 905-914.
Soltero, R. A., Sexton, L. M., Ashley, K. I., and McKee, K. O. (1994) “Partial and Full Lift
Hypolimnetic Aeration of Medical Lake, WA to Improve Water Quality.” Water
Research, 28 (11), pp. 2297-2308.
Speece, R.E., Rayyan, F., and Murfee, G. (1973) “Alternative Considerations in the Oxygenation
of Reservoir Discharges and Rivers.” Applications of Commercial Oxygen to Water and
Wastewater Systems, Center for Research in Water Resources, University of Texas at
Austin pp. 342-361.
Stefan, Heinz G., Bender, Merlynn D., Shapiro, Joseph, and Wright, David I. (1987)
“Hydrodynamic Design of a Metalimnetic Lake Aerator.” Journal of Environmental
Engineering, 113 (6), pp. 1249-1264.
Thomas, Jennifer A., Funk, William H., Moore, Barry C., and Budd, William W. (1994) “Short
Term Changes in Newman Lake Following Hypolimnetic Aeration with the Speece
Cone.” Lake and Reservoir Management, 9 (1), pp. 111-113.
Townsend, Thomas A. (1999). “The Seasonal Patterns of Dissolved Oxygen, and Hypolimnetic
Deoxygenation, in Two Tropical Australian Reservoirs.” Lakes and Reservoirs: Research
and Management, 4, pp. 41-53.
Wann, J.K., Chen, C.T.A., and Wang, B.J. (1997). “A Seasonally Anoxic Mountain Lake and
Active Fe Cycle in Tropical Taiwan.” Aquatic Geochemistry, 3, pp. 21-42.
Wetzel, R. G. (1975) Limnology, W. B. Saunders Company, Philadelphia, pp. 68-71.
Wirth, T. L. Knauer, D. R., and Smith, S. A. (1975) “Total and Hypolimnetic Aeration of Lakes
in Wisconsin.” Verh. Internat. Verein. Limnol., 19, pp. 1960-1970.
Wüest, A., Brooks, N. H., and Imboden, D. M.(1992) “Bubble Plume Modeling for Lake
Restoration.” Water Resource Research, 28 (12), pp.3235-3250.
13
Yagi, Akihiko (1996). “Manganese Flux Associated with Dissolved and Suspended Manganese
Forms in Lake Fukami-ike.” Water Research, 30 (8), pp. 1823-1832.
Zaw, Myint and Chiswell, Barry (1999). “Iron and Manganese Dynamics in Lake Water.” Water
Research, 33 (8), pp. 1900-1910.
14
CHAPTER 2: HYPOLIMNETIC OXYGENATION USING LINEAR BUBBLE-PLUME
DIFFUSERS IN TWO WATER-SUPPLY RESERVOIRS
Gantzer, P.A. and Little, J.C.
ABSTRACT
Hypolimnetic oxygenation of water-supply reservoirs is used to improve raw water quality.
Linear bubble-plume diffusers were installed in Spring Hollow Reservoir (SHR) and Carvins
Cove Reservoir (CCR). Diffuser operation promotes the distribution of oxygen throughout the
hypolimnion. A recently published linear bubble-plume model (Singleton et al., 2007) that
predicts the elevation of oxygen distribution during diffuser operation was further validated.
Also, the first field observations of a theoretically expected secondary plume were presented, and
were found to compare well to plume model predictions. Observations in both reservoirs
showed evidence of horizontal spreading that also correlated well with plume model predictions.
Finally, vertical oxygen spreading was observed below the diffuser elevation, showing
substantial penetration of oxygen into the sediment.
Key Words: Diffuser, Hypolimnion, Linear Bubble-Plume, Oxygenation
15
2.1 INTRODUCTION
Many lakes and reservoirs stratify thermally during the summer. As stratification occurs,
layers of water with distinct characteristics are created. The warmer, less dense epilimnion
forms the top layer. A transitional layer (known as the metalimnion or thermocline) with a
density gradient is established in the middle region where the water column temperature changes
from warmer to cooler. The cold, denser hypolimnion layer forms on the bottom. Figure 2.1
shows these regions corresponding to a typical temperature profile for a thermally stratified
water body. Two transition zones are observed, the metalimnion (between the epilimnion and
the hypolimnion) and the benthic region (Kim et al., 1983; Stauffer, 1986; Eckert et al., 2002)
(between the bulk hypolimnion and the sediment). The benthic region is identified in the
literature as the lower hypolimnion, commonly where dissolved oxygen (DO) is less than 2.0 mg
l-1. In this research, the benthic region is defined as the region where oxygen rapidly decreases
from the bulk concentration in the hypolimnion to the concentration at the sediment/water
interface (SWI), as shown in Figure 2.1.
The density gradient formed during thermal stratification causes oxygen transfer between
the epilimnion (in contact with the atmosphere) and the hypolimnion to dramatically decrease.
Oxygen depleting processes can completely consume the oxygen in the hypolimnion and cause
anoxia if sufficient oxygen is not available in the hypolimnion at the beginning of the stratified
period (Moore, 2003; Matthews and Effler, 2006; Beutel et al., 2007). Oxygenation systems are
used to meet discharge requirements in hydropower reservoirs to (Mobley, 1997), to minimize
eutrophication by controlling internal phosphorous loading in recreational lakes (Soltero et al.,
1994), and to address negative water quality associated with dissolved metals and algal blooms
in water-supply reservoirs (Jung et al., 1999).
Several studies have been conducted on hypolimnetic oxygen depletion and its effect on
lake and reservoir water quality (McQueen and Lean, 1986; Favre, 1991; Gibbons, 1994; Jaeger,
1994; Soltero et al., 1994; Gemza, 1995; Nordin et al., 1995; Beutel and Horne, 1999).
Hypolimnetic anoxia and corresponding water quality deficiencies have led to extensive research
regarding oxygenation devices and oxygen transfer characteristics (Speece et al., 1973; Ashley,
1985; Wüest et al., 1992; Little, 1995; Burris and Little, 1998; McGinnis and Little, 2002).
16
Several techniques have been employed to address hypolimnetic anoxia including air-lift
aerators, hypolimnetic oxygenation and artificial destratification. Bubble plumes release oxygen
bubbles directly into the hypolimnion as opposed to other oxygen transfer devices, such as down
flow bubble contactors (Moore, 2003) or air-lift aerators (Bernhardt, 1967), which entrain
hypolimnetic water and discharge oxygen enriched water through an engineered structure. When
designed appropriately, bubble-plume diffusers add oxygen to the deep waters of the
hypolimnion, using the induced mixing caused by the resulting plume to distribute the
oxygenated water within the hypolimnion without disrupting the thermal structure (Miller et al.,
2001; Socolofsky and Adams, 2003; McGinnis et al., 2004; and Singleton et al., 2007). The
weak plumes preserve stratification, but are sensitive to density variations in the water column.
Bubble plume dependence on thermal structure has been modeled for circular (Wüest et
al., 1992) and linear (Singleton et al., 2007) applications. The linear bubble-plume model
showed good correlation between model predictions and experimental observations for a primary
plume in a deep water-supply reservoir (Singleton et al., 2007). Secondary plumes, more
commonly observed in deep oceans, were modeled by Socolofsky and Adams (2003).
Secondary plumes are expected in shallower systems; however, research with field observations
is scarce. Mixing patterns related to plumes have been studied, but appear to be focused on nearfield studies and flow regimes within the plume (Miller et al., 2001; McGinnis et al., 2004).
Very little research has been performed regarding horizontal and vertical spreading of
oxygenated water beyond the immediate vicinity of the oxygenation device (Imboden and
Emerson, 1978; Colman and Armstrong 1983; Stauffer, 1985).
The focus of this research is to investigate the performance of the linear bubble-plume
diffusers installed in two water supply reservoirs. This paper presents:
1. further validation of a recently published linear bubble-plume model (Singleton et al.,
2007) to predict the elevation at which oxygen is added to the water column;
2. field observations of a theoretically-expected secondary plume structure;
3. experimental evidence of the vertical and horizontal spreading of oxygenated water
during bubble-plume operation, including dispersion of oxygen below the diffuser
elevation and subsequent penetration into the sediments.
17
2.2 MATERIALS AND METHODS
2.2.1 Study sites
Spring Hollow Reservoir (SHR) (Figure 2.2) and Carvins Cove Reservoir (CCR) (Figure
2.3) are man-made water-supply reservoirs for the City of Roanoke and surrounding counties and
are located on private, heavily forested watersheds in southwestern Virginia, USA. SHR is a
pump storage, side-stream reservoir that is supplied by withdrawing water from the Roanoke
River during high flow periods. CCR is supplied by two natural tributaries that flow through
agriculturally dominated lands and by two creeks from an adjoining watershed that are routed
through diversion tunnels. Linear bubble-plume diffusers have been installed in both reservoirs
to address negative water quality arising from hypolimnetic anoxia during summer stratification.
The characteristics of both reservoirs and their respective oxygenation systems are summarized
in Table 2.1. The two reservoirs are quite different. The hypolimnion in SHR is deeper and
shielded from the wind, while the hypolimnion in CCR is shallower and more strongly
influenced by wind-induced mixing.
2.2.2 Water column profiles
A Seabird Electronics SBE 19plus (4 Hz sampling rate) high resolution profiler (CTD)
was used to collect temperature, conductivity, and dissolved oxygen (DO) profiles in both
reservoirs throughout the year. The DO probe on the CTD has a response time of 1.4 seconds at
20°C, allowing data to be collected at 0.1 m increments. A Hydrolab DataSonde 4a multiprobe
was used in conjunction with the CTD to obtain profiles at a lower vertical resolution of 0.5 m.
Profiles were collected longitudinally along SHR at six locations spread over 800 m and
along CCR at seven locations spread over 2000 m. Lateral transect profiles were collected
across SHR and CCR at 60 and 80 locations, respectively, and with each sample location spaced
two meters apart.
2.2.3 Bubble-plume model
A linear bubble-plume model, validated by Singleton et al. (2007), was used to predict
the approximate elevation of oxygen addition in the water column during diffuser operations.
18
Input variables for the model are water column profiles (temperature, conductivity, and DO), gas
flow rate, percent oxygen content in the gas, initial bubble size, and diffuser characteristics
(depth, width, and length). Model outputs include oxygen addition, depth of maximum plume
rise (DMPR), the corresponding depth of equal density (ED), and predicted oxygen and
temperature profiles within the plume. DMPR is the height within the water column to which
the plume rises before detraining, while ED is the estimated depth to which the plume water
returns (the depth at which the ambient density is equal to that of the detraining plume). Note
that the estimated ED does not account for additional possible entrainment as the detraining
plume water sinks back through the water column. Figure 2.1 shows an example of a plume
(with arrows added to show typical flow patterns associated with bubble plumes), and the DMPR
and ED as they might appear with respect to the given temperature profile. For this research the
plume model is used to predict both primary and secondary plume formation, with the DMPR
and ED used to estimate the elevation range over which the detraining oxygenated water is
returned to the hypolimnion.
2.3 RESULTS AND DISCUSSION
2.3.1 Bubble-plume diffuser operation
During the 2005 diffuser operations, temperature and DO water column profiles were
collected from a location over the deepest point in both SHR and CCR (see Figures 2.4 and 2.5).
Temperature data were collected to monitor hypolimnetic warming, which gives an indication of
plume-induced mixing (Gantzer and Little, In Progress) and because the prevailing thermal
gradient is needed to predicted the behavior of the bubble plumes. Dissolved oxygen data were
collected to evaluate oxygen addition and subsequent spreading in the hypolimnion, as well as to
identify the elevation range of the detraining oxygenated plume water.
2.3.1.1 Temperature and mixing
Temperature profiles for both reservoirs show thermal stratification was preserved during
diffuser operation, although hypolimnion temperatures steadily increased. The observed
warming in both reservoirs was higher than background observations before either diffuser was
19
operated (data not shown). The accelerated warming was a function of the gas flow rate supplied
to the diffusers (Gantzer and Little, In Progress). Mixing in the hypolimnion during diffuser
operation is illustrated in the temperature profiles for CCR (Figure 2.5). Two observations are;
(1) hypolimnion temperature became more uniform and (2) thermocline sharpening occurred (the
slope of the temperature gradient through the thermocline decreased) during diffuser operation.
2.3.1.2 Oxygen and spreading
Oxygen profiles for both reservoirs show a steady increase throughout the hypolimnion
as well as in the benthic region. The diffusers are positioned 1.0 m and 0.5 m above the bottom
in SHR and CCR, respectively. This means that the diffusers were located above the benthic
region in both reservoirs. Increased DO in the benthic region therefore indicates vertical
dispersion of oxygen below the diffuser. In the hypolimnion, oxygen concentrations increased
more rapidly near the center of the hypolimnion indicating that more oxygen was added at these
elevations. Taking the plume model into consideration, the highest DO concentrations should
occur between the DMPR and the ED, which is the elevation range within which the oxygenated
plume water is expected to be returned to the water column.
2.3.2 Primary bubble-plume
Transect profiles were collected in SHR during the second day of the 2004 diffuser
operation with an applied gas flow rate of 40 NCMH (25 SCFM). Data collection during the
initial days of diffuser operation were most valuable because they provided the greatest contrast
between the temperature and DO in the water column and those within the plume itself. This
allowed the plume structure to be observed with greater clarity. Figure 2.6 shows temperature
and oxygen with the DO represented as filled contours in the background and temperature
represented as lines in the foreground. The plume model predictions for width, DMPR and ED
are overlaid on the transect data showing the DMPR and ED at ~384 m and ~ 379 m,
respectively.
The region of increased DO in Figure 2.6 shows that the oxygenated water appears to
detrain from the plume between the elevations of the predicted DMPR and ED. The right side of
the plume appears to agree more with the model predictions than the left. Additionally, a smaller
20
secondary plume appears to have formed above the DMPR; but initial attempts to use the model
to predict this secondary plume were unsuccessful.
2.3.3 Secondary bubble-plume
CCR is one third the depth of SHR, which reduces the oxygen transfer efficiency from
the bubbles to the water in the plume. Additionally, the low applied gas flow rate (in both
reservoirs) substantially reduces the momentum of the plume, causing it to be strongly
influenced by the prevailing thermal structure of the water column. Once the plume reaches the
DMPR, the water detrains and falls away from the plume (the detraining plume water has greater
density than the ambient water at the DMPR), while any remaining bubbles continue to rise to
the surface, possibly forming another plume. This process of plume formation, acceleration and
detrainment continues until the bubbles completely dissolve, reach the surface, or come in
contact with a strong thermal gradient that inhibits further plume formation. If the bubbles are
not completely dissolved and the primary plume detrains below the thermocline, a secondary
plume should form. This appeared to be the case for Socolofsky and Adams (2003), whose
experiments showed clear phase separation as the first plume ended and the second plume began.
The CCR transect profiles showed the formation of secondary plumes, which were predicted
using the plume model.
There are two diffusers installed in CCR (Figure 2.7) that are about 60 m apart. Diffuser
A is positioned approximately one meter higher than Diffuser B. Transect profiles were
collected in CCR during the second day of the 2005 diffuser operation at an applied gas flow rate
of 68 NCMH (40 SCFM). As previously stated, data collection during the initial days of diffuser
operation provided the greatest contrast of temperature and DO. Figure 2.7 shows the
temperature and oxygen data with DO represented as filled contours in the background and
temperature represented as lines in the foreground. The plume model predictions for width,
DMPR and ED are overlaid on the transect data. It is evident from Figure 2.7 that each diffuser
forms both primary and secondary plumes.
The plume model predicted the primary DMPR for each diffuser at about 343 m and
340.5 m elevations for Diffusers A and B, respectively. Interestingly, the temperature profiles
also show a disruption at the primary DMPR elevations predicted by the model (Figure 2.7).
21
Temperature data across the plume show the cold water being drawn upwards, as expected. As
the momentum in the plume approaches zero, the detraining colder water then falls away from
the plume (Singleton et al., 2007). When the colder water starts to descend, the temperature
contours above the DMPR are not substantially altered (Figure 2.7).
The flow patterns in CCR are very complex because of the two diffusers each forming
primary and secondary plumes, compounded by the fact that the diffusers are positioned at
different elevations. In addition, it is expected that the flow patterns between the diffusers affect
each other. The resolution of the transect data was too low to accurately decipher clear flow
patterns. Figure 2.7 nevertheless shows that elevated oxygen concentrations generally occurred
between the highest DMPR and lowest ED locations, which corresponded to Diffuser B at 344.5
m and 338 m elevations, respectively. Coincidentally, the middle of this range (340.5 m)
corresponds to the elevation between model predictions for DMPR and ED for Diffuser A.
Therefore, based on model predictions, the oxygen addition and subsequent spreading should
occur between 338 m and 344.5 m elevations with the highest concentrations around 340.5 m
elevation, which is indeed the case (Figure 2.7).
2.3.4 Horizontal spreading
The oxygen profiles in Figures 2.4 and 2.5 show increased oxygen concentrations in the
middle of the hypolimnion for each sample day during diffuser operation. The plume model
(DMPR and ED) showed good correlation with respect to the elevation at which oxygen-rich
plume water is returned to the hypolimnion (Figures 2.6 and 2.7). Since Figures 2.4 and 2.5 are
from a single location, the profiles do not fully represent oxygen conditions throughout the entire
hypolimnion, nor do they demonstrate horizontal mixing beyond the diffuser. In order to verify
the extent of horizontal spreading of the oxygen in the hypolimnion, the plume model predictions
were overlaid on longitudinal profiles collected in SHR and CCR.
2.3.4.1 SHR
Longitudinal profiles were collected in SHR during 2007 diffuser operation at 1.3 NCMH
(0.75 SCFM) (Figure 2.8). The 7.0 ºC temperature contour is plotted to identify the upper limit
of the hypolimnion. The reservoir water-withdrawal elevation is also shown, and is located
22
above the thermocline (see Figure 2.8). The predicted DMPR and ED (aligned with the Y-axis)
and plume (width not drawn to scale) are superimposed. It should be noted that bubbles were
only apparent for about 60 m at the surface directly above the location of the diffuser, suggesting
that at the very low gas flow rate; only a short 60-m section of the diffuser was active.
The highest oxygen concentrations were observed in the vicinity of the active section of
the diffuser between the predicted DMPR (385 m) and the ED (372 m) (Figure 2.8). The weak
plume created at this applied gas flow rate was not observed to induce substantial mixing in the
hypolimnion (Gantzer and Little, In Progress). However, Figure 2.8 shows increased oxygen
following the bottom profile past the active diffuser section. This suggests that detraining
(oxygenated) water was spreading via a weak advection-like intrusion, following an elevation of
equal density. Examination of temperature profiles indicated that the DO contours followed the
same pattern as temperature ranging from 6.01 to 6.05 ºC (data not shown), confirming that the
oxygenated water spreads along a contour of equal density.
2.3.4.2 CCR
Longitudinal profiles collected in CCR for diffuser operations during 2006 at 34 NCMH
(20 SCFM) and during 2007 at 68 NCMH (40 SCFM) are shown in Figure 2.9 and 2.10,
respectively. The approximate hypolimnion boundary that corresponded most closely to a single
temperature contour is also plotted. The reservoir water-withdrawal elevations are also shown
with plume model predictions superimposed and showing the estimated DMPR and ED
elevations (aligned with the Y- axis) and plumes (width not drawn to scale).
Two images are shown for CCR to demonstrate the effect of the plumes on the water
column, as well as the effect created by withdrawing water from the reservoir. Water withdrawal
took place between 347.5 m and 344.4 m elevation in 2006 (Figure 2.9) and between 344.4 m
and 341.3 m elevation in 2007 (Figure 2.10). The currents generated at the withdrawal
elevations show lower DO water being drawn towards the deeper basin from upstream regions in
the reservoir. The effect is shown more clearly in Figure 2.9.
High DO concentrations are observed at the 342 m and 344 m elevations in CCR during
2006 and 2007 respectively, which correlate well with plume model predictions. Increased DO
concentrations upstream of the diffuser are also observed at the same elevations, which indicate
23
horizontal spreading. Since the applied gas flow rates to the diffusers in CCR were much higher
than in SHR, more mixing in the hypolimnion was observed (see Gantzer and Little, In
Progress). Mixing in CCR appears to have created stronger horizontal spreading or intrusions
than observed in SHR. Increased oxygen concentrations were observed along the bottom
between 800 m and 1500 m in CCR (Figures 2.9 and 2.10), but only between 150 m and 300 m
in SHR (Figure 2.8). Measurements in both SHR and CCR show oxygen spreading horizontally
to sediments at the same elevation as, or above the diffuser, but they do not provide evidence of
transport to the sediments below the diffuser.
2.3.5 Vertical spreading
Sediment core samples were collected and analyzed with a micro DO probe (Bryant et
al., In Progress) during 2006 diffuser operations in CCR. These data were used to determine
oxygen concentrations at the sediment/water interface as well as the extent of oxygen penetration
into the sediment (Figure 2.11). As previously mentioned, an increase in oxygen was observed
in the benthic region, which is below the diffuser elevation (Figures 2.4 and 2.5). During 2006,
the oxygenation system in CCR was shut down for a 30-day period between June 15 and July 14.
The sediment core samples showed anoxic conditions above the sediment, as is evident on July 5
(Figure 2.11). When diffuser operation resumed on July 14, oxygen was observed to again
penetrate the sediment (Figure 2.11). During diffuser operations following July 14, oxygen
concentrations at the sediment/water interface were relatively constant. However, oxygen
concentration in the overlying water column increased between July 12 and August 16, following
an increase in the applied gas flow rate. Since the diffuser is positioned 0.5 m above the
sediment surface and oxygen was observed to increase at the sediment/water interface (after
restarting the diffuser on July 14) and penetrate into the sediment, this provides clear evidence of
vertical transport of oxygen below the diffuser.
2.4 CONCLUSION
Linear bubble-plume diffusers were installed and successfully operated in two watersupply reservoirs in southwest Virginia. This work provided further evidence of the reliability of
a recently published bubble-plume model (Singleton et al., 2007) to predict the elevation of
24
oxygen distribution in the hypolimnion. The plume model predictions compare well with water
column data, showing oxygen addition occurring between the DMPR and the ED. Model
predictions also correlated well with experimental observations of a theoretically expected
secondary plume formed during diffuser operations in CCR. Furthermore, evidence of vertical
and horizontal spreading of oxygenated water during diffuser operation was presented, showing
oxygen penetration into the sediments located directly below the diffuser. This work has shown
(1) the reliability of the plume model as a useful tool to predict oxygen distribution during
diffuser operation and (2) that linear bubble-plume diffusers are effective at distributing oxygen
(especially below the diffuser and into the sediment).
2.5 ACKNOWLEDGEMENTS
We thank Mark Mobley at Mobley Engineering for the assistance in designing and
installing the diffuser and Bob Benninger and Gary Robertson at the Western Virginia Water
Authority for the financial support that made this research possible.
2.6 REFERENCES
Ashley, K. I. (1985) “Hypolimnetic Aeration: Practical Design and Application.” Water
Research, 19 (6), pp. 735-740.
Bernhardt, H. (1967) “Aeration of Wahnbach Reservoir Without Changing the Temperature
Profile.” Journal of American Water Works Association, 59, pp. 943-963.
Beutel, M. W. and Horne, A. J. (1999). “A Review of the Effects of Hypolimnetic Oxygenation
on Lake and Reservoir Water Quality.” Lake and Reservoir Management, 15 (4), pp.285297.
Beutel, Marc, Hannoun, Imad, Pasek, Jeff, and Kavanagh, Kristen Bowman (2007). “Evaluation
of Hypolimnetic Oxygen Demand in a Large Eutrophic Raw Water Reservoir, San
Vicente Reservoir, Calif.” Journal of Environmental Engineering, 133 (2), pp. 130-138.
Burris, V. L. and Little, J. C.(1998) “Bubble Dynamics and Oxygen Transfer in a Hypolimnetic
Aerator.” Water Scienc & Technology, 37 (2), pp. 293-300.
25
Colman, John A. and Armstron, David E. (1983) “Horizontal Diffusivity in a Small, Ice-covered
Lake.” Limnology and Oceanography, 28 (5), pp. 1020-1026.
Eckert, Werner, Imberger, Jorg, and Saggio, Angelo (2002) “Biogeochemical Response to
Physical Forcing in the Water Column of a Warm Monomictic Lake.” Biogeochemistry,
61, pp. 291-307.
Favre, R. P. (1991) “Hypolimnetic Aeration in Three Swiss Lakes.” 2nd International Symposium
Gas Transfer Water Surface ASCE, NewYork, pp. 660-669.
Gemza, A.(1995) “Some Practical Aspects of Whole Lake Mixing and Hypolimnetic
Oxygenation. Ecological impacts of aeration on lakes and reservoirs in southern
Ontario.” Lake and Reservoir Management, 11 (141), abstract.
Gibbons, H. L., Bogus, B., and Williams, G. (1994) “World’s Largest Attempt at Hypolimnetic
Aeration.” Lake and Reservoir Management, 9 (76), abstract.
Imboden, D. M. and Emerson, S. (1978) “Natural Radon and Phosphorus as Limnological
Tracers: Horizontal and Vertical Eddy Diffusion in Greifensee.” Limnology and
Oceanography, 23 (1), pp. 77-90.
Jaeger, D. (1994) “Effects of Hypolimnetic Water Aeration and Iron-phosphate Precipitation on
the Trophic Level of Lake Krupunder.” Hydrobiologia, pp. 433-444.
Jung, R., Sanders, J. O., Lai, H. H., and Lai, Jr. (1999) “Improving Water Quality Through Lake
Oxygenation at Camanche Reservoir.” North American Lake and Management Society
Annual Symposium, December 1, 1999.
Kim, Byung R., Higgins, John M., Bruggink, David J. (1983) “Reservoir Circulation Patterns
and Water Quality.” Journal of Environmental Engineering, 109 (6), pp. 1284-1294.
Little, J. C. (1995) “Hypolimnetic Aerators: Predicting Oxygen Transfer and Hydrodynamics.”
Water Research, 29 (11), pp. 2475-2482.
Matthews, David A. and Effler, Steven W. (2006). “Long-term Changes in the Areal
Hypolimnetic Oxygen Deficit (AHOD) of Onondaga Lake: Evidence of Sediment
Feedback.” Limnology and Oceanography, 51 (1, part 2), pp. 702-714.
McGinnis, Daniel F. and Little, John C. (2002) “Predicing Diffused-Bubble Oxygen Transfer
Rate Using the Discrete-Bubble Model.” Water Research, 36 (18), pp. 4627-4635.
26
McGinnis, D.F., Lorke, A., Wüest, A., and Little, J.C. (2004) ”Interaction Between a Bubble
Plume and the Near Field in a Stratified Lake.” Water Resources Research, 40 (10),
citation number W10206.
McQueen, D. J. and Lean, D.R.S. (1986) “Hypolimnetic Aeration: An Overview.” Water
Pollution Research Journal Canada, 21, pp. 205-217.
Miller, Theron G., Mackay, W. C., Walty, David T. (2001) “Under Ice Water Movements
Induced by Mechanical Surface Aeration and Air Injection.” Journal of Lake and
Reservoir Management, 17 (4), pp. 263-287.
Mobley, Mark H. (1997). “TVA Reservoir Aeration Diffuser System” Proceedings of the 1997
International Conference on Hydropower, August 5-8, pp 2010-2019.
Moore, Barry (2003). “Downflow Bubble Contact Aeration Technology (Speece Cone) for
Sediment Oxygenation.” Remediation of contaminated sediments – 2003 proceedings of
the Second International Conference on Remediation of Contaminated Sediments,
September 30, pp. all.
Nordin, R., Ashley, K., and Law, P. (1995) “Hypolimnetic Aeration of St. Mary Lake, British
Columbia, Canada.” Lake and Reservoir Management, 11 (176), abstract.
Singleton, V.L., Gantzer, P., Little, J.C. (2007). “Linear Bubble Plume Model for Hypolimnetic
Oxygenation: Full-scale Validation and Sensitivity Analysis.:” Water Resource Research,
43, W02405, doi:10.1029/2005WR004836.
Socolofsky, Scott A. and Adams, E. Eric (2003). “Liquid Volume Fluxes in Stratified Multiphase
Plumes.” Journal of Hydraulic Engineering, 129 (11), doi: 10.1061/(ASCE)07339429(2003) 12-:11(905), pp. 905-914.
Soltero, R. A., Sexton, L. M., Ashley, K. I., and McKee, K. O. (1994) “Partial and Full Lift
Hypolimnetic Aeration of Medical Lake, WA to Improve Water Quality.” Water
Research, 28 (11), pp. 2297-2308.
Speece, R.E., Rayyan, F., and Murfee, G. (1973) “Alternative Considerations in the Oxygenation
of Reservoir Discharges and Rivers.” Applications of Commercial Oxygen to Water and
Wastewater Systems, Center for Research in Water Resources, University of Texas at
Austin pp. 342-361.
Stauffer, Robert E. (1986). “Cycling of Manganese and Iron in Lake Mendota, Wisconsin.”
Environmental Science and Technology, 20 (5), pp. 449-457.
27
Wüest, A., Brooks, N. H., and Imboden, D. M.(1992) “Bubble Plume Modeling for Lake
Restoration.” Water Resource Research, 28 (12), pp.3235-3250.
28
2.7 FIGURES AND ILLUSTRATIONS
DMPR
ED
-10
-5
0
5
10
Figure 2.1 Schematic of water column regions with plume overlay.
Arrows indicate common flow patterns in bubble plumes. Depth of maximum plume rise
(DMPR) and elevation of equal density (ED) show bubble-plume model predictions.
29
Figure 2.2 Topographic map, bottom profile, sample locations, and diffuser position in Carvins Cove Reservoir.
30
Figure 2.3 Topographic map, bottom profile, sample locations, and diffuser position in Spring
Hollow Reservoir.
31
430
420
Elevation (m) _
410
400
390
380
370
360
6.0
11.0
16.0
21.0
Temperature (C)
26.0
31.0
2.0
4.0
6.0
8.0
-1
Dissolved Oxygen (mg l )
10.0
12.0
430
420
Elevation (m) _
410
400
390
380
370
360
2-Sep
6-Sep
9-Sep
16-Sep
27-Sep
4-Oct
18-Oct
29-Oct
15-Nov
Figure 2.4 Temperature profiles (top) and dissolved oxygen profiles (bottom) in Spring Hollow
Reservoir during 2005 oxygenation collected in front of intake tower.
32
360
Elevation (m) _
355
350
345
340
335
330
6.0
11.0
16.0
21.0
Temperature (C)
26.0
31.0
360
Elevation (m) _
355
350
345
340
335
330
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
-1
31-Jul
1-Aug
8-Aug
Dissolved Oxygen (mg l )
15-Aug
29-Aug
5-Sep
22-Sep
13-Oct
3-Nov
Figure 2.5 Temperature profiles (top) and dissolved oxygen profiles (bottom) in Carvins Cove
Reservoir during 2005 oxygenation collected in front of intake tower.
33
165
170
175
180
185
190
195
200
205
210
215
220
225
230
235
240
245
390
388
388
386
384
384
Elevation (m)
382
380
380
378
376
376
374
372
372
370
368
165
368
175
185
195
205
215
225
235
245
Distance (m)
Figure 2.6 2004 plume image in Spring Hollow Reservoir. DO is shown as shaded contours,
temperature is shown as line contours, and plume model predictions (aligned vertically and
horizontally) overlaid.
34
345
DMPR
341
Elevation (m)
Elevation (m)
343
339
ED
337
Diffuser A
Diffuser B
335
30
40
50
60
70
80
90
100
110
120
Distance (m)
Figure 2.7 2006 plume image in Carvins Cove Reservoir. DO is shown as shaded contours, temperature is shown as line contours,
and plume model predictions (aligned vertically and horizontally) are overlaid.
35
425
0
50
100
150
200
250
300
350
400
450
500
550
600
650
700
Spring Hollow Reservoir June 20, 2007
415
Elevation (m)
405
396
Withdrawal Elevation
395
385
375
DMPR
ED
365
0
100
200
300
400
500
600
700
Distance (m)
Figure 2.8 Oxygen concentration in SHR during 2007 diffuser operation.
DO is shown as shaded contours, temperature is shown as line contours, and plume model predictions (aligned vertically and
horizontally) are overlaid.
36
Carvins Cove Reservoir June 6, 2006
352
Elevation (m)
348
347.5
Withdrawal Elevation
Diffuser-A
Diffuser-B
344
344.4
Horizontal Spreading
340
DMPR
ED
336
0
200
400
600
800
1000
1200
1400
1600
Distance (m)
Figure 2.9 Oxygen concentration in CCR during 2006 diffuser operation.
Plume model predicted DMPR and ED (aligned vertically) are shown, as well as plume width (not to scale).
37
1800
Carvins Cove Reservoir June 26, 2007
352
Elevation (m)
348
Diffuser-A
Diffuser-B
Horizontal Spreading
344.4
344
Withdrawal Elevation
Horizontal Spreading
341.3
340
DMPR
ED
336
0
200
400
600
800
1000
1200
1400
1600
Distance (m)
Figure 2.10 Oxygen concentration in CCR during 2007 diffuser operation.
Plume model predicted DMPR and ED (aligned vertically) are shown, as well as plume width (not to scale).
38
1800
10
6-Jun
5-Jul
12-Jul
5
Depth (mm) _
19-Jul
16-Aug
19-Oct
0
120
0
100
-1
80
-2
60
-4
40
-5
20
-6
0
-7
-5
A
M
J
J
A
S
O
N
-10
-1
0
1
2
3
4
5
DO (mg l -1)
Figure 2.11 Oxygen concentration for sediment cores collected during 2006 diffuser operations in CCR.
39
6
7
2.8 TABLES
Table 2.1 Summary of parameters in Spring Hollow and Carvins Cove Reservoirs.
Parameter
Spring Hollow Reservoir
Max Depth
62 m (200 ft)
Surface Area
Volume
Hypolimnion Volume
Oxygen Demand
Side Slopes
Diffuser Length
Design Flow Rate
Design O2 Addition
O2 Transfer Efficiency
6
23 m (70 ft)
2
(150 acres)
3
(3.4 X 10 gal)
0.61 km
13 X 10 m
9
15% total volume
~50 kg day
Carvins Cove
-1
1.5 - 2 : 1
2
(1100 acres)
3
(6.2 X 10 gal)
2.5 km
6
24 X 10 m
9
25% total volume
-1
~430 kg day
8-20 : 1
620 m (2000 ft)
34 NCMH (20 SCFM)
-1
1250 m (4000 ft)
68 NCMH (40 SCFM)
-1
~1000 kg day
~2100 kg day
95% - 98 %
80% - 85 %
40
CHAPTER 3: EFFECT OF HYPOLIMNETIC OXYGENATION ON OXYGEN
DEPLETION RATES IN TWO WATER-SUPPLY RESERVOIRS
Gantzer, P.A., Little, J.C.
ABSTRACT
Oxygenation systems, such as linear bubble-plume diffusers, are used to replenish oxygen in the
hypolimnia of water-supply reservoirs. The diffusers induce mixing, which helps distribute
oxygen throughout the hypolimnion. Mixing, however, has been observed to increase
hypolimnetic oxygen demand (HOD) during system operation. Two water-supply reservoirs
(Spring Hollow Reservoir (SHR) and Carvins Cove Reservoir (CCR)) that employ linear-bubble
plume diffusers were studied. A recently validated plume model (Singleton et al., 2007) was
used to predict oxygen addition rates. The results were used together with observed oxygen
accumulation rates to evaluate HOD over a wide range of applied gas flow rates. Plume-induced
mixing correlated well with applied gas flow rates and was observed to increase HOD. Linear
relationships between applied gas flow rate and HOD were found for both SHR and CCR. HOD
was also observed to be independent of bulk hypolimnion oxygen concentration, indicating that
the increase in oxygen concentration is not the cause of the increase in HOD. Interestingly, over
the course of several years, oxygenation resulted in an overall decrease in background HOD as
well as a decrease in the induced HOD during diffuser operation. This indicates that there is less
carry over of latent oxygen demand from one year to the next. Despite the induced increase in
HOD, hypolimnetic oxygenation remains a viable method to replenish oxygen in thermallystratified water-supply reservoirs such as SHR and CCR.
Key Words: Anoxia, Diffuser, Hypolimnion, Linear-plume, Oxygenation
41
3.1 INTRODUCTION
Oxygen consuming processes that occur naturally in lakes and reservoirs include
oxidation of metals (Zaw and Chiswell, 1999; Matthews and Effler, 2006a), algal and aquatic
respiration (Nakamura and Stefan 1994) and aerobic decomposition of organic matter
(Mackenthun and Stefan, 1998; Moore, 2003; Higashino and Stefan, 2005; Matthews and Effler,
2006a). Lakes in temperate climates undergo seasonal stratification, creating a warmer less
dense epilimion, a transitionary thermocline or metalimnion, and a colder denser hypolimnion at
the top, middle, and bottom, respectively (Wetzel, 1975). The corresponding density gradient
limits oxygen transfer from the surface to the lower hypolimnetic water (Mackenthun and Stefan,
1998). Algae growing in the photic zone complete their life cycle and settle through the water
column. This organic detritus undergoes aerobic decomposition in the presence of oxygen,
which contributes to water column oxygen demand (WOD). Incompletely oxidized detritus
settles through the hypolimnion and deposits on the bottom, becoming incorporated into the
sediment and contributing to sediment oxygen demand (SOD) (Higashino and Stefan, 2005;
Matthews and Effler, 2006b; Beutel et al., 2007). The net effect of both WOD and SOD in the
hypolimnion is an overall hypolimnetic oxygen demand (HOD) (Nakamura and Stefan, 1994;
Moore et al., 1996; Beutel et al., 2007). HOD can completely deplete the oxygen in the
hypolimnion and cause anoxia (Burris et al., 2002; Moore, 2003; Matthews and Effler, 2006a;
Singleton and Little, 2006; Beutel et al., 2007) if it exceeds the amount of oxygen available in
the hypolimnion at the beginning of the stratified period. Anoxic conditions result in a shift in
redox potential as oxidation continues, using other terminal electron acceptors with lower
available energy (Madigan et al., 2003). This leads to metals reduction and release of dissolved
compounds from the sediment to the hypolimnetic water. Water quality deteriorates as the
anoxic period lengthens (Matthews and Effler, 2006a; McGinnis and Little, 2006).
Although oxygenation systems are designed to replenish oxygen in the hypolimnion, an
increase in HOD during periods of oxygenation is frequently observed. For example, oxygen
demand was: 1) ten times higher in an aerated hypolimnion compared to a non-aerated control
hypolimnion (Ashley, 1981); 2) clearly higher in an aerated lake compared to a control enclosure
without aeration (McQueen and Lean, 1984); 3) approximately double pre-aeration demand in
Medical Lake, Washington (Soltero et al., 1994); and 4) observed to increase three to four times
42
during aeration (Lorenzen and Fast, 1977; Moore et al., 1996). An increased oxygen
consumption rate during hypolimnetic aeration or oxygenation is termed induced oxygen
demand and is hypothesized to result from (1) increased SOD (Moore, 2003; Beutel et al., 2007)
and (2) increased WOD (Ashley, 1983).
Factors affecting SOD have been extensively studied in laboratory incubated sediment
samples (Moore et al., 1996; Mackenthun and Stefan, 1998; Moore, 2003; Beutel et al., 2007)
and using in-situ SOD chambers (Davis et al., 1987). Researchers have investigated the effect of
surficial versus deep sediments (Matthews and Effler, 2006b), a multi-layered diffusive
boundary (Mackenthun and Stefan, 1998), variations between laminar and turbulent boundaries
at the sediment-water interface (SWI) (Mackenthun and Stefan, 1998; Moore, 2003) and have
shown that changes in SOD are directly related to water velocities over the sediment (Jorgensen
and Revsbech, 1985; Mackenthun and Stefan, 1998; Moore, 2003; Beutel et al., 2007). Although
useful to gain an understanding, small-scale laboratory measurements are not representative of a
real reservoir or lake, and SOD chambers do not accurately capture the naturally occurring
conditions at the SWI. In addition, oxygenation systems do not necessarily affect the entire
sediment surface area (Moore et al., 1996). It has also been hypothesized that aeration or
oxygenation over several years may in fact decrease HOD by more completely satisfying oxygen
demand each year (both WOD and SOD) (McQueen and Lean, 1984; Matinvesi, 1996; Moore et
al., 1996; Matthews and Effler, 2006b), which means less “carry-over” of latent demand from
one year to the next. No comprehensive analysis of induced HOD and no evidence of the
potential long-term decrease in HOD has been published to date. In this research, a detailed
analysis of two full-scale linear bubble-plume systems, located in two different water-supply
reservoirs, and operated over a wide range of applied gas flow rates, is performed. The
objectives are to investigate 1) induced HOD as a function of gas flow rate in both reservoirs
showing the effect of a) plume-induced mixing and b) oxygen concentration in the bulk
hypolimnetic water; and 2) the long term reduction in both background and diffuser-induced
HOD.
43
3.2 MATERIALS AND METHODS
3.2.1 Study sites
Spring Hollow Reservoir (SHR) (Figure 3.1) and Carvins Cove Reservoir (CCR) (Figure
3.2) are water-supply reservoirs for the City of Roanoke and surrounding counties, located on
private heavily forested watersheds in southwestern Virginia, USA. SHR is a pump storage sidestream reservoir, supplied with water withdrawn from the Roanoke River. CCR is supplied by
two natural tributaries that flow from agriculturally dominated lands and two creeks from an
adjoining watershed that are routed through diversion tunnels. Linear bubble-plume diffusers
have been installed in both reservoirs to address negative water quality during summer
stratification. The characteristics of both reservoirs and their respective oxygenation systems are
summarized in Table 3.1. The two reservoirs are quite different. The hypolimnion in the deeper
SHR is relatively unaffected by the wind compared to the hypolimnion in the shallower CCR,
which is more strongly influenced by wind-induced mixing.
3.2.2 Data collection
A Seabird Electronics SBE 19plus (4 Hz sampling rate) high resolution profiler (CTD)
was used to collect conductivity, temperature, and dissolved oxygen (DO) profiles in both
reservoirs throughout the year. The DO probe on the CTD has a response time of 1.4 seconds at
20°C, allowing data to be collected at 0.1 m increments. A Hydrolab DataSonde 4a multiprobe
was used in conjunction with the CTD to obtain profiles at a lower vertical resolution of 0.5 m.
Data collection began in early spring when the reservoirs were accessible and continued
through destratification. SHR and CCR begin to stratify in April, although the thermal structure
typically only stabilizes in May. CCR destratifies in early November while SHR, being deeper,
often remains thermally stratified through January. Water column profiles were collected during
stratified periods on an average of 36 days in CCR (from 2004 to 2007) and 38 days in SHR
(from 2003 to 2007) each year, totaling over 4,800 individual profiles.
Water column profiles were collected along a longitudinal transect at six and seven
locations in SHR and CCR, respectively (Figures 3.1 and 3.2). These locations cover the entire
hypolimnion including locations both upstream and downstream of the diffuser, as well as in the
immediate vicinity of the diffuser. Lateral transects of water column profiles were also collected
44
covering 60 and 80 locations across the diffusers in SHR and CCR, respectively, to more
accurately characterize the bubble plumes (Singleton et al., 2007).
3.2.3 Data analysis
Water column profiles were collected weekly during stratification before diffuser
operation, and at an increased frequency during periods of diffuser operation. The sampling
strategy focused on capturing oxygen conditions in the hypolimnion, before, during, and after
periods of oxygenation. Calculations were performed to determine HOD. This was done by first
analyzing the temperature profiles to determine the boundary between the metalimnion and
hypolimnion and hence the hypolimnetic volume. The water column was then divided into
sections (based on longitudinal sample locations) and layers (based on vertical sample locations)
to determine the oxygen mass associated with the measured oxygen concentrations. Total
oxygen mass as well as volume-averaged oxygen concentrations were determined as a function
of time. In this way, oxygen mass, average oxygen concentration, and hypolimnetic volume
during each time period were used to determine the hypolimnetic oxygen demand based on mass
of oxygen (HODmass) as well as concentration of oxygen (HODconc). The procedure just
described was also used to establish the background HOD when the diffuser was not in
operation. During operation, however, oxygen is being added by the bubble plume. To account
for this, a previously validated plume model (Singleton et al., 2007) was used to estimate the
amount of oxygen added to the hypolimnion. This allowed the induced HOD to be determined
by difference. In a similar fashion, water column temperature profiles were used to determine
the rate of hypolimnetic warming. The rate of warming, which is related to mixing associated
with the diffuser operation, was then used to gain a better understanding of what controls the
induced HOD.
3.2.3.1 Determining location of thermocline
The definition of Wetzel (1975) was used to determine the boundary of the hypolimnion
as the point of intersection between lines drawn through the thermocline and bulk hypolimnion
temperature profiles, similar to the inflection method of Quinlan et al. (2005), which is inclusive
of the lower portion of the metalimnion. Energy exchange between the hypolimnion and
45
metalimnion results in downward thermocline movement throughout the year (Eckert et al.,
2002). The resulting warming of the hypolimnion by the metalimnion is coincident with oxygen
transfer between these layers (Davis et al., 1987; Eckert et al., 2002) and changes the amount of
DO in the hypolimnion. Defining the hypolimnetic volume in this way provides a consistent
basis for evaluating the effects of diffuser-induced mixing.
3.2.3.2 Oxygen content
Dissolved oxygen concentrations were recorded at six and seven sections along SHR and
CCR, respectively. The CTD profiles collected at 0.1 – 0.2 meter increments were converted to
a uniform 0.1 meter grid using linear interpolation between consecutive data points. Each
section was divided into 0.1 meter layers. This created a section (X) –layer (Z) grid that divided
the lake into cells with corresponding oxygen concentrations. The volume-weighted total mass
of oxygen in the hypolimnion was found by multiplying the oxygen concentration (DOx,z) by the
corresponding cell volume (Volx,z) and summing:
s
n
Mass = ∑∑ DOx , z ×Vol x , z
x =1 z =1
where:
Eq. 1
Mass
=
total volume-weighted oxygen mass (kg)
DOx,z
=
oxygen concentration in section (x) and layer (z)
Volx,z
=
cell volume corresponding to section (x) and layer (z)
s
=
number of sections
n
=
number of layers
Dividing mass by total volume yields the volume-weighted oxygen concentration thus
normalizing the data for comparison between years and between reservoirs. An analogous
approach was used to estimate the metalimnetic oxygen content.
46
3.2.3.3 Hypolimnetic oxygen demand (HOD)
HODmass is commonly calculated by dividing the mass differential by the time differential
between two consecutive sample days (Davis et al., 1987) or by plotting hypolimnion oxygen
content against time and finding the slope of a regression line through the data (Lorenzen and
Fast, 1977). Closer examination of these methods has revealed variations in HOD caused by
volume fluctuations from thermocline movement, as previously mentioned. To better
characterize oxygen consuming processes, two calculation methods were employed; (1) using the
regression method outlined by Lorenzen and Fast (1977) for oxygen mass and correcting for
volume fluctuations, and (2) also using the regression method, but based on oxygen
concentration, and multiplying by the averaged hypolimnion volume for the time period. These
two methods, which had an average error between them of less than one percent, were averaged
to determine net oxygen depletion (HODmass) and accumulation rates. Dividing HODmass by each
period’s respective hypolimnion volume, results in a volume-averaged hypolimnion oxygen
demand based on concentration, HODconc. An analogous approach was used to estimate the
metalimnetic oxygen demand (MOD).
3.2.3.4 Oxygenation and bubble-plume model predictions
Bubble plumes release oxygen bubbles directly into the hypolimnion in contrast to other
oxygen transfer devices (for example, the Speece Cone (Moore, 2003; Singleton and Little,
2006) or air lift aerator (Burris et al., 2002)), which entrain hypolimnetic water and discharge
oxygen enriched water through an engineered structure. When designed appropriately, bubbleplume diffusers add oxygen to the deep waters of the hypolimnion, using the detraining plume
and induced circulation to distribute the oxygenated water within the hypolimnion, while
preserving thermal structure (McGinnis et al., 2004; Singleton and Little, 2006; and Singleton et
al., 2007).
A linear bubble-plume model, validated by Singleton et al. (2007), was used to predict
the amount of oxygen added to the hypolimnion during diffuser operation in both SHR and CCR.
The model was used to predict the rate at which oxygen was added to the hypolimnion on each
day of operation when data were collected. Input variables for the model are water column
profiles (temperature, conductivity, and DO), gas flow rate, percent oxygen content in the supply
47
gas, initial bubble size, and diffuser characteristics (depth, width, and length). Model outputs
include oxygen addition, depth of maximum plume rise (DMPR) and corresponding depth of
equal density (ED), and the predicted oxygen and temperature profiles within the plume. DMPR
is the height within the water column to which the plume water rises before detraining, while
ED, which does not account for entrainment into the detraining plume water, is the estimated
maximum depth to which the plume water returns (the depth at which the ambient density is
equal to that of the detraining plume). The oxygen addition rate predicted by the plume model
was compared to hypolimnion accumulation rates to determine HODmass during periods of
diffuser operation.
3.2.3.5 Induced HOD
Induced HOD is calculated by comparing background oxygen depletion rates measured
before diffuser operations began with those observed during oxygenation. Depletion rates during
oxygenation are calculated from the difference between predicted oxygen addition (plume model
output) and observed hypolimnetic oxygen accumulation (water column analysis). The actual
addition rate is expected to be reasonably constant because (1) flow control was maintained by
an Alicat Scientific digital mass flow controller and 2) the bulk DO concentration (Cbulk) of ~10
g m-3 is much lower than the saturated DO concentration (Csat) in the deep water of the
hypolimnion at approximately three and seven bar for CCR and SHR, respectively. An increase
in the bulk dissolved oxygen concentration of a few g m-3 is therefore small relative to the
overall concentration driving force (Csat – Cbulk). Therefore, diminished oxygen accumulation
rates can only be caused by an increased rate of oxygen consumption.
3.2.3.6 Hypolimnetic mixing
Mixing throughout the water column occurs naturally due to wind driven processes and
seiching (Eckert et al., 2002). Natural background mixing results in heat exchange between the
metalimnion and hypolimnion, and subsequently causes thermocline movement and hypolimnion
warming. Oxygenation also induces mixing, which is revealed by (1) increased hypolimnion
warming (Soltero et al., 1994; Thomas et al., 1994), (2) increased turbulence (Ashley 1981,
1983), (3) increased SOD (Moore et al., 1996, Moore, 2003; Beutel et al., 2007), and (4) the
48
tendency to create a uniform temperature throughout the hypolimnion. Oxygenation induced
mixing is therefore quite similar to natural hypolimnetic warming.
The hypolimnion sensible heat content is obtained by calculating the temperature
difference between consecutive sample days for each layer. Comparing sensible heat throughout
the water column, during background conditions and at various diffuser flow rates, provides a
means to determine the degree of induced mixing.
3.2.3.7 Hypolimnetic warming
The rate of hypolimnetic warming is calculated similar to oxygen content described
above. CTD profiles were converted to uniform 0.1 meter layers for each section. The resulting
section (X) – layer (Z) grid divided the reservoir into cells with corresponding temperature data.
The change in sensible heat for each cell was found by multiplying the temperature difference
between sample days (Tx,y+1,z - Tx,y,z) by the corresponding cell mass (mx,y,z) and the sensible heat
coefficient (c), and then summing over each layer:
s
p
Q = ∑∑ m
z
where:
x =1 y =1
x, y,z
× c × (T
x , y +1, z
− T x, y,z )
Eq. 2
Qz
=
change in sensible heat of layer (z) (kJ)
mx,y,z
=
water mass for section (x), day (y), at layer (z) (kg)
c
=
4.190 kJ kg-1 ºC-1
Tx,y,z
=
cell temperature for section (x), day (y), at layer (z) (ºC)
s
=
number of sections
p
=
sample day
Dividing sensible heat by time (the number of days during the sample period) yields a rate of
change of sensible heat for each layer, which is then plotted as a function of elevation. Summing
the rate of change of heat over all hypolimnion layers and normalizing by hypolimnion volume
generates the total rate of change of heat per unit hypolimnion volume, which is used to evaluate
hypolimnetic warming for different applied gas flow rates.
49
3.3 RESULTS AND DISCUSSION
3.3.1 Hypolimnetic anoxia
The oxygenation system in SHR was not operated in 2001, allowing baseline conditions
without diffuser operation to be established. DO profiles collected between mid July and the end
of December (from a single location at the deepest point in the reservoir) are plotted as oxygen
isopleths in Figure 3.3. Selected temperature isopleths ranging from 10 – 12 ºC are also shown,
indicating the boundary between the metalimnion and hypolimnion. Figure 3.3 shows that
oxygen depletion in the hypolimnion is driven by SOD at the bottom, as well as WOD from
detritus settling through the metalimnion into the hypolimnion. In addition, detritus
accumulating in the metalimnion causes MOD, which reduces the amount of oxygen available
for subsequent detrital degradation. This means that algal detritus settling through the
metalimnion during the latter part of the summer exerts a higher WOD in the hypolimnion,
because it is not degraded to the same extent as that settling during the earlier part of the
summer.
As shown in Figure 3.3, hypoxic conditions in the hypolimnion progress upwards from
the bottom over time. This demonstrates the impact that SOD has on oxygen in the hypolimnion,
which coincides with observations reported by Davis et al. (1987), that SOD contributes to ~80%
of HOD during early stratification. Algal growth in the epilimnion occurs between Julian day
220 and 260, and subsequently exerts an oxygen demand in the metalimnion as the algae settle
and begin decomposing. Detrital buildup in the metalimnion occurs because settling velocities
decrease as particles sink from warmer into colder water. The settling velocity of a detritus
particle may be estimated using Stokes Law, or
Ψ
where:
ψ
=
2 g r 2 ( ρ p − ρ w)
9μ
=
settling velocity (m s-1)
50
Eq. 3
g
=
gravitational acceleration (m s-2)
ρp
=
particle density (kg m-3)
ρw
=
water density (kg m-3)
μ
=
dynamic viscosity (kg m-1 s-1)
As an illustration, temperatures in SHR in August are commonly 23 ºC in the epilimnion and 5
ºC in the hypolimnion. Assuming a 140 μm particle size with a density of 1100 kg m-3 [particle
size ranges from 20-200 μm and densities range from 1030-1230 kg m-3 (Agusti et al., 1987;
Weilenmann et al., 1989)], the estimated settling velocity decreases from 0.41 to 0.27 meters
day-1 as the particle sinks through the metalimnion. This means that detritus enters the
metalimnion faster than it exits. The accumulating organic material therefore exerts a
disproportionate oxygen demand at this depth, which can lead to what is known as a
“metalimnetic minimum” in DO concentration.
The decreased oxygen levels in the upper hypolimnion shown in Figure 3.3 suggest that
detritus appeared to be entering the hypolimnion as early as mid-August (~day 225), however;
DO concentrations indicate that detritus was present in the hypolimnion by early October (~day
260). With the lower hypolimnion being essentially anoxic, the dominant contributor to HOD
shifts from SOD to WOD during late stratification (Davis et al., 1987). SOD is limited at this
stage by the large volume of anoxic water above the sediment. By the end of October (~day
300), the metalimnion was also anoxic, limiting further detrital decomposition in that region.
Subsequently, settling detritus enters the hypolimnion, which is close to being completely
anoxic. The detritus settles to the bottom and becomes incorporated in the surface sediment
where it degrades aerobically causing additional SOD. Although the focus thus far has been on
the degradation of organics, there are other factors that contribute to HOD. Figure 3.3 shows the
net effect on the water column from all oxygen depleting processes. HOD is the combination of
WOD, which is primarily detrital decomposition, and SOD, which encompasses biological and
chemical processes (Davis et al., 1987). For this research all oxygen consuming process are
combined in HOD.
51
3.3.2 HOD and gas flow rates
As stated previously, the change in HODmass prior to the start of diffuser operation
represents the background depletion rate. During diffuser operation, HODmass was calculated as
the difference between observed water column accumulation and the oxygen addition predicted
by the plume model. For example, an 8.5 NCMH (5 SCFM) oxygen flow rate applied to the
diffuser in SHR yielded a predicted oxygen addition rate of 266 kg day-1. The observed oxygen
accumulation rate in the hypolimnion during this period was 181 kg day-1. The calculated
HODmass during the period was therefore ~85 kg day-1.
The oxygenation systems were operated at gas flow rates ranging from 2 to 50 NCMH (1
to 30 SCFM) and 17 to 102 NCMH (10 to 60 SCFM) in SHR and CCR, respectively. HODmass
and the corresponding applied gas flow rates are plotted in Figure 3.4, revealing a surprisingly
simple linear relationship for both SHR and CCR. The intercept for both regression lines
represents the background HODmass for each reservoir. The increase in HODmass above the
background represents induced HODmass. The relationship between induced demand and applied
flow rate is 22 and 6 times flow rate in NCMH for CCR and SHR, respectively. For example,
during the initial years following oxygenation at SHR, background HODmass was observed to be
~50 kg day-1. For every 1 NCMH increase, the demand increases approximately 6 kg day-1. At
17 NCMH the HODmass is ~150 kg day-1; 50 kg day-1due to background depletion and 100 kg
day-1 induced by operating the diffuser. Likewise, the same flow rate applied to CCR would
yield an induced demand of ~370 kg day-1, with an overall HODmass of ~790 kg day-1.
Increasing the gas flow rate to the diffuser increases both the degree of mixing in the
hypolimnion as well as the oxygen concentration, and both of these consequences could be
expected to increase the HOD. For example, increasing the DO concentration increases the
driving force for oxygen transfer across the diffusive boundary layer (DBL), which would
increase the flux of oxygen into the sediments. The increased turbulence could also decrease the
thickness of the DBL at the sediment water interface, which would also increase the flux of
oxygen into the sediments (Jorgensen and Revsbech, 1985; Mackenthun and Stefan, 1998;
Moore, 2003; Beutel et al., 2007). To get a rough idea as to which of these two effects
(concentration versus mixing) is most important; we first examine the rate of diffuser-induced
hypolimnetic warming caused by the bubble-plume.
52
3.3.3 Hypolimnetic warming
As previously described, temperature profiles were analyzed during each flow period to
determine the rate of hypolimnetic warming. Figure 3.5 shows warming rate versus elevation for
the range of gas flow rates applied to the SHR diffuser. The data for each period of diffuser
operation are truncated at the hypolimnion boundary. The higher rates of warming in the upper
hypolimnion indicate warmer water from the metalimnion mixing with the upper hypolimnion.
It also clearly shows the higher warming rates throughout the hypolimnion at higher gas flow
rates. As higher gas flow rates are applied, more heat is added to the hypolimnion confirming
that the plume causes an increase in mixing. A similar effect was observed in CCR although the
data are not shown here.
To more closely relate mixing (observed as hypolimnetic warming) to gas flow rates,
total warming rates per unit hypolimnion volume (KJ m-3 day -1) were calculated for both SHR
and CCR, as shown in Figure 3.6. Background warming (at 0 NCMH) for SHR is shown over a
range of values, indicating that hypolimnion warming is at times affected by external conditions.
The hypolimnetic warming rates during diffuser operations for SHR were calculated over a range
of applied gas flow rates between 2003 and 2007. Data for CCR are also plotted over a wide
range of applied gas flow rates observed between 2005 and 2007. As stated previously, CCR is
more strongly affected by wind, which is evident in the variability of warming rates at equivalent
applied gas flow rates. Figure 3.6 nevertheless reveals a relationship between applied gas flow
rate and diffuser-induced mixing in the hypolimnion. Overall, the relationship between
hypolimnetic warming and gas flow rate is quite similar to that between HODmass and gas flow
rate, suggesting that mixing induced by the bubble plume is a primary contributor to the increase
in HOD.
3.3.4 Effect of bulk oxygen concentration on HOD
Mass transfer theory suggests that the driving force for sediment oxygen demand is
regulated by the movement of water over the sediment, coupled with the oxygen concentration
driving force between the bulk water and the sediment (Cbulk – Csed) (Jorgensen and Revsbech,
1985). SHR’s low background HOD allows several months of data to be collected before
oxygenation is necessary. Figure 3.7 shows volume-averaged DO in the metalimnion and
53
hypolimnion of SHR. The 2004 – 2006 data represents DO during summer stratification, but
prior to diffuser operation. The 2007 data are included even though the diffuser was operated
continuously, albeit at a low flow rate, during this year. The gas flow rate applied to the diffuser
during 2007 was only 1.3 NCMH (0.75 SCFM), a flow rate which is believed to induce
negligible mixing or oxygen demand.
Closer examination of Figure 3.7 indicates that the background depletion rates are
essentially the same in both metalimnion and hypolimnion for all years, with the exception of
2007 where the DO in the hypolimnion was maintained relatively constant. In 2007, the
hypolimnion DO averaged 11.9 mg l-1 (standard deviation of 0.2 mg l-1) during stratification
revealing a balance between oxygen addition and depletion. Uniform and continuous oxygen
addition coupled with steady oxygen consumption resulted in a constant oxygen concentration.
Despite the elevated oxygen concentration maintained in the hypolimnion, the depletion rate did
not change. The metalimnion data also show a slightly lower depletion rate than in previous
years. The decreased oxygen consumption rate in the metalimnion could be caused by increased
oxygen transport from the hypolimnion or by reduced organic loading.
Traditionally it is hypothesized that higher oxygen concentrations fuel increased
depletion rates (Jorgensen and Revsbech, 1985). However, increased oxygen concentrations in
the bulk water did not yield elevated depletion rates in the hypolimnion or the metalimnion. In
fact, the opposite was observed with decreased HODconc over time despite higher oxygen
concentrations. Table 3.2 summarizes initial DO conditions and corresponding concentration
depletion rates (HODconc and MODconc) in SHR, as shown in Figure 3.7. Table 3.2 results are
plotted in Figure 3.8 showing decreasing HODconc in SHR between 2003 and 2007. Because the
increased oxygen concentrations did not affect HODconc, it can be concluded that induced HOD
is primarily caused by mixing. The relatively constant background depletion rates shown in
Figure 3.7 and Table 3.2 for 2004 – 2006 further suggest that HOD is a zero-order process, with
respect to oxygen concentration. In addition, the higher initial concentration for 2004 – 2006
also has no apparent effect on the subsequent depletion rate, again suggesting a zero-order
consumption process.
54
3.3.5 Effective depth
Ashley (1983) suggested that the increase in oxygen demand in Black Lake was a result
of hypolimnetic aeration increasing the “effective depth” of the hypolimnion. The idea is that
settling material is stirred up, prolonging the residence time of the organic detritus in the aerobic
decomposition zone. In order to evaluate this idea, sediment traps were deployed in CCR. The
traps were positioned two meters above the sediment at locations directly over the diffuser
(sample locations CC, CB, and CE) and at locations 600 and 1150 meters away from the diffuser
(sample locations C2 and C3). The sediment traps were monitored bi-weekly for total solids
between September and October. The results in Table 3.3 show increased deposits in the
sediment traps directly over the diffuser compared to those positioned further away. These
results suggest that settling particles in the vicinity of the diffuser remain suspended in the water
column longer, as they continue to be circulated from mixing induced by the bubble plume.
Organic material suspended longer in the water column experiences more complete oxidation,
leading to increased WOD and thus increased HOD.
3.3.6 Long term reduction of Background HOD
As stated previously, it has been hypothesized that oxygenation over several years may
decrease HOD (McQueen and Lean, 1984; Matinvesi, 1996; Moore et al., 1996; Matthews and
Effler, 2006a). To test this hypothesis, HODconc and MODconc were analyzed in SHR between
2003 and 2006 during periods when the diffuser was not operated. The oxygenation system in
SHR was operated for two-week periods in June and again in August in 2003. The period
between diffuser operations and following the August operation were analyzed for HODconc. In
2004 – 2006 HODconc was calculated during similar periods of stratification before the diffuser
was operated. In 2007 HODconc was calculated as the difference between hypolimnion
accumulation and the amount added as predicted by the bubble-plume model. Since the 2003 to
2006 data represents HODconc during periods without oxygenation, and the 2007 data do not
include any induced HOD, these rates all represent background HODconc during the five year
period. MODconc represents the averaged rates observed in the metalimnion during the same time
periods. Figure 3.8 is a plot of MODconc and HODconc over time, showing a decreasing trend in
55
background HODconc. Interestingly, Figure 3.7 also shows that the initial DO concentration was
higher each year in both the metalimnion and the hypolimnion.
It can be argued that hypolimnion volume may affect oxygen depletion rates because a
different volume will encompass a different sediment surface area. Hypolimnion volumes were
determined over the same time periods, but there was no statistical relationship between HODconc
and hypolimnetic volume. It can also be argued that decreased HODconc is caused by decreased
organic loading. If less material is settling through the water column, there would be less oxygen
consuming organics settling through the hypolimnion. However, if MODconc remains relatively
constant, indicating steady organic loading, but the HODconc decreased anyway, then the decrease
can only be accounted for by factors contributing to HOD. MODconc analyzed in conjunction
with the hypolimnion data shows a slight increase over time (Figure 3.8). The increased
MODconc suggests that organic loading to SHR did not change. Therefore, it is concluded that
the reduced oxygen demand is due to reduced HOD (both SOD and WOD) and is not affected by
sediment surface area related to hypolimnion volume fluctuations or by changes in organic
loading. The regression line in Figure 3.8 for the HOD shows a steady (linear) decrease in
HODconc, suggesting that long-term oxygenation in SHR has lead to a decrease in HODconc.
3.3.7 Long term reduction of induced HOD
Although the decreasing background HODconc in SHR shows a benefit of long-term
oxygenation, historical data for CCR are not available. To further investigate the long term
affect of oxygenation on HOD, induced HODconc was investigated. Induced HODconc was
analyzed in SHR (2004 – 2007) and in CCR (2006 – 2007) over a variety of flow rates. HODconc
is plotted over time with the applied gas flow rate labeled on each data point, as shown in Figure
3.9. HODconc was calculated as the difference between bubble-plume model predictions and
hypolimnion accumulation. Figure 3.9 shows that induced HODconc decreased over time in both
SHR and CCR for equivalent gas flow rates applied to the diffuser. In addition, higher gas flow
rates in more recent years induced approximately the same HODconc found in earlier years. For
example, SHR diffuser operation in 2006 at 8.5 and 17 NCMH resulted in similar HODconc rates
in 2005 at flow rates of 3.4 and 8.5 NCMH, respectively. Likewise CCR HODconc values during
56
2007 at flow rates of 51 and 85 NCMH were similar to HODconc values during 2006 at flow rates
of 34 and 51 NCMH.
3.4 CONCLUSION
Hypolimnetic oxygenation systems were operated in two reservoirs (SHR and CCR) over
a wide range of applied gas flow rates. Temperature and oxygen profiles were collected to
monitor hypolimnion warming and oxygen accumulation rates. A linear-bubble plume model
was used to estimate the rate of oxygen addition to the hypolimnion. Accelerated warming was
observed in both reservoirs, but was not substantial enough to significantly disrupt thermal
stratification. The rate of warming was a function of the applied gas flow rate, indicating that
hypolimnion warming is a consequence of diffuser-induced mixing. HOD was also observed to
increase with applied gas flow rate. The increase in HOD was observed to be independent of
oxygen concentration in the hypolimnion. Although induced mixing increases HOD, long-term
oxygenation appears to decrease the background HOD as well as the induced HOD over time.
Background HOD in SHR decreased linearly from 2003 to 2007. Induced HOD in SHR and
CCR was observed to be lower in later years compared to equivalent flow rates applied in earlier
years. Despite the increased oxygen demand and warming during diffuser operation, the
oxygenation systems were able to successfully replenish hypolimnetic oxygen in both reservoirs.
The results indicate that hypolimnetic oxygenation is a viable method to manage source waters in
water-supply reservoirs such as SHR and CCR.
3.5 REFERENCES
Agusti, S., Duarte, C. M., and Kalff, J. (1987), Algal Cell Size and the Maximum Density and
Biomass of Phytoplankton, Limnology and Oceanography, 32 (4), 983-986.
Ashley, K. I. (1981), Effects of Hypolimnetic Aeration on Functional Components of the Lake
Ecosystem, Masters of Science Thesis, University of British Columbia.
Ashley, K. I. (1983), Hypolimnetic Aeration of a Naturally Eutrophic Lake: Physical and
Chemical Effects, Canadian Journal of Fisheries and Aquatic Sciences, 40 (9), 13431359.
57
Benoit, G. and Hemond, H. F. (1996), Vertical Eddy Diffusion Calculated by the Flux Gradient
Method: Significance of Sediment – Water Heat Exchange, Limnology and
Oceanography, 41 (1), 157-168.
Beutel, M., Hannoun, I., Pasek, J., and Kavanagh, K. B. (2007), Evaluation of Hypolimnetic
Oxygen Demand in a Large Eutrophic Raw wAter Reservoir, San Vicente Reservoir,
Calif., Journal of Environmental Engineering, 133 (2), 130-138.
Burris, V. L., McGinnis, D. F. and Little, J. C. (2002), Predicting Oxygen Transfer and Water
Flow Rate in Airlift Aerators, Water Research, 36, 4605-4615.
Davis, W. S., Fay, L. A., and Herdendorf, C. E. (1987), Overview of USEPA/Clear Lake Erie
Sediment Oxygen Demand Investigations during 1979, Journal of Great Lakes Research,
13 (4), 731-737.
Eckert, W., Imberger, J., and Saggio, A. (2002), Biogeochemical Response to Physical Forcing
in the Water Column of a Warm Monomictic Lake, Biogeochemistry, 61, 291-307.
Higashino, M. and Stefan, H. G. (2005), Sedimentary Microbial Oxygen Demand for Laminar
Flow Over a Sediment Bed of Finite Length, Water Research, 39, 3153-3166.
Jorgensen, B. B. and Revsbech, N. P. (1985), Diffusive Boundary Layers and Oxygen Uptake of
Sediments and Detritus, Limnology and Oceanography, 30 (1), 111-222.
Lorenzen, M. W. and Fast, A. W. (1977), A Guide to Aeration / Circulation Techniques for Lake
Management, Ecol. Res. Ser. EPA-600/3-77-004. US. Environmental Protection Agency.
Mackenthun, A. A. and Stefan, H. G. (1998), Effect of Flow Velocity on Sediment Oxygen
Demand: Experiments, Journal of Environmental Engineering, 124 (3), 222-230.
Madigan, M. T., Martinko, J. M., and Parker, J. (2003), Brock Biology of Microorganisms Tenth
Edition, Pearson Education Inc., New Jersey, 115-116, 576.
Matinvesi, J. (1996), The Change of Sediment Composition During Recover of Two Finnish
Lakes Induced by Waste Water Purification and Lake Oxygenation, Hydobiologia, 335,
193-202.
Matthews, D. A. and Effler, S. W. (2006a), Assessment of Long-term Trends in the Oxygen
Resources of a Recovering Urban Lake, Onondaga Lake, New York, Lake and Reservoir
Management, 22 (1), 19-32.
58
Matthews, D. A. and Effler, S. W. (2006b), Long-term Changes in the Areal Hypolimnetic
Oxygen Deficit (AHOD) of Onondaga Lake: Evidence of Sediment Feedback, Limnology
and Oceanography, 51 (1, part 2), 702-714.
McGinnis, D. F. and Little, J. C. (2002), Predicting Diffused-Bubble Oxygen Transfer Rate using
the Discrete-Bubble Model, Water Research, 36, 4627-4635.
McGinnis, D.F., Lorke, A., Wüest, A., and Little, J.C. (2004), Interaction Between a Bubble
Plume and the Near Field in a Stratified Lake, Water Resources Research, 40 (10),
citation number W10206.
McQueen, D. J. and Lean, D. R. S. (1984), Hypolimnetic Aeration: Changes in Bacterial
Populations and Oxygen Demand, Archiv Fur Hydrobiologie, 99 (4), 498-514.
Moore, B. C., Chen, P. H., Funk, W. H., and Yonge, D. (1996), A Model for Predicting Lake
Sediment Oxygen Demand Following Hypolimnetic Aeration, Water Resources Bulletin
American Water Resources Association, 32 (4), August 1996.
Moore, B. (2003), Downflow Bubble Contact Aeration Technology (Speece Cone) for Sediment
Oxygenation, Remediation of contaminated sediments – 2003 proceedings of the Second
International Conference on Remediation of Contaminated Sediments, September 30, all.
Nakamura, H. and Stefan, H. G. (1994), Effect of Flow Velocity on Sediment Oxygen Demand:
Theory, Journal of Environmental Engineering, 120 (5), 996-1016.
Quinlan, R., Paterson, A. M, Smol, J. P., Douglas, M. S.V., and Clark, B. J. (2005), Comparing
Different Methods of Calculating Volume-weighted Hypolimnetic Oxygen (VWHO) in
Lakes, Aquatic Science, 67, 97-103.
Singleton, V. L. and Little, J. C. (2006), Designing Hypolimnetic Aeration and Oxygenation
Systems – A Review, Environmental Science & Technology, 40, 7512-7520.
Singleton, V.L., Gantzer, P., Little, J.C. (2007), Linear Bubble Plume Model for Hypolimnetic
Oxygenation: Full-scale Validation and Sensitivity Analysis, Water Resource Research,
43, W02405, doi:10.1029/2005WR004836.
Soltero, R. A., Sexton, L. M., Ashley, K. I., and McKee, K. O. (1994), Partial and Full Lift
Hypolimnetic Aeration of Medical Lake, WA to Improve Water Quality, Water
Research, 28 (11), 2297-2308.
59
Thomas, J. A., Funk, W. H., Moore, B. C., and Budd, W. W. (1994), Short Term Changes in
Newman Lake Following Hypolimnetic Aeration with the Speece Cone, Lake and
Reservoir Management, 9 (1), 111-113.
Weilenmann, U., O'Melia, C. R., and Stumm, W. (1989), Particle Transport in Lakes: Models
and Measurements, Limnology and Oceanography, 34 (1), 1-18.
Wetzel, R. G. (1975), Limnology, W. B. Saunders Company, Philadelphia, 68-71.
Zaw, M. and Chiswell, B. (1999), Iron and Manganese Dynamics in Lake Water, Water
Research, 33 (8), 1900-1910.
60
3.6 FIGURES AND ILLUSTRATIONS
Figure 3.1 Bathymetry, bottom profile, diffuser locations, and sampling stations in Carvins Cove Reservoir.
61
Figure 3.2 Bathymetry, bottom profile, diffuser location, and sampling stations in Spring
Hollow Reservoir.
62
425
16.0
420
420
14.0
415
12.0
410
410
10.0
Elevation (m)
405
8.0
400
400
395
390
380
390
4.0
385
3.0
380
375
370
200
370
220
240
260
280
Julian Day (2001)
Figure 3.3 Oxygen isopleths in SHR during 2001 summer stratification.
Temperature isopleths represent the lower thermocline boundary.
63
300
320
340
6.0
5.0
2.0
1.0
0.0
3500
2500
CCR
y = 21.8x + 418.2
2000
R = 0.9
-1
HODmass (kg day ) _
3000
2
1500
1000
SHR
y = 6.2x + 47.4
500
2
R = 0.9
0
0
20
40
60
80
100
120
Oxygen Flow Rate (NCMH)
Figure 3.4 Hypolimnetic oxygen demand in SHR (2003-2007) and CCR (2006-2007) as a
function of applied gas flow rate.
The y-intercept represents the background HODmass.
64
1320
396
1300
390
1280
384
1260
378
1240
372
1220
366
1200
-3E+06
-2E+06
-1E+06
0E+00
1E+06
2E+06
Elevation (ft) _
Elevation (m) _
402
3E+06
-1
Heat Flux (kJ day )
2
4
8
17
34
47 (NCMH)
Figure 3.5 Hypolimnetic warming in SHR as a function of elevation and applied gas flow rate.
65
450
350
-3
-1
Heat Flux (kJ m day ) _
400
300
250
200
150
100
50
0
0
20
40
60
80
100
Flow Rate (NCMH)
Carvins Cove Reservoir
Spring Hollow Reservoir
Figure 3.6 Total warming rates per unit hypolimnion volume in CCR and SHR as a function of
applied gas flow rate.
66
14.0
Dissolved Oxygen (mg l
-1
) _
12.0
10.0
8.0
6.0
4.0
2.0
0.0
Jan-04
Jan-05
Jan-06
Jan-07
Jan-08
Date
Hypolimnion
Metalimnion
Figure 3.7 Volume-averaged DO in the metalimnion and hypolimnion in SHR (2004-2007).
Data shows background concentration rates observed before diffuser operation for 2004 to 2006.
2007 data shows continuous diffuser operation at 1.3 NCMH thus maintaining a relatively
constant DO concentration over time.
67
-1
0.05
HODconc and MODconc (mg l
0.06
-1
day ) _
0.07
0.04
0.03
0.02
0.01
0.00
Jan-03
Jan-04
Jan-05
Jan-06
Jan-07
Jan-08
Date
Hypolimnion
2007 Hypolimnion (Diffuser ON)
Metalimnion
Figure 3.8 Background hypolimnion and metalimnion oxygen demand observed in SHR
between 2003 and 2007.
68
0.9
101
0.8
85
day
-1
) _
0.7
0.6
HODconc (mg l
-1
0.5
68 68
0.4
51
34
85
68
0.3
34
51
0.2
0.1
17
8
8
8
4 2
3
34
17
0.0
Jan-04
Jan-05
Jan-06
Jan-07
1
Jan-08
Date
SHR (04)
SHR (05)
SHR (06)
SHR (07)
CC (06)
CC (07)
Figure 3.9 Net oxygen loss (HODconc) in SHR and CCR over time with applied gas flow rate (in
NCMH) shown on data points.
69
3.7 TABLES
Table 3.1 Summary of reservoir and oxygenation characteristics for SHR and CCR.
Parameter
Max Depth
Surface Area
Spring Hollow Reservoir
62 m
2
0.61 km
(200 ft)
(150 acres)
Volume
Hypolimnion Volume
Oxygen Demand
Side Slopes
Diffuser Length
Design Flow Rate
Design O2 Addition
13 × 106 m3
(3.4 × 109 gal)
15% total volume
~50 kg day-1
1.5 - 2 : 1
620 m
(2000 ft)
34 NCMH
(20 SCFM)
~1000 kg day-1
O2 Transfer Efficiency
95% - 98 %
70
Carvins Cove Reservoir
23 m
2
2.5 km
(70 ft)
(1100 acres)
24 × 106 m3
(6.2 × 109 gal)
25% total volume
~430 kg day-1
8-20 : 1
1250 m
(4000 ft)
68 NCMH
(40 SCFM)
~2100 kg day-1
80% - 85 %
Table 3.2 Summary of initial DO and depletion rates for hypolimnion and metalimnion in SHR.
Hypolimnion
Date
Initial
DO
(mg l-1)
Depletion Rate
(mg l -1 d-1)
Aug-03
Nov-03
Oct-04
Aug-05
Aug-06
Aug-07
9.2
12.7
5.4
6.7
8.8
11.9
0.049
0.047
0.046
0.036
0.039
0.030
71
Metalimnion
Initial
DO
(mg l-1)
Depletion Rate
(mg l -1 d-1)
8.1
6.8
7.6
9.5
10.9
0.047
0.050
0.052
0.057
0.037
Table 3.3 Summary of sediment trap data collected in CCR between September and October
2006.
Distance
from
diffuser
Position
over
diffuser
Total Solids (mg l -1)
Begin (CC)
Middle (CB)
End (CE)
700 meters
(C2)
1150 meters
(C3)
25-Sep
14,700
25,500
13,650
Date (2006)
6-Oct
13,800
23,100
16,300
20-Oct
27,200
25,700
20,900
3,400
5,000
5,400
9,400
9,600
4,000
72
CHAPTER 4: CONTROLLING IRON AND MANGANESE IN A WATER SUPPLY
RESERVOIR USING HYPOLIMNETIC OXYGENATION
Gantzer, P.A. and Little, J.C.
ABSTRACT
Dissolved metals such as iron (Fe) and manganese (Mn) are commonly found in water-supply
reservoirs if hypolimnetic hypoxia occurs during summer stratification. This study provides
detailed results of Mn and Fe behavior following the installation of a hypolimnetic oxygenation
system in Carvins Cove Reservoir, a water-supply impoundment operated by the Western
Virginia Water Authority. During oxygenation, manganese concentrations were observed in
benthic waters immediately adjacent to the sediment, but were very low in the bulk hypolimnion.
Eliminating hypoxic conditions during stratification and driving the oxic/anoxic boundary closer
to or below the sediment/water interface decreases metal concentrations in the hypolimnion.
These results show that source-water control of dissolved metals can be accomplished with
hypolimnetic oxygenation.
Key Words: Anoxia, Bubble plume, Hypolimnion, Hypoxia, Manganese, Oxygenation
73
4.1 INTRODUCTION
Dissolved metals such as iron (Fe) and manganese (Mn) may be found in water supply
reservoirs as a result of transport from the sediments (Hoffmann and Eisenreich, 1981; Balzer,
1982; Zaw and Chiswell, 1999). Increased concentrations of the dissolved, reduced forms of
these metals coincide with hypoxic conditions in the hypolimnion during thermal stratification in
the summer (Yagi, 1996; Wann et al., 1997).
Manganese is the fifth most abundant element on the earth’s surface and is the second
most abundant transition element after iron, yet it is one of the least well understood (Nealson et
al., 1988). Manganese has several oxidation states, and has slow oxidation kinetics in the low
pH environments typical of natural waters (Balzer, 1982). It is autocatalytic, and not only coprecipitates, but has strong binding capabilities (Nealson et al., 1988). Manganese is persistent
in soluble forms despite unfavorable thermodynamics (Balzer, 1982) and is thus more easily
recycled back to the water column (Wann et al., 1997) than iron. Manganese reduction occurs
rapidly when oxygen is depleted and is especially prevalent at the oxic/anoxic boundary.
Manganese redox processes are a function of both biogeochemical and chemical oxidation
(Nealson, et al., 1988). Mn is a continuous oxygen consumer and a nuisance contaminant to the
water treatment industry (Stauffer, 1986) because it is easily reduced and transported from the
sediment to the overlying water column. As a result, it is difficult to remove (oxidize and
precipitate) from the water column and bury in bottom sediments. Iron, on the other hand, is
easily oxidized in the presence of oxygen (via chemical oxidation alone) at the oxic/anoxic
boundary (Schaller et al., 1997) and is therefore much easier to control than Mn. Despite
potentially effective removal of dissolved metals in the water treatment process, recent studies
have shown that Mn can re-dissolve in sedimentation basins because of the formation of anoxic
conditions in the settled sludge (Budd et al., 2007).
Several techniques have been employed to address hypolimnetic hypoxia and the
subsequent release of dissolved metals, including air-lift aerators, hypolimnetic oxygenation and
artificial destratification (Singleton and Little, 2006). The literature provides a wealth of
knowledge about Mn behavior in natural systems (Nealson et al., 1988), but Mn behavior during
hypolimnetic oxygenation is not well understood. For example, Zaw and Chiswell (1999)
74
reported an initial decline in Mn concentrations during artificial aeration, but also reported an
unexplained increase in Mn during subsequent years.
The Western Virginia Water Authority (WVWA) installed an oxygenation system in
Carvins Cove Reservoir (CCR) in 2005. In this research, we provide detailed results of Mn and
Fe behavior in both soluble and particulate forms at various locations in the reservoir. We
consider oxygen concentration and distribution within the reservoir, mixing induced by the
oxygenation system, and the effect that the location of the oxic/anoxic boundary has on dissolved
metals and their subsequent mobility during both stratified and destratified periods.
4.2 MATERIALS AND METHODS
4.2.1 Carvins Cove Reservoir
Carvins Cove Reservoir (CCR) (Figure 4.1) is a man-made, monomictic, eutrophic,
water-supply reservoir for the City of Roanoke and surrounding counties in southwestern
Virginia, USA. CCR is located on a private, heavily forested watershed and is supplied by two
natural tributaries that flow from agriculturally dominated lands, and by two creeks from an
adjoining watershed that are routed through diversion tunnels. Two linear bubble-plume
diffusers (considered as a single oxygenation system) were installed to improve water quality
during summer stratification. The characteristics of the reservoir and the oxygenation system are
summarized in Table 4.1.
4.2.2 Data collection and analysis
Monthly water samples were collected between 1999 and 2005 by CCR staff at each
intake elevation at the water intake tower. There are six withdrawal elevations positioned 3.1 m
(10 ft) apart, with the highest located at an elevation of 353.6 m (1160 ft). Samples were
analyzed for Fe and Mn by the colorimetric spectrophotometer method (Clesceri et al., 1998).
After the oxygenation system was installed, the sampling strategy was modified to more
closely monitor Fe and Mn in the reservoir. This included water column samples as a function of
depth as well as pore water samples from a single location in 2006 (CC) and two locations in
2007 (CC and C3) (see Figure 4.1). CC is located at the deepest point in the lake, which is
75
approximately 150 m (500 ft) in front of the intake tower and has a bottom elevation of 335.3 m
(1100 ft). C3 is located approximately 2000 meters upstream of CC and has a bottom elevation
of 341.4 m (1120 ft). Water column samples were collected using a 1.2-l Kemmerer bottle at the
same elevations as previously used by CCR staff. One additional elevation (335.3 m (1100 ft)),
located in the benthic region of the water column, was added following the installation of the
oxygenation system. The benthic region is defined as the lower end of the steep dissolved
oxygen (DO) gradient (on a DO-versus-elevation plot) just above the sediment-water interface
(SWI). This region generally extended from 0.1 to 0.3 m above the SWI. Water column
samples for total metal analyses were transferred from the Kemmerer bottle directly to preacidified plastic bottles. Samples for soluble metal analyses were filtered through 0.45 μm
Millipore filter paper before being transferred to pre-acidified plastic bottles. All samples
collected during 2006 and 2007 were analyzed for metal concentrations using the inductively
coupled plasma (ICP) method (Clesceri et al., 1998).
4.2.3 Water column profiles
A Seabird Electronics SBE 19plus (4 Hz sampling rate) high resolution profiler (CTD)
was used to collect conductivity, temperature and dissolved oxygen profiles throughout the year.
The DO probe on the CTD has a response time of 1.4 seconds at 20°C, allowing data to be
collected at about 0.1 m increments. A Hydrolab DataSonde 4a multiprobe was used in
conjunction with the CTD to obtain profiles at 0.5 m increments.
CCR begins to stratify in April, although the thermal structure typically stabilizes in May
and is destratified in early November. Water column profiles were collected along a longitudinal
transect at seven locations, as shown in Figure 4.1. These locations cover the entire
hypolimnion, including locations both upstream and downstream of the diffuser. Water column
profiles were used to identify the thermocline position (as defined by Wetzel (1975)), to
determine metalimnion and hypolimnion oxygen content (Gantzer and Little, 2008; In
Preparation) and also to estimate the location of the oxic/anoxic boundary near the SWI.
76
4.2.4 Diffuser operation
The oxygenation system was installed and first operated in August 2005 and has been in
continuous operation with the exception of a one month test period (between June 15 and July
14, 2006) when the oxygen supply was turned off. The diffuser was operated at flow rates
ranging from ~8 to 100 normal cubic meters per hour (NCMH) with the applied gas flow rate
based on oxygen requirements needed to maintain elevated DO throughout the hypolimnion.
4.3 RESULTS AND DISCUSSION
4.3.1 Historical observations
Contour plots (using Kriging for interpolation) based on monthly water column samples
of Fe and Mn are shown in Figures 4.2 and 4.3, respectively. The DO concentration isopleth of
5.0 mg l-1 is superimposed as a blue line labeled ‘DO 5’. Figures 4.2 and 4.3 show that:
1. soluble metal concentrations fluctuated annually depending on whether the reservoir
was either stratified or destratified;
2. during stratified periods, low DO promoted the transport of soluble metals to the bulk
water; and
3. during destratified periods, the uniformly high DO and stronger mixing decreased the
soluble metals to low concentrations.
It is clear from Figure 4.2 that Fe reached problem levels following the annual decrease
in dissolved oxygen. When oxygen in the bottom water was replenished during fall turnover,
soluble Fe quickly decreased. The rapid oxidation of Fe in the presence of oxygen has been
observed by many researchers (Balzer, 1982; Stauffer, 1986; Wann et al., 1996; Schaller et al.,
1997; Burns, 1998; Zaw and Chiswell, 1999; Ismail et al., 2002). In contrast, Mn oxidation is
not determined by thermodynamics alone (Balzer, 1982; Nealson et al, 1988), and the behavior
of Mn in response to hypolimnetic oxygen requires closer examination.
4.3.2 Comparison between stratified and destratified periods
The data in Figures 4.2 and 4.3 has been reduced to the historical data collection
frequency and only provides a general overview of soluble metals in CCR. The more recent Mn
77
data are therefore plotted in Figure 4.4 (without any distortion due to interpolation), summarizing
soluble Mn concentrations in the benthic region as well as at three other elevations within the
hypolimnion for both 2006 and 2007. The terms ‘stratified’ and ‘destratified’ are also used to
denote summer and winter conditions, respectively. During the destratified period:
1. wind mixes the entire water body, creating turbulence down to the SWI;
2. DO remains near saturation (with respect to the surface) in the water column and
down to the SWI; and
3. turbulence and elevated DO at the SWI drives the oxic/anoxic boundary deeper into
the sediment (Balzer, 1982).
During destratification, soluble metal concentrations were very low, as shown in Figure
4.4 from December 2006 through April 2007. Low Mn concentrations are thought to be caused
by the deeper location of the oxic/anoxic boundary (Balzer, 1982; Katsev et al., 2007) as well as
large-scale dilution (Yagi, 1996). As oxygen penetrates deeper into the sediment, transport of
soluble metals to the water column is decreased because of slow diffusion within the sediments,
as well as oxidation occurring within the near-surface sediments (Balzer, 1982; Nealson et al.,
1988; Yagi, 1996). In the event that soluble metals do reach the water column, they come into
contact with a large volume of well-mixed, well-oxygenated water and are rapidly diluted and/or
oxidized.
During the stratified period:
1. the hypolimnion is isolated from the epilimnion as a result of the vertical density
gradient, and is relatively quiescent, with low turbulence (Schoemann et al., 1998);
2. DO-consuming processes in both the sediment and the bulk hypolimnion deplete
available oxygen, potentially leading to hypoxia (Gantzer and Little, 2008; In
Preparation); and
3. the quiescence and low DO at the SWI causes the oxic/anoxic boundary to move out
of the sediments and into the water column (Balzer, 1982; Stauffer, 1986; Wann et
al., 1997; Katsev et al., 2007), allowing soluble metals in the sediment to be released
into the overlying water (Hoffman and Eisenreich, 1981; Balzer, 1982; Yagi, 1996;
Schaller et al., 1997; Wann et al., 1997; Schoemann et al., 1998).
As the oxic/anoxic boundary moves out of the sediment, soluble Mn is transferred more
easily into the water column (Balzer, 1982; Stauffer, 1986; Nealson et al. 1988; Schaller et al.,
78
1997; Wann et al., 1997) and has been observed to persist in the reduced form despite the
presence of oxygen (Balzer, 1982). Manganese concentrations are initially observed to be higher
in the benthic region, probably because transport is restricted to diffusion (Stauffer, 1986;
Nealson et al., 1988; Johnson et al., 1996; Schoemann et al., 1998; Katsev et al., 2007), with
little to no advection because of the relatively quiescent state of the hypolimnion.
When the oxygenation system is in operation, mixing in the hypolimnion is primarily
governed by the operation of the diffuser (Gantzer and Little, 2008; In Preparation). While
variations in the diffusive boundary layer at locations both near to and far from the diffuser
indicate turbulence in the hypolimnion, the data also show that the degree of induced mixing is
lower than that observed during destratification (Bryant and Little, 2008; In Preparation). Water
column profiles collected with the CTD were used to estimate the relative location of the
oxic/anoxic boundary. The CTD has a stainless steal cage around it that allows it to be
positioned within a few centimeters of the SWI. To check these measurements, bottom DO
profiles obtained with the CTD were compared to micro-sensor DO profiles obtained from
extracted sediment cores. The bottom DO measured with the CTD was found to be essentially
the same as the DO measured just above the sediment using the oxygen microsensor (Bryant and
Little, 2008; In Preparation). Figure 4.5 shows bottom DO recorded by the CTD as well as the
volume-weighted average hypolimnion DO calculated from CTD profiles. The bottom DO is an
arithmetic average of measurements made at all seven sample locations (shown in Figure 4.1)
with error bars representing the 1st and 3rd quartile. Figure 4.5 shows that the bottom DO tracks
the average hypolimnion DO and that the difference between them generally decreases with
greater turbulence in the hypolimnion. This is especially evident during destratification, when
the two lines coincide. Elevated DO levels and turbulence during the destratified period
therefore appear to push the oxic/anoxic boundary well below the SWI. It is also possible that
lower sediment oxygen demand in the winter (when the readily available organic matter that
settled during the summer has been degraded) facilitates deeper penetration of the oxic/anoxic
boundary during destratification.
79
4.3.3 Diffuser operations in 2006
Extensive Mn data were collected in 2006. Although the diffuser was in operation during
the early part of the summer, it was turned off to simulate the hypoxic sediment conditions that
develop after the reservoir becomes stratified. After one month, the diffuser was turned back on
again. The diffuser remained in operation for the rest of the year continuing through the winter,
although the flow rate of oxygen was substantially decreased during the destratified period.
Figure 4.5a shows the flow rate of oxygen supplied to the diffuser as a function of time while
Figure 4.6 provides an overview of oxygen concentrations and the corresponding total Mn levels
in the hypolimnion during 2006.
As shown in Figure 4.5a and Figure 4.6, the diffuser was turned off for one month
between June 15 and July 14, 2006 to observe Mn behavior as oxygen is depleted at the SWI.
During this period, the bottom-water DO rapidly dropped below 5.0 mg l-1, as shown in Figure
4.6. Total Mn increased at the lower elevation (338.3 m), which is ~ 3 m above the bottom, but
decreased at the upper elevations (341.4 m and 344.4 m). The increase at the lower elevation is
believed to be caused by outward diffusion of soluble Mn from the benthic region as well as
particulate Mn settling from the upper elevations. Because the diffuser was turned off, mixing
decreased, thus enabling suspended particulate Mn in the upper regions of the hypolimnion to
settle. A mass balance on the upper hypolimnion (344.4 m and 341.4 m) reveals that total Mn in
this region decreased by approximately 700 kg during this one month period.
The weighted-average soluble and particulate Mn is shown in Figure 5b. During the first
two weeks following diffuser shut down, both soluble and particulate Mn decrease. After this
two-week period, when the bottom DO has been substantially depleted, the soluble Mn increases
rapidly, although Figure 6 shows that this only occurs at the lower elevation (338.3 m). Even
though the reduced Mn comes in contact with elevated oxygen concentrations, it remains in the
soluble form due to slow oxidation kinetics (Balzer, 1982; Nealson et al., 1988).
Within ten days of restarting the oxygenation system, the Mn in the hypolimnion was
almost completely mixed. This mixing is revealed in Figure 4.6 as the Mn concentrations at
three hypolimnion elevations converge. The higher values at the lower elevation (338.3 m) were
clearly mixed with the lower values at the higher elevations (341.1 and 344.4 m). This relative
uniformity in bulk hypolimnion Mn concentration was observed at all times when the diffuser
was in operation and is evident in both Figure 4.4 (soluble Mn) and Figure 4.6 (total Mn). This
80
diffuser-induced mixing was observed during operation of the oxygenation system (Gantzer and
Little, 2008, In Preparation).
Despite increasing DO concentrations in the hypolimnion after the oxygenation system
was turned back on, the soluble Mn concentrations continued to increase, most likely due to
horizontal transport of Mn. Although this phenomenon will be described in more detail later, it
appears to occur because of Mn release from hypoxic sediments at higher elevations in the
reservoir. These hypoxic sediments are in contact with low DO water, especially at the
metalimnetic minimum (Gantzer and Little, 2008; In Preparation). The metalimnetic minimum
is clearly shown in Figure 4.6 as the large pale section in the middle of the DO contour plot. The
changing DO concentration in the metalimnetic minimum is also plotted in Figure 4.5b.
One month after the oxygenation system was turned back on, soluble Mn in the
hypolimnion began to decrease at the same time as the DO in the metalimnion began to increase
(see Figure 4.5b). Soluble Mn is presumable no longer being supplied from hypoxic sediments.
In addition, soluble Mn decreased while particulate Mn increased, suggesting that the rate of Mn
oxidation increased. The decrease in metalimnion DO coincides with detritus entering the
hypolimnion. When detritus settles out of the metalimnion and into the hypolimnion, the
demand in the metalimnion diminishes, allowing the DO to be replenished. Sediment traps
placed in the hypolimnion showed increased rates of Fe, Mn, TOC, and total solids accumulation
during this period, with total solids accumulation shown in Figure 4.6. The increase in the rate
of oxidation of Mn coincided with an increase in solids collected in the sediment traps. This
suggests that Mn oxidation may have been accelerated due to an increase in the amount of
organic matter in the hypolimnion, which is known to facilitate Mn oxidation (Johnson et al.,
1996; Wann et al., 1997; Schoemann et al., 1998; Zaw and Chiswell, 1999). However, the rate
could also have increased due to the sustained exposure to the elevated oxygen concentration, or
could perhaps have been triggered by oxidation and subsequent co-precipitation (Mn oxidation is
autocatalytic).
By October, soluble Mn concentrations at the SWI had increased to ~36 mg l-1 (Bryant
and Little, In Preparation) while soluble Mn concentrations in the bulk hypolimnion and the
benthic region decreased substantially. In order for Mn to increase at the SWI and decrease in
the benthic region and water column: (1) Mn must be removed by oxidation and precipitation in
the benthic region and (2) diffusion out of the sediment must be suppressed. It is clear that
81
oxidation and precipitation occurred. In order to suppress Mn diffusion out of the sediment, it
would require movement of the oxic/anoxic boundary deeper into the sediment (Balzer, 1982;
Stauffer, 1986; Wann et al., 1997; Katsev et al., 2007). Taking into consideration existing
conditions at the SWI, including elevated DO, newly deposited organics, and the steep gradient
of Mn concentration between the overlying benthic region and the SWI (increasing from 2 mg l-1
in the benthic region to 36 mg l-1 at the SWI), it appears that: (1) the oxic/anoxic boundary was
located in the sediment (turbulence induced by the diffuser promoted the penetration of oxygen
into the sediment), thus limiting the mobility of soluble Mn into the water column and (2) rapid
Mn oxidation was occurring at the SWI as a result of the combination of elevated oxygen
concentrations, organic matter deposition and possible microbial activity.
Historically, Mn concentrations decreased substantially following destratification,
presumably as a result of dilution, oxidation and precipitation (Yagi, 1996). In 2006, however,
CCR Mn concentrations were actually lower prior to destratification (which occurred in early
November) than in previous years (shown in Figures 4.3 and 4.4). During the destratified period
between 2006 and 2007, peeper results indicate a large deposit of Mn at the SWI that is more
than 100 times higher than the concentrations observed in the benthic region or anywhere else in
the water column (Bryant and Little, In Preparation). During the stratified period, Mn present at
the SWI has been observed to be the source of Mn to the benthic region (Hoffman and
Eisenreich, 1981; Balzer, 1982; Yagi, 1996; Schaller et al., 1997; Wann et al., 1997; Schoemann
et al., 1998). During the destratified period, despite the elevated levels at the SWI, Mn
concentrations in the benthic region were equal to those in the bulk water, indicating that the
diffusive flux out of the sediment is being suppressed. Soluble Mn concentrations at the SWI
decreased from ~30 mg l-1 to ~10 mg l-1 during destratification; however, Mn concentrations in
the benthic region and bulk water did not increase above 0.05 mg l-1. Apparently, conditions in
the water column (elevated DO, turbulence at or near the SWI, movement of the oxic/anoxic
boundary into the sediment, organics, and microbial activity) minimized Mn concentrations in
both the benthic region and the bulk water. This suggests that a large quantity of previously
deposited Mn is oxidized and retained in the sediment during destratification, and is thus no
longer capable of being transported to the overlying water via diffusion.
82
4.3.4 Diffuser operations in 2007
A comparison of the 2006 and 2007 results in Figure 4.5 shows that both soluble and
particulate Mn levels in the bulk hypolimnion were substantially lower in 2007 despite benthic
region concentrations being similar for both years. The primary difference is the fact that the
oxygenation system was turned off in 2006, but was in continuous operation during 2007. In
addition, Figure 4.3 shows that Mn concentrations in the upper hypolimnion were somewhat
higher than observed immediately below. These mid-water-column Mn levels are likely due to
Mn coming from hypoxic sediments upstream of the deeper basin. Overall, the 2007 data show
that:
1. Mn continued to be transported from the SWI to the benthic region (but not higher in
the water column) during oxygenation;
2. dissolved Mn levels in both the bulk hypolimnion and in the benthic region were
reduced; and
3. Mn concentrations in the upper hypolimnion were influenced by horizontal transport
of Mn from upstream hypoxic sediments.
4.3.4.1 Mn transport from SWI to benthic region
During the 2007 stratified period, Mn at the SWI was observed to be approximately five
times higher than in the benthic region (Bryant and Little, In Preparation). This indicates that
Mn continued to be transported from the sediment to the benthic region during oxygenation,
although Mn was not significantly transported to higher elevations. Similar to 2006, the
dissolved Mn increase in the benthic region did not occur until June. The delay in the presence
of Mn in the benthic region indicates the apparent movement of the oxic/anoxic boundary into
the water column. Figure 4.5 shows that the range of bottom DO and the difference between the
bottom DO and the hypolimnion DO increased, suggesting upward movement of the oxic/anoxic
boundary. This delayed shift is supported by the findings of Balzer (1982) and Katsev et al.
(2007), which show that mobilization of Mn is much slower when the oxic/anoxic boundary is
located within the sediment. Between April and June, DO concentrations were still elevated
everywhere in the reservoir (see Figure 4.5), thus supporting the theory of the oxic/anoxic
boundary being well below the SWI. Increased Mn in the benthic region between June and
83
August suggests that the oxic/anoxic boundary may have been positioned above the SWI, thus
allowing diffusion of dissolved Mn into the benthic waters.
4.3.4.2 Soluble Mn during 2007
The proportion of dissolved relative to total Mn was lower during 2007 stratification than
during 2006, showing that more Mn was in the oxidized form during 2007. From 2006 to 2007,
average soluble Mn decreased by ~ 30% in the bulk water and by ~ 15% in the benthic region.
Figures 4.4 and 4.5 show dissolved Mn in the bulk hypolimnion remained low throughout the
2007 stratified period. The difference between total Mn and soluble Mn levels during 2007
stratification indicates that even though Mn may have been transported to the bulk water, the
majority of it was oxidized. Total Mn increased in the bulk hypolimnion during October 2007,
similar to 2006, although not to the same extent. Total Mn concentrations again decreased as the
metalimnetic minimum was eliminated, supporting accelerated removal such as co-precipitation
of Mn with detritus, as the organics settle through the hypolimnion.
4.3.4.3 Horizontal transport
Manganese concentrations increase in the upper regions of the hypolimnion in both 2006
and 2007 (Figure 4.3). To further understand this phenomenon, the mass of Mn at each elevation
was calculated for 2006. The results indicate that when the diffuser was turned off, Mn present
at the lower (benthic) elevation can reasonably be assumed to be the source of Mn to the upper
elevations (Figure 4.7). Once the diffuser was turned on, however, the mass of Mn in the benthic
region (335.3 m) was observed to be significantly lower than that at the higher (344.4-m)
elevation. The increased mass of Mn at the higher elevation far exceeded that present at the
lower elevations. The Mn at the upper elevation therefore appears to come from an upstream
source where the hypolimnetic water is in contact with hypoxic sediments. This peak in the
middle of the water column resembles the results observed by Wann et al. (1997), who suggested
that the increased Mn was caused by lateral transport from shallower sediments, resulting in a
natural phenomenon called geochemical focusing (Balzer, 1982; Stauffer, 1986; Schaller et al.,
1997).
84
To further examine the potential horizontal transport, Mn data were collected at two
locations in 2007 that were spaced approximately 2000 m apart: one over the deep basin with a
bottom elevation of 335.3 m (CC) and one in a shallower region with a bottom elevation of 341.4
m (C3). The data are shown in Figure 4.8. The water column samples were collected at the
341.4-m elevation, which was in the benthic region at C3 and in the bulk hypolimnion region at
CC. Manganese concentrations at C3 were higher than CC throughout the first half of the
stratified period, but increased significantly in August, once the applied gas flow rate to the
diffuser was reduced and the DO at C3 began to decrease (Figure 4.8). C3 is located far enough
away from the diffuser that it does not experience the same magnitude of diffuser-induced
mixing or levels of DO in the benthic region that were observed in other areas more directly
influenced by the diffuser. DO levels in the benthic region at C3 dropped below 5 mg l-1,
although they did not reach complete anoxia. Therefore, if both DO and induced mixing were
observed to diminish at C3, it is reasonable to assume that this trend continued further along the
reservoir, thus promoting the diffusion of dissolved Mn from the sediment to the overlying
waters. The location of the hypolimnion in the region of C3 in 2007 suggests that anoxic
conditions would be present at the SWI. The most likely cause of increased Mn at CC in the
bulk hypolimnion is therefore the horizontal transport of Mn from hypoxic sediments at C3 or
further up the reservoir.
4.4 CONCLUSION
This research studied metals behavior in CCR and confirmed the sediments as a major
source of dissolved Fe and Mn to the water column. Furthermore, it showed that DO must be
maintained throughout the water column (and down to the SWI) to minimizing Mn transport to
the bulk hypolimnion water. Dissolved metal concentrations during destratification are low and
appear to be a result of decreased transport from the sediment as well as dilution. The decreased
transport is believed to coincide with the movement of the oxic/anoxic boundary into the
sediment, which is facilitated by elevated oxygen concentrations and turbulence at the SWI.
During stratification, the oxic/anoxic boundary moves into the water column because of
diminished oxygen transfer from the overlying water. As the oxic/anoxic boundary moves
upwards, dissolved Mn can be more easily transported to the bulk of the hypolimnion.
85
Oxygenation increased the DO concentrations throughout the hypolimnion and down to the SWI,
but induced a lower degree of mixing and turbulence than during destratification. While the
lower degree of turbulence does not promote deep penetration of oxygen into the sediment, it
does appear to affect the location of the oxic/anoxic boundary sufficiently to restrict substantial
transport of soluble Mn to the bulk water of the hypolimnion. As a result, metals concentrations
in the hypolimnion during summer stratification were lower than previously observed during
destratification. While not completely simulating destratified conditions (which is undesirable
because it would mix the entire reservoir), the oxygenation system was nevertheless able to
suppress substantial transfer of Mn from the benthic region to the bulk water, indicating that
source-water control of dissolved metals can be accomplished with hypolimnetic oxygenation in
water-supply reservoirs such as CCR.
4.5 REFERENCES
Balzer, Wolfgang (1982). “On the Distribution of Iron and Manganese at the Sediment/Water
Interface: Thermodynamic Versus Kinetic Control.” Geochimica et Cosmochimica Acta,
46, pp. 1153-1161.
Budd, George, Knocke, William, Hanchak, John, Rice, Craig, and Billman, Mellisa (2007).
“Manganese Control Issues with Changing Treatment Sequences – Need for
Understanding Manganese and Related Profile in a Plant.” VWEA & VA-AWWA JOINT
ANNUAL MEETING WATER JAM 2007, September 16-20, pp. all.
Burns, Frank L (1998). “Case Study: Automatic Reservoir Aeration to Control Manganese in
Raw Water Maryborough Town Water Supply Queensland, Australia.” Water Science
and Technology, 37 (2), pp. 301-308.
Chiswell, Barry and Zaw, Myint (1991). “Lake Destratification and Speciation of Iron and
Manganese.” Environmental Monitoring and Assessment, 19, pp. 433-447.
Clesceri, Lenore S., Greenberg, Arnold E., Eaton, Andrew D. (1998). Standard Methods for the
Examination of Water and Wastewater 20th Edition, American Public Health Association,
Washington, DC, pp. 3-2 and 3-44.
86
Hoffmann, Michael R. and Eisenreich, Steven J. (1981). “Development of a ComputerGenerated Equilibrium Model for the Variation of iron and Manganese in the
Hypolimnion of Lake Mendota.” American Chemical Society, 15 (3), pp. 339-344.
Ismail, R., Kassim, M.A., Inman, M., Baharim, N.H., and Azman, S. (2002). “Removal of Iron
and Manganese by Artificial Destratification in a Tropical Climate (Upper Layang
Reservoir, Malaysia).” Water Science and Technology, 46 (9), pp. 179-183.
Johnson, Kenneth S., Coale, Kenneth H., Berelson, William M., and Gordon, R.Michael (1996).
“On the Formation of the Manganese Maximum in the Oxygen Minimum.” Geochimica
et Cosmochimica Acta, 60 (8), pp. 1291-1299.
Katsev, Sergei, Chaillour, Gwenaelle, and Sundby, Bjorn (2007). “Effects of Progressive
Oxygen Depletion on Sediment Diagenesis and Fluxes: A Model for the Lower ST.
Lawrence River Estuary.” Limnology and Oceanography, 52 (6), pp. 2555-2568.
Nealson, Kenneth H., Tebo, Bradley M., and Rosson, Reinhardt A. (1988). “Occurrence and
Mechanisms of Microbial Oxidation of Manganese.” Advances in Applied Microbiology,
33, pp. 279-318.
Schaller, Tobias, Moor, H. Christopher, and Wehrli, Bernhard (1997). “Sedimentary Profiles of
Fe, Mn, V, Cr, As, and Mo as Indicators of Benthic Redox Conditions in Baldeggersee.”
Aquatic Sciences, 59, pp. 345-361.
Schoeman, V., W. de Baar, H.J., M. De Jong, J.T., and Lancelot, C. (1998). “Effects of
Phytoplankton Blooms on the Cycling of Manganese and Iron in Coastal Waters.”
Limnology and Oceanography, 43 (7), pp. 1427-1441.
Singleton, V. L. and Little, J. C. (2006). “Designing Hypolimnetic Aeration and Oxygenation
Systems – A Review,” Environmental Science & Technology, 40, pp. 7512-7520.
Stauffer, Robert E. (1986). “Cycling of Manganese and Iron in Lake Mendota, Wisconsin.”
Environmental Science and Technology, 20 (5), pp. 449-457.
Wann, J.K., Chen, C.T.A., and Wang, B.J. (1997). “A Seasonally Anoxic Mountain Lake and
Active Fe Cycle in Tropical Taiwan.” Aquatic Geochemistry, 3, pp. 21-42.
Wetzel, R. G. (1975) Limnology, W. B. Saunders Company, Philadelphia, pp. 68-71.
Yagi, Akihiko (1996). “Manganese Flux Associated with Dissolved and Suspended Manganese
Forms in Lake Fukami-ike.” Water Research, 30 (8), pp. 1823-1832.
87
Zaw, Myint and Chiswell, Barry (1999). “Iron and Manganese Dynamics in Lake Water.” Water
Research, 33 (8), pp. 1900-1910.
88
4.6 FIGURES AND ILLUSTRATIONS
Figure 4.1 Bathymetry, bottom profile, diffuser locations, and sampling stations in Carvins Cove Reservoir.
89
36529
36712
36895
37078
37261
37444
37627
37810
37993
38176
38359
38542
38725
38908
39091
39274
356
3.0
Diffuser Installed
Aug 1, 05
352
Off
(Jun 15, 06)
2.5
On
(Jul 12, 06)
2.0
Elevation (m)
1.5
348
1.0
0.5
344
0.4
0.3
340
0.1
336
Jan-00
Jan-01
Jan-02
Jan-03
Jan-04
Figure 4.2 Soluble Fe in Carvins Cove Reservoir (2000-2008).
90
Jan-05
Jan-06
Jan-07
Jan-08
0.0
356
Off
(Jun 15, 06)
Diffuser Installed
Aug 1, 05
Elevation (m)
352
4.0
On
(Jul 12, 06)
3.0
348
2.0
1.0
344
0.5
340
0.1
336
36600
Jan-00
36800
37000
Jan-01
37200
Jan-02
37400
37600
Jan-03
37800
38000
Jan-04
Figure 4.3 Soluble Mn in Carvins Cove Reservoir (2000-2008).
91
38200
38400
Jan-05
38600
38800
Jan-06
39000
39200
Jan-07
39400
Jan-08
0.0
344.4 m (1130 ft)
5.0
341.4 m (1120 ft)
3.0
2.0
338.3 m (1110 ft)
Bottom (Benthic)
Destratification
4.0
Destratification
Manganese (mg l
-1
)_
6.0
1.0
_
0.8
0.4
0.0
1-Apr
1-Jun 1-Aug 1-Oct
1-Dec 1-Feb 3-Apr
Date (2006-2007)
Figure 4.4 Soluble Mn in Carvins Cove Reservoir during 2006 and 2007.
92
3-Jun 3-Aug 4-Oct
4-Dec 3-Feb
12.0
120
10.0
100
8.0
80
6.0
60
4.0
40
2.0
20
0.8
0.7
0.6
8.0
0.5
6.0
0.3
4.0
0.2
2.0
0.1
0.0
Mar-06
Jul-06
Nov-06
Mar-07
Jul-07
Nov-07
-1
10.0
Manganese, Mn (mg l ) _
12.0
-1
Dissolved Oxygen, DO (mg l ) _
14.0
0
Destratification
0.0
Flow Rate (NCMH) _
140
Destratification
-1
Dissolved Oxygen, DO (mg l ) _
14.0
0.0
Mar-08
Date
Weight Averaged Hypolimnion DO
Averaged Soluble Mn
Bottom DO
Averaged Solid Mn
Flow Rate
Metalimnion DO
Figure 4.5 Hypolimnion DO, bottom DO, and applied gas flow rate (top) and average soluble
and particulate hypolimnion Mn concentrations and metalimnion DO (bottom).
93
354
1160
344.4 m
341.1 m
338.3 m
351
Cummulative Total Solids (mg/ l)
12.0
1150
10.0
Thermoclin
e
1140
Boundary
ON
344
56,000
38,000
341
1130
Elevation (ft)
Elevation (m)
347
8.0
6.0
1120
4.0
OFF
25,000
338
1110
2.0
11,000
335
1-Jun
2,100
1-Jul
1-Aug
4,900
1100
1-Sep
1-Oct
1-Nov
Date (2006)
Figure 4.6 Total Mn concentration as well as total solids accumulated in sediment traps, superimposed on DO contours.
94
0.0
Total Manganese Mass (kg) _
2400
1800
1200
600
0
15-Apr
15-Jun
15-Aug
15-Oct
Date (2006)
344.4
341.4
Figure 4.7 Total Mn mass as a function of elevation.
95
338.3
335.3
15-Dec
1.05
_
0.75
0.15
-1
Mn Concentration (mg l ) _
0.45
0.10
0.05
0.00
21-Mar
21-May
21-Jul
20-Sep
20-Nov
Date (2007)
CC
C3
Figure 4.8 Soluble Mn collected at 338.3 m elevation at both CC and C3.
96
20-Jan
4.7 TABLES
Table 4.1 Summary of reservoir and oxygenation system characteristics.
Parameter
Value
Maximum Depth
23 m (70 ft)
2.5 km2 (1100 acres)
Surface Area
24 × 106 m3 (6.2 × 109 gal)
Volume
Hypolimnion Volume
~25% of total volume
~430 kg day-1
Background Oxygen Demand
Side Slopes
Diffuser Length
Design Flow Rate
8-20:1
1250 m (4000 ft)
68 NCMH (40 SCFM)
~2100 kg day-1
Design O2 Addition
O2 Transfer Efficiency
80 % - 85 %
97