2. Penobscot River Mercury Study Panel - Radiochemical analyses of sediment cores from the Penobscot River and Estuary, Peter H. Santschi, PI, Bryce Johnson, Kathleen Schwehr, Patrick Louchouarn, co-PIs, 2009-2011 ($330,400). This court mandated collaborative project between USM, TAMUG and the Flett Laboratory will evaluate the potential of natural attenuation of the mercury contaminated Penobscot River and Estuarine System in Maine through determination of sedimentation and sediment mixing rates. Summary: This court mandated collaborative project between USM, TAMUG and the Flett Laboratory will evaluate the potential of natural attenuation of the mercury contaminated Penobscot River and Estuarine System in Maine through determination of sedimentation and sediment mixing rates. The gradual reduction of surface sediment mercury concentrations by deposition of cleaner particulates should lead to a gradual reduction in the rate of methyl mercury (MeHg) production in the ecosystem, which would be followed by lower MeHg concentrations in biota. This natural attenuation option would obviously also be very cost effective because it would require no active remediation. Methods to measure sediment accumulation rates are very well established. The age of sediments can be determined by the use of several radioisotopes that have been deposited from past atmospheric nuclear testing events (cesium-137, plutonium-239, 240) as well as ongoing deposition of naturally occurring radioisotopes (beryllium-7, lead-210). The concentration profiles of these isotopes versus depth is used to determine the rate of long term sediment accumulation. These isotopes can also be used to identify locations where sediments, which are resuspended often by currents as they move down the river, are being temporarily deposited.