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(245) The Long Term Fate of PCBs in San Francisco Bay.
Davis, Jay*,1, 1 San Francisco Estuary Institute, Oakland, CA, USA
ABSTRACT- PCBs are currently a high regulatory priority in San Francisco Bay and the subject of a TMDL. In work funded by the Regional Monitoring Program for Trace Substances in the San Francisco Estuary (RMP), a simple mass budget model was developed as a first step toward understanding the long term fate of PCBs in San Francisco Bay. The model treats the whole Bay as one box with two compartments (water and sediment) and estimates PCB fluxes in and out of these compartments. The principal value of this model is in showing how predicted trends respond to ranges of input values, including PCB loads. The most influential parameters were degradation half-life in sediment, Kow, outflow, average PCB concentration in sediment, and depth of the active sediment layer. Moderately influential parameters included organic carbon content of suspended solids, sediment burial mass transfer coefficient, and Henry's Law constant. The model indicates that sustained long term loading of 10 kg yr-1 would prevent the total PCB mass in the Bay from ever dropping below one tenth of the present mass. With a sustained loading of 20 kg yr-1, the total PCB mass would never fall below about one quarter of the present mass. Based on the best estimates for all input parameters and the observed rate of PCB decline from 1982 to 2000, the model suggests that annual external loads to the Bay during this period were in the 0 to 20 kg range. Even in this first formulation, the PCB mass budget has been of value in summarizing the existing state of knowledge on contaminants and processes in the Bay, predicting the response of contaminant concentrations in the Bay to management actions and natural processes, identifying and prioritizing data gaps, and communicating RMP results.
Key words: PCBs, mass budget, San Francisco Bay, estuary
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