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DEVELOPMENT OF A TMDL FOR PCBS IN SAN FRANCISCO BAY

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California placed all San Francisco Bay segments on its [303(d)] list of impaired waters in 1998 based on a fish-consumption advisory due to the concentrations of polychlorinated biphenols (PCBs) in sport fish caught throughout the Bay. Coincident with the listing, the California Regional Water Quality Control Board, San Francisco Bay Region (Water Board) began the development of the total maximum daily load (TMDL) for this class of persistent compounds. There are obvious challenges associated with establishing a TMDL for legacy pollutants such as PCBs, but fortunately, the Water Board has realized opportunities to build upon past studies and to engage San Francisco Bay area stakeholders in a collaborative effort to focus new resources on filling critical data gaps necessary to build a meaningful solution to the impairment.

Our strategy for the PCBs TMDL is founded on the premise that PCBs are associated with hot spots around the margins of the Bay and within drainage areas of runoff discharges from urban areas around the Bay. Many of these sediment hot spots have been previously identified and characterized, so the challenge has been to analyze the linkage between PCBs in sediments and the aquatic food web. Integral to this effort has been the development of a food web model and studies designed to determine the active sediment layer. We have also collaborated with the San Francisco Estuary Institute on the development of a mass balance model of PCBs in the Bay.

Although the dominant mass of PCBs are in the sediments of the Bay, the PCBs contamination will persist as long as there are ongoing discharges of PCBs to the Bay that exceed PCBs removal mechanisms. The model has allowed us to put this into perspective. The model, as expected, is very sensitive to the depth of the active sediment layer, and predicts that with an assumed active sediment layer of fifteen centimeters, steady-state levels will maintain as long as inputs to the system total 80 kilograms a year. In other words, even though the total mass of PCBs in the sediments of the Bay is on the order of magnitude of tens of thousands of kilograms, as long as ongoing sources exceed 80 kilograms, the Bay may never recover.

As part of our source assessment, we used an indicator based on concentrations of PCBs in embedded sediments at the mouths of tributaries and in urban storm drain systems. Although we observe PCBs in most areas tested, certain tributaries and storm drain clearly exhibit higher levels of PCBs indicative of possible hot spots in waterways, storm drains, and/or on land. We have established a TMDL and allocation that reflect conservation application of our understanding of sources and knowledge gained from our modeling efforts. We have also crafted an adaptive implementation plan that calls for early actions focusing on remediation or management of in-bay, in-waterway, in-storm drain, and/or on-land hot spots and efforts to resolve key uncertainties and remaining challenges with the TMDL and its implementation.
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Document Type: Research Article

Publication date: 01 January 2005

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