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Slow Down on Quicksilver
Mercury creeping up the Bay food chain from fish to herons to seals has the S.F. Regional Water Quality Control Board ready to say "enough is enough." Their response was to propose, in draft form this June, a total maximum acceptable daily load for the Bay, which applies to all sources.
Mercury levels as high as 0.9 ppm (parts per million) have been found in local sharks and 0.3 ppm in striped bass (compared to an average of 0.1 ppm for fish from all U.S. waters); studies have also found the metal in night herons, clapper rails and harbor seals. In the Bay, this diffuse and changeable metal - also known as quicksilver - crops up in both water and sediments and comes from diverse sources, among them runoff from abandoned mercury mines, wastewater and industrial discharges, and deposition from air pollution.
The proposed total load - "TMDL" - is a watershed-based approach to clean up required under the Clean Water Act to help the Bay comply with federal standards, says the Board's Kim Taylor. It looks at all sources of mercury and tries to limit the total amount entering the Bay by proposing target effluent concentrations and an offset program (a hybrid between a mitigation bank and a tradable loads program) for dischargers that would help pay for remediation of the largest sources of mercury - abandoned mines. Mercury entering the Bay from abandoned mines in the Sacramento River watershed is "one or two orders of magnitude higher" than all other inputs combined, says Taylor.
Clean up of some abandoned mines, such as New Almaden in the South Bay (a Superfund site) has already begun. Clean up of another problem area far upstream from the Bay - the mine-riddled Sacramento River watershed - is much more daunting. When mercury from mine tailings in this watershed runs off into rivers and creeks in the rainy season, it adheres to riverbanks and beds, and in high flows becomes re-suspended and flushed into the Bay. In the rainy winter of 1997, the U.S. Geological Survey measured mercury flowing into the Bay from the Sacramento River and Yolo Bypass at levels as high as 32 kg/day. (For comparison, a dry season loading would average around 0.2 kg/day.) High levels in Yolo County's Cache Creek (which flows into the Bypass) prompted the Central Valley Regional Board to list the creek in its proposed regional toxic hot spot clean up plan, out of concern about potential bioaccumulation in the new Yolo Wildlife Refuge.
Further downstream, many of the hot spots in San Francisco Bay mud (such as Point Potrero in Richmond) are vestiges of old industrial activities, says Taylor, adding that over time much of the mercury will be buried in the Bay's deepest sediments. Still, because of constant new inputs, currents that stir up sediment layers, and activities like dredging, she explains, "we've got a problem with our mercury water quality objective every time the wind kicks up."
The real problem is that re-suspended inorganic mercury (from mine runoff) can be transformed into methyl mercury - the form taken up by fish and other organisms. "The biggest unanswered question for us in controlling potential inputs is what are we doing that increases the rate of methyl mercury formation and uptake in the food chain?" says Taylor. "Everything from dredging to creating new wetlands stirs up the deeper layers. And you're adding bacteria which transform it into organic forms."
Even though inputs from Bay dischargers may be small compared to abandoned mine runoff, they're not off the hook since the mercury they discharge is dissolved mercury - which more easily becomes methyl mercury. So what will the TMDL mean for dischargers? One option is to change effluent limits for deep- and shallow-water dischargers to make them equal (deep-water dischargers currently get a 10:1 dilution credit) while making limits more stringent overall. The Board is proposing long-term averages of between .02 and .05 ppb; current limits are .025 and .012 ppb.
Chuck Batts, of the Bay Area Dischargers Association, says the TMDL seems like a viable long-term strategy, but that "the devil's going to be in the details." He thinks bigger questions are going to be "how to deal with the sediment re-suspension problem and runoff from mercury mines." Environmentalists could not be reached for comment.
With the TMDL, says Taylor, the Board wants to make sure effluent limits reflect "state-of-the-art pollution prevention programs and plant operations, and long-term variability in loading." After the draft TMDL public comment period expires on September 1, the details, including how an offset program will work, will be hammered out in a pilot study. Offsets could be defined, for example, in dischargers' NPDES permits, according to Taylor. The challenge will be putting all the information together and making a program that makes sense, admits Taylor. "Our goal is to control all controllable sources."
Contact: Kim Taylor (510)622-2426 or Joe Domagalski, USGS (916)278-3077
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