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Delta Fish Carry Contaminants

Anglers casting a line out into Delta waters may not want to eat every last fish they catch. In a study published by the S.F. Estuary Institute this September (see Now On-Line), most of the fish scientists pulled out of Delta rivers and sloughs exceeded screening values for the protection of human health for at least one contaminant - only four out of 28 locations yielded a "clean" catch. Health advisories are already in effect for Bay fish.

In the nutshell, analysis of white catfish and largemouth bass caught in 1998 found that about half the fish had levels of potential concern for mercury, a third for PCBs, and a quarter for DDT. Great variations occurred among locations. While mercury concentrations averaged about 290 parts per billion in the Central Delta, the Northern Delta and lower Sacramento River soared to more like 650 ppb (screening value = 300 ppb). The biggest surprise however, turned out to be the San Joaquin River, where levels were nearly as high as in the Sacramento. "No one expected it, but no one had ever looked in the San Joaquin before," says researcher Jay Davis. Both watersheds contain historic mining debris laced with the mercury used to release gold from its ore.

The San Joaquin also proved heavy on the DDT. Two spots in the lower river had concentrations of 389 and 407 ppb in the catfish, as compared to a low of 42 ppb elsewhere (screening value = 100 ppb). Levels of PCBs in the catfish ranged from a low of 8 ppb in the Middle River at Bullfrog to a high of 102 ppb (screening value=20 ppb) in the Smith Canal, an isolated backwater where someone must have once spilled or disposed of PCBs, says Davis, adding that PCB problem areas are generally quite localized.

Davis is still scratching his head over why the Central Delta, where waters from two rivers high in mercury mix, came out so clean. "It's a good mystery," he says, "especially since the literature suggests that the transition zone between fresh and salt water is a region of enhanced mercury methylation." Methylation is a process by which mercury turns into a chemical form more available for uptake by organisms.

The Institute's findings jibe well with ongoing results from CALFED-funded studies of methylmercury distribution in clams and small fish (Slotton, UC Davis) and water (Foe, CVRWQCB). As for the differences between the Estuary's upper and lower reaches, Davis says the results confirm that mercury is high in both regions, and as high in fresh water as in salt; that PCBs are higher and more widespread in the Bay, which makes sense given its more industrial and urban history; and that DDT is more of a problem in the Valley. "The good news is that the Central Delta is cleaner than we thought," he says. Contact: Jay Davis (510)231-9539 ARO

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