The group headed by researchers at UC Santa Cruz found high levels of methylmercury in underwater flows at Stinson Beach in Marin County, north of San Francisco, and at Elkhorn Slough in Monterey County, south of San Francisco.
The study suggests that groundwater may be as big a source of mercury in coastal waters as mercury deposited from atmospheric pollution.
Methylmercury accumulates in the food chain, leading to levels in some sea food that can be dangerous if too much is consumed.
"The big question for public health is, 'Where is all the mercury in seafood coming from?'" says coauthor Russell Flegal, a professor of environmental toxicology at UC Santa Cruz. "What we have shown is that methylmercury is coming from groundwater in California at surprisingly high levels."
Atmospheric deposition of mercury has tripled in the past 200 years. But the study of mercury in groundwater is comparatively new.
The study is to be published in Environmental Science and Technology.