When researchers wanted to test largemouth bass at Lower Crystal Springs Reservoir for mercury levels, the reservoir's managers in San Francisco figured the scientists were simply looking for a clean sample to compare with toxic results at other spots.

Instead, the study showed that the fish in the San Mateo County lake - which collects rainwater as well as water piped in from Yosemite's Hetch Hetchy reservoir - had some of the highest mercury levels in the state.

Now, the San Francisco Public Utilities Commission, which oversees Crystal Springs and the rest of the sprawling network that supplies drinking water to 2.5 million people in San Francisco, San Mateo, Santa Clara and Alameda counties, is trying to find the source of the heavy metal, a neurotoxin that can cause developmental damage in children and brain, lung and kidney problems in adults.

"It was a big surprise," said Tim Ramirez, manager for natural resources and land management at the commission's water enterprise division. "We're going to jump on it and try to find out what's going on."

No swimming

Mercury contamination in predator fish like the bass does not indicate that the water itself is unsafe for drinking or swimming - though the 22.6 million-gallon Crystal Springs has long been off-limits to swimming, fishing and boating to protect drinking water quality. The naturally occurring mineral becomes hazardous when it interacts with bacteria that thrive in low-oxygen environments. The bacteria change the mercury into methyl mercury, which is consumed by microscopic organisms at the low end of the food chain. As each creature is in turn eaten by a bigger creature, the mercury becomes more concentrated.

The study was done by researchers at the San Francisco Estuary Institute, who released the results of the landmark, $1.5 million, two-year project in June. They sampled sport fish at nearly 300 popular fishing lakes in California for methyl mercury, PCBs, DDT and other contaminants. Twenty-one percent of the lakes, including Crystal Springs, had at least one fish species with mercury concentration above 0.44 parts per million - a level considered unsafe for ingestion by young children and women of child-bearing age. Less than 1 percent of the lakes had similar levels of PCBs, a set of highly noxious chemicals now banned but once used commonly in electronic components and coolants.

Understanding risks

The data is the first step in a broader effort to understand chemical pollution in California's lakes, as well as risks it poses to humans and wildlife.

At many of the lakes, the mercury results weren't wholly unexpected. San Jose's Almaden Lake, which had the highest fish mercury levels in the state, is in a geological zone rich with mercury, extracted during the 1800s for its ability to bind with gold. Fishing is allowed at Almaden, but signs in several languages warn against eating fish pulled from that lake and others in the area. To help control mercury levels at Almaden, the Santa Clara Valley Water District is aerating the water - adding oxygen to reduce the bacteria that mixes with mercury to form the toxic methyl mercury.

The Crystal Springs watershed, however, doesn't have a history of mercury mining, said the report's lead author Jay Davis, senior scientist at the institute.

"It is one of the more contaminated lakes in a pristine watershed," he said.

Pollution from afar

Mercury in the Crystal Springs area may be coming from a source other than local sediments, according to Davis and Ramirez. Both men theorize that instead of leaching out of rocks and soil, the mercury may be wafting across the Pacific Ocean from China. China relies heavily on energy from coal-fired plants, a major source of mercury pollution.

"It's still possible that geology in the (Crystal Springs) watershed could contribute to the contamination we see, but it seems like a case where atmospheric deposition might play a role," Davis said.

Through the institute's partnership with the State Water Resources Control Board, scientists will continue to monitor the mercury levels in fish at Crystal Springs and elsewhere. The San Francisco PUC, meanwhile, is consulting with other regional water and land management agencies about the Crystal Springs results. Ramirez said the utility is particularly interested in air-monitoring programs in the Central Valley that are tracking west-to-east air pollution patterns.

"It's not of immediate concern to people right now, but we want to know what's happening," Ramirez said. "That said, if it's airborne pollution from a global source, that's going to be hard for us to do something about."