Cities Key Source of Toxics in Bay, Study Finds

But more advanced testing techniques show cities are the key source, according to Jay Davis, senior environmental scientist at the San Francisco Estuary Institute in Oakland. The volume of mercury flushed from the Central Valley may be half what had been estimated a decade ago.

"Historically, we thought (the Sacramento and San Joaquin) were the dominant inputs of contaminants in the bay, but our thinking has shifted," Davis said. "The tributaries from the urban landscapes are contributing more."

Higher levels of mercury are of particular concern, because when ingested by people and animals the heavy metal can cause a host of physical problems, including neurological damage. Tests of striped bass from the bay, a so-called "indicator species," show mercury levels at an average of 0.4 parts per million in 2009, Davis said, well above the 0.2 parts per million level considered safe for moderate consumption.

One of most mercury-laden waterways in the region remains the Guadalupe River, which flows from the Santa Cruz Mountains and the site of the now-defunct New Almaden Quicksilver Mining District, one of the largest historic mercury mining operations in North America. Various local agencies have partnered with one another with the intention of cutting mercury levels in the Guadalupe, from controlling erosion along river banks to adding oxygen to local reservoirs. That process helps prevent mercury from converting to methylmercury, the chemical form most frequently eaten by and concentrated in fish.

Efforts are also under way to slash the most visible types of contamination: cigarette butts, Styrofoam packaging and plastic bags. Last year, the San Francisco Bay Regional Water Quality Control Board approved a measure requiring municipalities in Alameda, Contra Costa, San Mateo and Santa Clara and Solano counties to reduce the amount of trash in storm water runoff 40 percent by 2014.

"Without a question, trash has become one of our highest priority water quality concerns," said Tom Mumley, assistant executive officer at the water board. "It affects the creeks, the shoreline and contributes to ocean pollution. The public simply has to create less trash, and cities need to capture more of it."