More of the pollution getting into San Francisco Bay is coming from those living around it than from distant megafarms and old gold mines, new research shows.
The San Francisco Estuary Institute's annual report on the health of the Bay turns around conventional wisdom that most of the pollution comes from the vast Central Valley watershed.
"That's been a major shift in our understanding in the last 10 years," said Jay Davis, a senior scientist at the San Francisco Estuary Institute and the editor of the annual report, called "The Pulse of the Estuary 2010." "Ten years ago, we didn't have good hard data."
One of the studies in the Pulse report said that a decade ago, scientists estimated that 24 percent of the sediment and pollution getting into the Bay came from urban creeks and stormwater. That figure is now 56 percent.
Among the pollutants flowing into the Bay from creeks and storm drains are PCBs, mercury, pesticides, metals and toxic compounds called polycyclic aromatic hydrocarbons, which come from exhaust pipes, barbecues and fireplaces.
But Davis said there's good news in that.
Last year, water quality regulators issued rules to address stormwater pollution in the Bay Area. The rules, Davis said, should help, much as measures and spending in the 1970s led to major reductions in the amount of sewage and industrial pollution that was getting into the Bay.
The new stormwater regulations encourage low-impact
"It should lead to significant improvements in runoff quality," said Thomas Mumley, assistant executive officer at the San Francisco Bay Regional Water Quality Control Board. "That's the plan, but we are careful about not overburdening municipalities with economic constraints."
In the past, scientists believed the bulk of Bay pollution came from the Central Valley, which made sense because the watershed covers 40 percent of the state and is the source of 90 percent of fresh water running into the Bay.
But recent studies that compare pollution from the Central Valley, the Guadalupe River watershed in San Jose and a small watershed in Hayward are turning that perception around.
Ten years ago, regulators thought 70 percent of the mercury coming into the Bay came from the Central Valley. Now, they say, less than 40 percent comes from the valley, and more than 40 percent is coming from urban creeks.
There could be several reasons for the shift.
First, the notion that most of the pollution was coming from the upstream watersheds could have been wrong. But it is also possible that earlier estimates were correct and pollution from upstream declined, Davis said.
In last year's Pulse report, scientists reported that the amount of upstream sediment that had been pouring into the Bay since the Gold Rush suddenly and dramatically dropped about 10 years ago. Scientists think that the pool of erodible sediment was exhausted by about 2000, and Davis said much of the pollution that comes with sediment could have declined at the same time.
Mike Taugher covers the environment. Contact him at 925-943-8257.