The number of underground storage tanks leaking pollutants into the county’s groundwater and soil has been slashed by half over the last decade, according to a report released last month.
As much as 85 percent of the leaks contaminate the aquifer, which lies 10 to 30 feet below the surface throughout much of the county, said Dean Peterson, county Director of Environmental Health. Typically buried 10 to 15 feet below the ground, the storage tanks are primarily used by fueling stations to store gasoline, he said.
The tightening of federal and state regulations in the 1980s has force the phasing out of older tanks for those that are double-walled and have leak sensing technology, said Chuck Headlee of the Regional Water Quality Control Board.
The number of tanks being investigated or cleaned up in the county dropped to 414 in 2006, from 814 in 1996, according to the report, “Indicators for a Sustainable San Mateo County.”
The county closes another 24 cases a year on average, Peterson said.
San Francisco currently has about 230 such sites and closes as many as 100 cases a year, said Stephanie Cushing of the San Francisco Public Health Department.
Peterson attributed the lower closure rate in San Mateo County to more thorough monitoring after clean up and the conservation “closure” of cases. “When we close a site we don’t want to ever have to open it up again,” Peterson said. He estimated that about 41 of the 414 sites pose a significant risk of seeping into the aquifer and polluting the soil with contaminants such and benzene, a known carcinogen, or MTBE, a chemical used to make cleaner burning fuel.
“Clearly we are making progress, but we have a long way to go,” Supervisor Rich Gordon said.
While 400 contaminated sites is a far cry from the stated goal of zero, officials said the problem would be worse if more Peninsula residents depended on wells for their drinking water. “The majority of the [contaminated] sites are in communities that are traditionally industrial, so this is not necessarily a major countywide problem,” Gordon said.
Because cities with the largest number of underground leaks — Redwood City, San Carlos, San Mateo, and South San Francisco — rely on tap water from the Hetch Hetchy system, the county has the luxury of time when it comes to thoroughly cleaning a site, Peterson said.
And, with more and more of those sites being bought and turned into residential infill developments, ensuring they’ve been properly decontaminated is a priority, he said. “We are working very closely with the parties that are responsible for the leaks, keeping them on a tight time table to pump ground water or remove fumes through vacuum pumping.”
Peterson estimated that reducing the number of underground leaks to zero would take another 10 years. The millions of dollars in decontamination costs are paid for by using a federal 0.1-cent tax on each gallon of gas purchased, Headlee said.