Engineers sniffing out cause of gas leaks 

Engineers are investigating a recent rise in methane gas levels in the Oyster Point area — once the site of a 57-acre landfill that took in both wet and dry waste.

Methane, a flammable, naturally occurring gas, which is lighter than air, could prove a safety risk if it were to collect in an area such as a manhole or basement and given a sufficient spark to ignite it, Public Works officials said.

The investigation by Terra Engineers is to determine whether the methane is coming from the old landfill and if the clay cap laid over the landfill in 1975 is leaking and the gas migrating, Public Works Director Terry White said. The increased levels were coming from test wells located along the western portion of the old landfill, along Gull Drive.

"If it is (leaking), then we’ll be faced with a problem of how do we get that under control," White said. "The danger is if you get a big enough concentration and it starts finding its way into a basement or parking structure."

When Gull Drive was built in 1995, more than 4,000 cubic yards of waste, which included steel drums, was removed and the cap extended to the road, according to staff reports.

Melody Castillo, a guest services manager for the Inn at Oyster Point located on Marina Boulevard, said she hadn’t heard of such a problem and that they hadn’t experienced anything out of the ordinary.

"Definitely it’s a concern, but it’s something we’ll look into," Castillo said.

The inn has a crawl space under the first floor but it’s ventilated, severely limiting any methane build-up, said Bob Kirby, the principal of Terra Engineers, the firm investigating the increase.

A permanent monitoring well will be installed by Gull Drive to verify levels, which are tested currently by seven wells, Kirby said.

In San Mateo County, there are 15 closed landfills, all of which monitor methane levels, said Dean Peterson, the Environmental Health director for the county. Four of those sites actively collect it, which is done when there’s enough gas to do so. Three of those — Colma, Burlingame and Brisbane — burn it when it reaches appropriate concentrations.

The county’s high water table acts as a barrier to any migrating subterranean gases and limits its production. But it also seals it in, Peterson said.

"There’s not as much but for a longer period of time," he said.

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