--------------------------- Methane Leak Halts Work In New Marin Subdivision Some construction stopped to ensure safety Jim Doyle, Chronicle Staff Writer Wednesday, February 28, 2001 ------------------------------------------------------------------------ A Marin County subdivision where home buyers have been warned of underground MTBE contamination has halted some construction because explosive methane gas from an old military dump is seeping up near home sites. Although workers are putting the final touches on 80 newly built homes, construction of dozens more have been put on hold while air tests are conducted at Hamilton Meadows at the former site of Hamilton Air Force Base in Novato. "There's been some indication that the methane gas is migrating toward our property" from the former dump, said Thom Gamble, executive vice president of Shea Homes, one of the nation's largest home builders. However, he added, recent tests of unoccupied homes have shown no evidence that the highly flammable gas has entered any homes. "We are trying to be proactive to make sure that the residents are being protected," said Gamble, who has hired a consultant to monitor the air in the homes. Almost one-third of 235 planned homes at Hamilton Meadows have been built, and more than a dozen have been occupied. As part of the final two phases, houses will be built closer to both the former dump -- located 200 feet from the closest home sites -- and a toxic plume of MTBE in the groundwater. None of the current residents must move, Gamble said, because the methane is away from the completed homes. When the Air Force vacated the old base in 1975, the area was divided among the Army, Navy and Coast Guard. The Army first detected high levels of methane gas at the edge of the former dump in September 1999, but later tests produced lower levels. Army and state officials decided that it would be safe to build homes nearby, and gave the developer the go-ahead. Military consultants and state water quality officials also concluded that the plume of MTBE, or methyl tertiary butyl ether, beneath the subdivision would not endanger residents -- as long as they did not plant fruit trees, dig wells or expose themselves to polluted groundwater. The gasoline additive had leaked from underground storage tanks at a Navy gas station. In a Feb. 19 letter to home buyers, Shea Homes said it had "voluntarily delayed construction of our new homes in closer proximity to the landfill in order to determine what mitigation measures, if any, should be considered" to deal with the methane problem. Construction is expected to be delayed at least until the U.S. Army Corps of Engineers comes up with a plan in mid- to late March for containing the methane. TAKING IT IN STRIDE Gamble said that one new homeowner had demanded immediate air monitoring in her home but that others were taking the matter in stride. "I'm a little nervous about it, but I'm a holistic person -- I'm concerned about the air I breathe at the gas station," said Mary Ramsay, who was house- sitting at one of the homes yesterday. Although the earlier tests showed methane levels to be safe, more tests performed last summer revealed significant concentrations of the gas near almost 30 lots at Hamilton Meadows. The Army's gas probes showed traces of methane seeping from the ground inside a buffer zone of 150 to 200 feet between the landfill and the home sites. More recent tests confirmed these results. Jim McAlister, a project manager for the Army Corps of Engineers, said the methane levels were higher than the Army had anticipated, but not extreme. "I personally don't believe there's a safety risk there," McAlister said. "I don't think there's an explosive hazard. A municipal landfill generates much more methane." Methane, an odorless and colorless gas, is commonly generated by decomposing dumps and also natural conditions such as bay mud and oil fields. The gas can cause an explosion if ignited in confined spaces. The old military dump contains construction debris, fuel residue, heavy metals, solvents, PCBs and pesticides. The Air Force stopped using the landfill in 1974 and capped it with clay and high-strength plastic in 1995. McAlister said the gas seemed to be escaping the landfill from a layer of soil between the bottom of the plastic cap and groundwater. SEARCH FOR SOLUTIONS The Army has hired a consultant to look at ways to fix the problem. Among the possibilities are digging a trench around the landfill and depositing gravel in it; planting trees to get oxygen into the soil; using an extractor to burn off the gas; and piping oxygen underground to help break down the methane. The Army's plan must be approved by the state Department of Toxics and Substance Control and the California Regional Water Quality Board. Shea Homes' Gamble said the Army should install vents for the methane and a containment wall to prevent the gas from getting any closer to his home sites. "The Army has capped a landfill, and it's an improper capping," Gamble said. "They're the ones responsible. I think everyone is in agreement that there should have been a venting system." The question of whether the old military base had been cleaned up enough for homes to be built there fell to federal and state regulatory agencies. Marin County and Novato city planners were barred from deciding whether the land was a suitable place for people to live. But Novato played an active role in fostering the military's reuse plan for the airfield. The parcel for Hamilton Meadows was sold first by the Army to the city of Novato, which then sold it to a development partnership, which then sold it to Shea Homes about 18 months ago. Novato planners had envisioned a golf course and playing fields on top of the former landfill, but those plans have been put on hold. "There isn't anything that's going to happen there until we get the methane resolved," McAlister said.