SPOKANE, Wash. (AP) - An aquifer that supplies drinking water to 400,000 people in two states is threatened by millions of gallons of stored petroleum products and leaks from car motors, experts say.

A leak in December at a Burlington Northern and Santa Fe Railway Co. refueling depot near Hauser, Idaho, raised awareness of the Rathdrum Prairie-Spokane Valley Aquifer.

Although the extent of the spill is still being assessed, most experts agree it poses little long-term threat to the region's drinking water supply.

What is more troubling is the amount of petroleum stored in aging tanks and dripping motor oil and gasoline that runs off parking lots and streets into the aquifer.

"They reflect a problem at least as big as the depot," said Stan Miller, who retired last year as Spokane County's water resources manager.

Miller calculates that as much as 40,000 gallons of motor oil and gasoline made their way into the aquifer last year through parking lot and street runoff.

At least 30 million gallons of petroleum sit atop the aquifer in Spokane County in aging storage tanks, many of them single-walled, the Spokesman-Review reported.

The potential for contamination is compounded by the region's rapid population growth, which is taking large swaths of prairie.

The aquifer is recharged by lakes and springs in Idaho. That state remains the only one without its own underground storage tank inspection laws, and spends little on prevention and enforcement.

A recent inspection by the U.S. Environmental Protection Agency found that only one in three underground fuel storage tanks in Idaho complied with federal laws.

The compliance rate was 52 percent in Washington, which is still 20 percentage points below the national average.

The aquifer's drinking water meets federal health requirements, but only because the aquifer flows with enough water to mask the problem and flush much of the pollution, said Al Isaacson, a hydrologist from Coeur d'Alene, Idaho.

"People don't think about how fragile this aquifer is," Isaacson said. "If it's contaminated, what are we going to do?"

Studies, including a $3.5 million federal and state data-gathering effort, are under way on the aquifer.

The Idaho Department of Environmental Quality also is beginning to identify pollution that occurred before record keeping began in the 1970s, said Geoff Harvey, the agency's waste and remediation manager in Coeur d'Alene.

Northern Idaho's growth spurt is prompting the work, Harvey said.

The aquifer's high rate of recharge made it easier for businesses and residents to treat the ground as a convenient dumping site.

"This aquifer's been bailing us out," Harvey said. "The question to the public is how long can we go on sweeping the petroleum under the rug?"

There's cause for optimism, aquifer experts say.

Kootenai County zoning requirements have helped reduce nitrate contamination from septic tanks. Tougher laws in Washington have largely removed the threat of old, leaky underground storage tanks.

The odds against the aquifer seem high, especially with the combination of four fuel-storage tank sites, two major petroleum pipelines and countless small fuel storage tanks, said Rachael Paschal Osborn, a Spokane attorney who has worked with the group Friends of the Aquifer and the local chapter of the Sierra Club.

"It's just shocking the number of tanks and pipelines we have crisscrossing the aquifer," she said. "And nobody's really keeping a very close watch on those."