A caravan of trucks from the U.S. Geological Survey, snaking across Virginia collecting samples of water from 160 community wells, made a stop Tuesday morning in Middleburg.

In addition to collecting a sample of water from one of Middleburg's two operating town wells, the USGS hydrologists came armed with charts and satellite imagery maps showing the geology of the areas where wells are to be sampled. Just as important, perhaps, the hydrologists showed a willingness to hold forth informally about Virginia's recently imperiled water supply and the purpose of their sampling.

The field team will take samples of water from town wells in about 40 locations in the Piedmont, including Purcellville, Marshall, Bealeton and Remington.

"It's like we're doing a travelogue of the state," said David L. Nelms, a hydrologist in the Richmond office of the USGS and a member of the team.

The goal of the survey--titled "Virginia Aquifer Susceptibility Study: Dating of Ground Water for Source Water Assessment Screening"--is to determine which wells tend to pump contaminants from surrounding rock and soil.

The Virginia Department of Health and the U.S. Environmental Protection Agency will help the USGS pay for the $900,000 study, which was mandated by Congress as part of a nationwide assessment of the quality of ground and surface water.

Samples of the Virginia well water will be tested in laboratories around the world for the presence of chlorofluorocarbons, tritium, helium and other "environmental tracers." The presence of the tracers will tell scientists the age of the water.

Water that contains chlorofluorocarbons is a few days to 50 years old; water that contains carbon-14 is 1,000 to 30,000 years old.

Knowing the age of water will help scientists trace the path that it took--and which tracers it intercepted--as it traveled from the air to the underground aquifers and fissures, and back up again through the pumping action of a well.

"What we're trying to figure out is the susceptibility of the supply to the near-contaminants," Nelms said. "It's been said before: Ground water is an archive of what's been in the atmosphere."

But Virginia's geography is complicated--in the Piedmont, it is a jumble of ancient fissured granite--and two wells on adjoining properties might behave very differently. One might draw down the nearby contaminants while the other one doesn't, Nelms said.

As the study proceeds, scientists also will find out, for example, the time needed for water supplies to recharge in different parts of the state--that is, how long rainwater takes to sink into the ground, travel into underground aquifers or rock fissures and eventually into the area from which a well draws its water.

Alice Love, the town administrator of Middleburg, said the study will provide useful data--particularly to communities where wells are sampled--above and beyond the information that the scientists are seeking.

In Middleburg, for example, only two of the three town wells are in use, and the town is close to using every drop of the 131,200 gallons the two wells can pump each day. The data collected will help the town manage the water supply in the future.

"We have a fairly critical situation in terms of being up against our pumping limits," said Love, who noted that Middleburg is the only community in Loudoun County that is still on mandatory water-use restrictions as a result of the prolonged drought in the area. Mandatory restrictions that went into effect Aug. 1 in eastern Loudoun were lifted last week after several days of steady rain.

"It's a whole lot more data than we could ever afford to get ourselves," Love said. "Getting any data is going to be vital to our future existence."

The USGS presentation attracted about a dozen people, including Supervisor James G. Burton (I-Mercer), who said he was gathering facts for the upcoming Loudoun County session on water supplies. Locals have dubbed the event, to be held Oct. 5 at the Loudoun County Government Center, the "water summit."