But instead of tremors, this instrument called a gas chromatograph records the volatile organic compounds found in the Barnes aquifer, the reservoir of groundwater that underlies Easthampton, Southampton, Westfield and Holyoke.

In particular, the line indicates the presence and concentration of trichloroethylene, or TCE, which has contaminated groundwater in a portion of the aquifer.

"This is the first level of analysis," said John Swallow, whose company, Pine & Swallow Associates of Groton, has been hired by the state Department of Environmental Protection. He is working with state environmental experts to determine the extent and the source or sources of the TCE contamination.

Tracking TCE, a search that involves pinpointing the location of a colorless liquid in groundwater deep underground, might look like guesswork. But in fact since the investigation began in February, it has followed a step-by-step procedure that includes the analysis of hundreds of water samples, each of which provide a clue about where the TCE is located and where it might be coming from.

Swallow aid the TCE contamination has been tracked along a narrow 4.5 mile stretch, starting at the Hendrick Street wellfield in Easthampton and extending south to the northeastern edge of Westfield.

Tests have been conducted on about 300 private wells, of which about 30 have detectable levels of TCE, according to Richard Green, section chief for the DEP's Bureau of Site Management and Permitting in Springfield.

A dozen of these wells have water that exceeds the federal drinking water standards of 5 parts per billion, including a few with levels of about 33 or 34 parts per billion, said Green.

The DEP has provided these homes with bottled water and plans to install filtration systems in homes that do not meet safe drinking water standards. However, state officials have recommended communities, like Southampton, consider extending water lines to those homes as the best long-term solution to the problem.

The DEP has ranked the TCE contamination as the top cleanup priority for public water sources in western Massachusetts.

This search has its genesis in 1988 after traces of the cancer-causing chemical were found several years earlier in Easthampton's Hendrick Street wellfield, a major source of the town's municipal water supply. As a result of that contamination, Easthampton last year finished construction of a $2 million water treatment plant, which filters the contaminant from the water supplied t residents.

The Routine

Each morning Swallow meets with Cathy Chamberlain, an environmental analyst with the DEP and the western region coordinator of its Site Discovery Program, which manages groundwater resources, identifying and cleaning up contaminated public water systems statewide.

As the coordinator of the 10-week investigation into the local TCE contamination, Chamberlain consults with Swallow to decide n the scope of the day's field work, including which private wells will be tested and where monitoring wells will be drilled.

Swallow pointed to a dozen-or-so vials of water next to the gas chromatograph, samples recently collected from a number of streets in the southeastern corner of Southampton, from private wells and from about 70 monitoring wells in the area.

Water samples can be obtained at varying depths within these wells, providing a "profile" of the TCE contamination, where it's located and at what concentrations.

After these samples are analyzed in the mobile laboratory in Easthampton, selected samples are sent to a commercial lab to confirm the initial results and for further testing. A portion of these samples are then sent to a state laboratory in Lawrence for additional testing, he said.

A specialist in hydrogeology and water analysis, Swallow explained that the mobile laboratory makes it possible to analyze water samples quickly to determine where the next day's testing should take place. Rapid analysis also ensures that homeowners with contaminated wells are made aware of the problem and supplied with an alternative source of water as soon as possible, he said.

According to Swallow, the search for the TCE contamination also involves study of the region's geology using such sources as aerial photographs, assessors maps and water resource atlases. Applying the principles of hydrogeology, this information helps determine the direction and scope of the testing, he explained.

Testing is now concentrated near the western border of Holyoke, in the Dupuis Road area, at the possible site of a former dump, said Green. As yet, no sources for the contamination have been identified, he said.

Swallow said the TCE appears to be following the flow of groundwater in that portion of the Barnes Aquifer along a narrow channel which extends in a northerly direction from Westfield to where it enters the Connecticut River near the Oxbow in Easthampton.

Swallow explained TCE may be found in one well and not in another on a neighboring property because the contamination is confined along a narrow corridor within the aquifer and occurs at varying depths, depending on the characteristic of the aquifer and its flow patterns. Thus a 200-foot-deep well may intersect with the "cylinder of contaminated water" in the aquifer while a shallower one a short distance away does not, he said.

Because groundwater tends to move very slowly and because the TCE concentrations have reached an equilibrium in the groundwater, Swallow said he does not expect the contaminant levels in private wells in the area to change drastically in the near future.

Chamberlain said the investigation's major concern is the impact of the TCE contamination on the public health and the environment.

"We can't identify the extent of the contamination, identify the source and clean up the problem, " said Chamberlain. If a site is identified as a source, its owner or former owner can be held liable for the cost of the cleanup.

After 15 years of tracing similar environmental pollutants, Swallow said he is optimistic the source or sources of the contamination can be found.

"I've never been on a project where a plume (a contaminant's trail) was found that we haven't been able to identify the source," said Swallow.

Once a source is located, the site and the polluted portion of the aquifer can be cleaned up and the aquifer can begin the process of cleaning itself, he said.

Because the state has identified the TCE contamination as one of the major environmental problems in the state, he added, "A lot of time and money are being brought to the problem."

To those homeowner who have found they have tainted well water, he said, "There's a lot of support for their situation. They're not alone with this problem."