Simulation of TCE, PCE and BTEX Contamination at a USEPA Superfund Site, St. Thomas, USVI

In response to rising levels of TCE, PCE and BTEX in the water supply the USEPA shut down all production wells in the largest groundwater basin on the island of St. Thomas, USVI. The chemicals originated from multiple sources including dry cleaners (TCE and PCE), gasoline stations (BTEX), and other industrial and commercial sources (other VOCs). Withdrawals from the aquifer were estimated to range between 500,000 to 1,000,000 gallons per day.

Earthfx staff conducted an analysis of groundwater flow and contaminant migration in the basin. The purpose of the study was to assess the likely path of contaminants at the time they were introduced into the groundwater system (pumping conditions) as well as the likely path of contaminants at the present time (non-pumping conditions). The finding would be used as a basis for a lawsuit against the responsible parties.

A numerical model, based on the USGS MODFLOW code (Figure 2), was developed for the study area based on the interpretation of available hydrogeologic and geophysical data. The model simulated flow in the entire upper Turpentine Run Basin, although attention was focused primarily on the VOC source area. The model was calibrated to observed pre-development water levels and then verified against recent water table maps.

The model was used to provide estimates of rates and direction of groundwater flow under non-pumping and pumping conditions (Figure 3a). Finally, a particle-tracking technique was used prepare figures that pathways that dissolved TCE and other VOCs would follow from identified source locations to potential discharge points under non-pumping and pumping conditions (Figure 3b). Dirk Kassenaar and E.J. Wexler served as expert witnesses in pre-trail hearings; the lawsuit was settled out-of-court.