ARCHIVED REPORTS_XR0011482 - Laserfiche WebLink

Home Browse Search

been open to the atmosphere . Because of the low levels original- ly measured and the attenuation of concentrations resulting from volatilization, the exposures and consequent risks associated with these soils are negligible. Small concentrations of hydrocarbon constituents (other than benzene) were detected in the ground water beneath the Stockton site. 'There is significant utilization of ground water in the vicinity of the site, including potable uses. In August 1987, the concentrations of benzene, toluene and xylene in five domes- tic wells within 0.25 mile of the site were below the MCL and MCLGs for drinking water (Table 6) . Thus, the site was not the source of unacceptable risks at that time . However, a ground- water model for contaminant transport predicts that in three years the steady-state concentration in the nearest well might be as high as 0.036 mg/L benzene and 0.0005 mg/L toluene. While the benzene concentration predicted by the model is above the MCL, ithe actual level is expected to be lower by virtue of unmodeled effects of sorption, biodegradatiun, and volatilization. One major uncertainty in the ground-water modeling concerns the rate of transport of contaminants through the clay layers over the water table into the shallow ground water. This rate may well be so l,)w that contaminant concentrations in ground water directly beneath the former tank pit will decrease rapidly with time. In any case, the fact that benzene has not appeared in any nearby receptor well at detectable levels (>0.0005 mg/L) suggests that attenuating mechanisms have caused a major reduction of contamin- ation--at least 10 times as much as modeled above with RESSQ. For this reason, the risk associated with continued ground-water transport of hydrocarbon contaminants from beneath the site is considered to be -very low, if not negligible. - 23