SITE INFORMATION AND CORRESPONDENCE

Home Browse Search

DRAFT sulfate concentrations are about 70 ppm and 170 ppm respectively while concentrations of these constituents at the core of the plume are "non detect" and in down-gradient groundwater nitrate concentrations are "non detect" while sulfate concentrations are only 10 to 20 ppm. Thus, available data provide evidence that the dissolved phase hydrocarbon plume in site groundwater is attenuating and biodegrading via iron, sulfate and nitrate reduction. In considering possible impacts to current and future beneficial uses of groundwater, three factors are relevant: 1) according to Department of Water Resources (DWR) Bulletin 146 (1967) and petitioner's March 1998 sample analyses, shallow groundwater in the vicinity of the site is of"inferior quality," 2) the groundwaters monitored by site wells are not in direct hydraulic communication with deeper groundwater aquifers or local supply-yells, and 3) as discussed above, the plume is stable and concentrations are decreasing as a consequence of natural attenuation. A review of DWR Well Drillers' Reports for water supply wells located in the vicinity of the site show that these wells are constructed with sanitary seals which extend to depths of 150 to 200 feet below ground surface. The purpose of this method of well construction is to exclude poor quality shallow groundwater(which generally contains elevated concentrations of nitrate, chloride, and TDS). The presence of this water in the Victor Formation is a consequence of the easterly intrusion of poor quality water underlying the Delta and the Stockton Deep Water Ship Channel over the past 50 years or so. An analysis of water level elevation data for site wells also shows an absence of the seasonal fluctuations one would expect if the groundwater monitored was in direct hydraulic communication with local production wells. Hence, the localized volume of affected water is not currently being used and, due to its inferior quality, is not likely to be used in the future. Furthermore, historic groundwater monitoring data, in addition to showing plume stability and decreasing constituent concentrations, shows that the poor quality of the shallow groundwater enhances the on-going bioremediation, consequently reducing the residual petroleum hydrocarbon mass in the subsurface. Because constituent concentrations will continue to decline as the mass of residual petroleum hydrocarbons to