SITE INFORMATION AND CORRESPONDENCE

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attenuate the petroleum hydrocarbons present in site soil and groundwater. ConcentratioQ(RAF T dissolved oxygen in groundwater from wells MW-1, MW-2, MW-3, and MW-6 (the four shallow (A 4 a.� wells that have historically defined the core of the site's groundwater plume) were less than N 40 0.5 ppm. Concentrations of dissolved oxygen in groundwater from up-gradient, cross-gradient, and down-gradient monitor wells ranged from 2.24 to 5.49 ppm. Thus the low dissolved oxygen A/6416e5 concentration associated with the location of the release relative to the higher concentrations at C'`''' distance further demonstrates the on-going biodegradation and attenuation of the hydrocarbon /0r plume. P,/i1j ' 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 d� 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 wells, 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 Charnel 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 decrease over time, continued monitoring of site groundwater would serve no useful purpose. The only way to ensure complete removal of lingering, residual,detectable 0 concentrations of petroleum constituents in site groundwater would be to excavate an additional 11