SITE INFORMATION AND CORRESPONDENCE

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Measurements of dissolved oxygen in groundwater samples collected from site wells in February 1999 provide the basis to conclude that aerobic biodegradation is also acting to attenuate the petroleum hydrocarbons present in site soil and groundwater. Concentrations of dissolved oxygen in groundwater from wells MW-1, MW-2, MW-3, and MW-6 (the four shallow wells that have historically defined the core of the site's groundwater plume) were less than 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 concentration associated with the Iocation of the release relative to the higher concentrations at distance further demonstrates the on-going biodegradation and attenuation of the hydrocarbon plume. In considering possible impacts to current and future beneficial uses of groundwater, three factors are relevant: 1) according to Department of Water Resources (D WR) 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 wells, and 3) as discussed above, the plume is stable and concentrations are decreasing as a consequence of natural attenuation. The presence of poor quality water(which generally contains elevated concentrations of nitrate, chloride, and TDS) 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 decrease over time, continued monitoring of site groundwater would serve no useful purpose. The only way to ensure complete removal of lingering, residual, detectable concentrations of petroleum constituents in site groundwater would be to excavate an additional several thousand cubic yards of affected soil to depths in excess of 50 feet. This would entail a 11