SITE INFORMATION AND CORRESPONDENCE

Home Browse Search

that these wells are constructed with sanitary seals which extend to depths of about 100 to 200 feet below groundsurface. 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 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 substantial disruption of streets, businesses, sewers and other utilities while producing little or no benefit to current or anticipated beneficial uses of the minimal area of groundwater not meeting Basin Plan objectives. i Furthermore, if complete removal of detectable traces of petroleum constituents becomes the standard for UST corrective actions, the statewide technical and economic implications will be enormous. For example, disposal of soils from comparable areas of excavation throughout the state would greatly impact already limited landfill space. In light of the minimal if any benefit to be gained, the evidence of continuing attenuation of residual petroleum concentrations, and the precedent that would be set by requiring additional excavation at this site, attaining background water quality in this limited area is not feasible. While it is impossible to determine the precise level of water quality that will be attained given the residual petroleum constituents that remain at the site, in light of all the factors discussed above, a level of water quality will be attained that is consistent with the maximum benefit to the people of the state. The final step in determining whether cleanup to a level of water quality less stringent than background is appropriate for this site requires a determination that the alternative level of water quality will not result in water quality less than that prescribed in the relevant Basin Plan. Pursuant to SWRCB Resolution No. 92-49, a site may be closed i if the Basin Plan requirements will be met within a reasonable time frame. In this particular case, as discussed above, TPH-g and BTEX in the shallow groundwater in immediate contact with the limited residual petroleum hydrocarbon constituents adsorbed to soils will likely remain above, and thus violate, the Basin Plan's objectives in a localized volume of surrounding groundwater for a significant period of time. This time period could be anywhere from a few decades for BTEX to degrade below MCLs to hundreds of years for that limited volume of groundwater in immediate i 6