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WHF Environmental Consulting was subsequently dismissed from the site by the responsible <br /> party. <br /> Following recommendations from PHS/EHD and USTCF the responsible party solicited bids from <br /> qualified consultants to continue groundwater monitoring activities as required and evaluate the potential <br /> effectiveness of the original soil vapor extraction system. The suspect system internal combustion engine <br /> and blower motor was reported to be inadequate to effectively address the contamination residing at the <br /> site. <br /> Depth to groundwater observed in monitor wells at this site changed from 54 feet below surface grade to <br /> 26 feet below surface grade since November of 1993 . Data from cumulative tables indicate that ground <br /> water in monitor wells now exceeds the screened intervals (drown wells). Overall ground water elevation <br /> changes have likely contributed to dilution of observed contaminant concentrations over time in wells with <br /> screen intervals from 45 to 70 feet below grade surface (bgs). As a result of elevation changes <br /> groundwater has recently contacted soil horizons in which known significant concentrations of petroleum <br /> hydrocarbons reside (soil vapor extraction well field treatment area). Groundwater has recently begun to <br /> enter the screened interval of all existing soil vapor extraction wells onsite. Groundwater samples collected <br /> from these vapor extraction wells have been analyzed at state certified laboratories. Analytical data reveals <br /> the significant impact to groundwater resources. <br /> Benzene, a known carcinogen, has been detected in groundwater water collected from these soil vapor <br /> extraction wells at 91 ,000 ug/I. Benzene concentrations in monitor wells have reached 170,000 ug/l. The <br /> current California Primary Maximum Contaminant Level for benzene in water supplies is 1 .0 ug/1. Fuel <br /> oxygenates, possible human carcinogens, have also been detected in several monitor and soil vapor <br /> extraction wells on site. Methyl tertiary butyl ether (MTBE) has reached a concentration of 79,000 ug/I in <br /> groundwater collected and analyzed from soil vapor extraction wells. The current California Drinking <br /> Water Action Level (DHS) for MTBE is 35 ug/l. <br /> The observed concentrations of petroleum hydrocarbon constituents in groundwater, a significant source of <br /> drinking water in Stockton and San Joaquin County, poses a potentially unacceptable health risk. The <br /> potential risk pathway is the proximity of this site to municipal well fields. California Water Services <br /> Company owns and operates a shallow municipal supply well near this site built in 1953 designated as <br /> Station 56. According to operations staff at California Water Services well Station 56 screened interval <br /> (bowl) begins at 140 feet below surface grade. MTBE and Benzene contamination in ground water has <br /> been confirmed in samples from monitor wells screened as deep as 70 feet below surface grade at this site <br /> providing minimal vertical separation from the municipal well. The well is less than 2500 feet from the <br /> contaminant source. California Water Services Company supports PHS/EHD in our efforts to conserve, <br /> protect, and reduce impacts to groundwater resources thereby controlling unnecessary risks to consumers. <br /> These conditions warrant additional monitor well installation and site assessment at this site. <br /> Given the documented environmental persistence and rapid migration potential of MTBE, carcinogenic <br /> properties of benzene, elevated concentrations of these constituents, increasing degradation of known <br /> beneficial use waters resulting from ground water elevation changes in conjunction with the unauthorized <br /> release, and the potential risk pathway, an engineered remediation system has been recommended by the <br /> consulting firm (Condor Earth Technology) and approved by PHS/EHD. <br /> The Underground Storage Tank Cleanup Fund corrective action cost pre-approval correspondence dated <br /> October 23, 1997 regarding this engineered remediation system presents the following provisions: <br /> • PROVISION 1: Demonstrate that the contaminant plume has been adequately assessed as required by <br /> San Joaquin County Health Services. <br /> • PROVISION 2: Demonstrate that the cleanup system is feasible, cost effective and able to achieve the <br /> desired level of soil and groundwater cleanup required by San Joaquin County Health Services, and <br /> Section 2725, Chapter 16, of the Underground Storage Tank Regulations (Article 11). <br /> 2 <br /> f <br />