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i Human Health Risk Assessment Former Mobil Oil Bulk Plant 04-343 p 500 East Grant Line Road Tracy, California The average difference between the maximum and minimum depths to ground water for Monitoring Wells AW-1 through AW-9 was 0.76 feet during the period for which monitoring data were obtained. The maximum ground water level fluctuation ob,-erved in the monitoring wells occurred in Monitoring Well AW-4 (maximum depth 11.22 feet [10121192]; minimum depth 10.17 feet [1122/93]). The minimum ground water level fluctuation observed in the monitoring wells occurred in Monitoring Well AW-7 (maximum deptt: 10.25 feet [10/21/92]; minimum depth 9.63 feet [8121/91]). ' 1.2.5.2 Local Ground Wateruali Q ty In December 1992, ground water samples were obtained from Monitoring Wells AW-4 and AW-7 and analyzed for general inorganic and physica' quality using EPA Methods 300 (chloride, nitrate, and sulfate), 340.2 (fluoride),6010(ahl.ninum, iron, manganese, copper, silver, and zinc), 7060 (arsenic), and 7470 (mercury). Monitoring Well AW-4 is ' located upgradient of the former storage tank areas, and Monitoring Well AW-7 is located downgradient of the former storage tanks areas. The results of the general inorganic and physical quality analyses indicated that near-surface ground water contains concentrations of constituents that exceed primary drinking water standards for aluminum, and secondary drinking water standards for iron, manganese, sulfate, nitrate, chloride, and fluoride. A summary of the ' general inorganic chemical and physical quality analyses for grounO water is presented in Table 1-1. 1.2.6 Summary of Site Investigations ' The former bulk storage and distribution plant was dismantled in 1988. All structures, including one gasoline underground storage tank and four aboveground diesel fuel storage tanks, were ' removed from the property. Initial assessment activities conducted by ]Kaprealian Engineering Incorporated (KEI) in August 1988 involved the collection and analysis of one ground water sample and two soil samples. The results of the initial assessment i,idicated the presence of petroleum hydrocarbons in soil and ground water. Since AugLlst 1988, considerable characterization and remediation activities have been conducted at the site. The most notable of these assessment activities involved the installation of 48 exploratory soil borings to ChAraCterl7.e. hvflrnc.:irhnn rnnr,.ntratinn, in ,nil A tntil .,f AA --,r;+" --A i,, —1 " I vimoiLLV anu iz- u11J1LG Jull UUllllg'N i 30-0136-11 1-5 1 w