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�h Y Ms. Lori Duncan Page 2 of 9 } San Joaquin County Environmental Health Department August 25,2004 s extraction (SVE)well screen intervals begin between 5 and 25 feet bgs, span the saturated- unsaturated soil boundary, and extend to a maximum depth of 45 feet bgs. Although constructed as extraction wells with large slot sizes(0.05 inch to 0.10 inch)and gravel filter packs, these wells are usable as monitoring points for the groundwater interface. Groundwater has been measured between 8 and 14 feet bgs in Well VW-5S. This anomalous water level is believed to be a localized perched aquifer in the former underground storage tank (UST)complex, resulting from porous and permeable backfill in the UST excavation pit underlain by relatively impermeable ` clayey silts. Appendix B contains monitoring well and remediation well construction information. Currently, groundwater is being monitored and sampled quarterly from all existing monitoring wells and SVE wells. As of August 2004, all three air sparging(AS)wells (AS-1 through AS-3) have been added to the quarterly groundwater monitoring program. All site wells were purged prior to sampling. Groundwater flow across the site is generally eastward,ranging from northeast to southeast, with hydraulic gradients ranging from 0.002 feet/foot(ft/ft)to 0.03 ft/ft(see Table 1 and Figure 5). Site History Site investigations began in October 1990 with the drilling of five soil borings (B-1 through B-5). Soil samples were collected and analyzed for petroleum hydrocarbons, including total petroleum } hydrocarbons as gasoline(TPHg) and benzene. Hydrocarbon impacts were detected in 6 of the 18 samples collected. The highest concentrations detected were 1,400 milligrams per kilogram (mg/kg)for TPHg and 1.2 mg/kg for benzene. The investigation indicated that hydrocarbon- } impacted soil was limited to the vicinity of the UST complex (Appendix Q. In October 1991,four on-site groundwater monitoring wells (MW-I through MW-4), and three SVE wells(VW-1, VW-2, and VW-3)were installed. Soil samples collected identified maximum TPHg and benzene concentrations of 340 mg/kg and 0.031 mg/kg, respectively. These results also indicated that petroleum hydrocarbon impacted soil was limited to the area of the UST complex (Appendix Q. s In 1992, RESNA conducted an SVE test at the site. The test concluded that the site soil could be remediated using SVE, with each well having a radius of influence of 30 feet. �1 In 1993, two additional monitoring wells(MW-5 and MW-6) and one SVE well(VW-4) were installed. Soil samples did not contain detectable amounts of TPHg. One sample from B-15 had �i detectable concentrations of benzene(0.024 mg/kg). During the boring of B-18, a product line was accidentally fractured, resulting in the release of product into the soil. VW-4 was installed adjacent to B-18 for future mitigation of the released product(Appendix Q. In 1994,EMCON evaluated the site as part of the prioritization of remediation efforts at ARCO sites in San Joaquin County. EMCON calculated the total petroleum hydrocarbon mass at the site at approximately 383 pounds(lbs). Hydrocarbon distribution figures generated during this evaluation are presented in Appendix C. K:IWprocessUS BP ARCQ10054812004 SC \Site Conceptual Model V2.doc