SITE INFORMATION AND CORRESPONDENCE

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Former Discovery Chevrolet June 17, 1998 Tracy,California SWRCB Closure Anneal Letter In September 1994, GeoStrategies installed three groundwater monitoring wells (MW-1, MW- 2 and MW-3) at the site, and in April 1997, Horizon installed two downgradient groundwater monitoring wells (MW-4 and MW-5) to evaluate for the presence of hydrocarbons in the soil and groundwater beneath the site. Groundwater samples from wells MW-1 and MW-2 have been found to contain concentrations of TPHg up to 17,000 parts per billion(ppb), and benzene up to 3,100 ppb; however, the recent trend has shown a significant decrease in these concentrations. Groundwater samples from well MW-3 have been found to contain no detectable concentrations of TPHg, BTEX, MTBE or TOG, and concentrations of TPHd up to 200 ppb. Groundwater samples from downgradient wells MW-4 and MW-5 have been found to contain no detectable concentrations of TPHg and BTEX since the initial well sampling. Confirmation testing by EPA Method 8260 have indicated that MTBE detected in groundwater by EPA Method 8020 were false positive results. In 1997, groundwater samples collected and analyzed from wells MW-I and MW-2, located within the dissolved hydrocarbon plume, indicated lower concentrations of nitrate, sulfate, and ferric iron than the concentrations detected in perimeter wells MW-3, MW-4 and MW-5. The bioparameters indicated that intrinsic bioremediation is a viable and active method of natural remediation occurring beneath the site. The overall historical trend has been a reduction in hydrocarbon concentrations in the groundwater beneath the site. Additional sampling and delineation work was performed in April 1997 and February 1998. In April 1997, hand borings SA through SK were drilled around and in the former UST excavation to a depth of approximately 8 feet bsg. Selected soil samples collected from the hand borings were analyzed for TPHg and BTEX. The highest concentrations of TPHg and BTEX in soil were reported in the vicinity of the former product lines adjacent to the southwest corner of the former UST excavation. In February 1998, soil borings GP-1 through GP-10 were advanced and sampled using a truck-mounted Geoprobe® direct push rig. At the request of the SJC PHS/EHD, 26 soil samples and 10 water collected from the soil borings were analyzed for TPHg, BTEX, MTBE, and fuel oxygenates. Again, the highest concentrations in soil were reported in the vicinity of the former product lines adjacent to the southwest comer of the former UST excavation. The highest concentrations of TPHg, BTEX, and MTBE in groundwater were reported in the vicinity of the former product lines adjacent to the southwest corner of the former UST excavation, and along the west side of the excavation area. Based on the soil and groundwater sample analytical results, the TPHg- and benzene-impacted soil and groundwater have been defined vertically to a depth of 15 feet and laterally beneath the site. Based on mass balance calculations, the amount of gasoline hydrocarbons in the subsurface soil and water were estimated to be approximately 40 gallons. Historical laboratory analytical +� soil and groundwater results are included in Appendices #2 and #3 included with this letter. The shallow groundwater being monitored beneath the site is within the uppermost water- bearing or water table zone, is generally present between approximately 9 to 11 feet bgs, and generally has a flow direction toward the north-northwest beneath the site. Reported subsurface materials encountered during drilling at the site consisted primarily of fine-grained soils such as silty to sandy clay, clayey silt, and clayey sand. Because of the generally fine- grained nature of the shallow water-bearing materials and poor historical land-use practices, 0617jmun.wrb 2 HORIZON ENVIRONMENTAL INC.