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Former Discovery Chevrolet March 20, 1998 Tracy,California Problem Assessment Report the latest groundwater quarterly monitoring results for December 18, 1997 are indicated rr:� below each well on the cross-sections. .1 Petroleum H dr Garb ns in Soil Analytical results of soil samples for TPHg and benzene were used to construct isoconcentration maps of TPHg at 8 to 10 feet bsg (Figure 6) and benzene at 8 to 10 feet bsg (Figure 7). Analytical results from soil samples collected at 5 feet bsg detected 320 ppm TPHg only in boring GP-6, and benzene ranging from 0.0066 ppm to 0.67 ppm in borings GP-4 through GP-7. Based on the soil sample analytical results and the shallow depth to groundwater, the area of TPHg- and benzene-impacted soil has been defined both laterally and vertically. The analytical results of soil samples for the oxygenated fuel additive compounds indicated MTBE ranging from 0.0070 ppm to 0.021 ppm in borings GP-4, GP-6, and GP-8 in the vicinity of the former UST cavity. The oxygenated compounds DIPE and ETBE were detected only in the soil samples from boring GP-5 at 0.22 and 0.20 ppm, res ecivelY• The oxygenated compound tertiary-butanol was detected only in borings GP-4, GP-5, and GP-7. The tertiary-butanol concentrations in borings GP-4 (0.0052 ppm and 0.0069 ppm) and GP- 5 (0.24 ppm) were at and very slightly above their respective laboratory detection limits. The tertiary-butanol concentrations in borings GP-7 were 0.063 ppm and 0.032 ppm. No TAME was detected in any of the soil samples. The low concentrations of oxygenated fuel additive compounds detected in the soils beneath this site are negligible. 6.1 ss Balance of Petroleum nstit is in Soil The area of concern for gasoline hydrocarbons in the soil is the area shown on the TPHg Isoconcentration Map (Figure 6). The 1,000 ppm isoconcentration contour generally reflects the area of subsurface soil impacted by fuel hydrocarbons from the former product line, while the 100 ppm isoconcentration contour generally reflects the area of subsurface soil impacted by the spread of dissolved fuel hydrocarbons along the surface of the groundwater. 3001-12.par 8 HORIZON ENVIRONMENTAL INC.