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i' j09 April 2002 AGE-NC Protect No 98-0497 1 Page 2 of 6 ' 23 PREVIOUS INVESTIGATIONS On 29 September 1999,two soil probe borings(P-1 and P-2)were advanced at the site as part of site assessment activities related to the two former USTs Total petroleum hydrocarbons quantified as gasoline(TPH-g) and diesel(TPH-d)were detected in soil samples collected from probe boring P-1 at maximum concentrations of 1,900 mg/kg and 10,000 mg/kg, respectively, laboratory review of t' the gasoline and diesel range chromatograms indicated that the sample chromatograms are more indicative of a diesel impact Benzene, toluene, ethylbenzene and total xylenes (BTEX) were detected in soil samples collected from P-1, at maximum concentrations of 0 047 mg/kg benzene, 0 21 mg/kg toluene, 2 8 mg/kg ethylbenzene and 8 3 mg/kg xylenes The oxygenated ether methyl tertiary butyl ether (MTBE) was detected in soil samples collected from P-1 at concentrations ranging as high as 0 05 5 mg/kg No other target analytes were detected during the laboratory analysis tof selected soil samples On 14 and 15 August 2000,four soil borings(P-3,MW-1,MW-2,MW-3)were advanced at the site, borings MW-1 through MW-3 were completed as ground water monitoring wells Soil samples collected from the four borings did not contain detectable concentrations of TPH-g,but TPH-d was detected in soil samples P3-100 and MW1-75 at concentrations of 7 8 mg/kg and 40 mg/kg, ' . respectively Xylenes was detected in sample P3-75 (0 018 mg/kg) and MTBE was detected in sample MW 1-75 (0 025 mg/kg) No other target analytes were detected in the soil samples analyzed Petroleum hydrocarbon compounds were not detected in soil samples collected from soil borings MW-2 and MW-3,limiting the northern and southern extent of contaminated soil Table 1 contains analytical results of soil samples collected during soil boring advancment Figures 5 and 6 illustrate the inferred vertical and horizontal extent of petroleum hydrocarbon contamination at the site Quarterly monitoring of the ground water monitoring well network has been performed at the site since the third quarter 2000 TPH-g and TPH-d have been detected in monitoring well MW-1 at concentrations as high as 5,700 µg/1 and 120,000 gg/l, respectively Minor concentrations of ethylbenzene and xylene have been detected sporadically in samples collected from MWA No ' contaminates of concern have been detected in monitoring wells MW-2 and MW-3 since monitoring began,limiting the northern and southern extent ofpetroleurn hydrocarbon contamination in ground water Table 3 contains analytical results from previous quarterly ground water monitoring events Figure 7 illustrates the inferred extent of hydrocarbon contamination in ground water at the site - 2 4 GROUND WATER DEPTH AND FLOW DIRECTION Depth to ground water was approximately 61 feet below surface grade(bsg)during during this phase ' of investigation Review of historical depth to ground water maps (San Joaquin County Flood i ' Advanced GeoEnvzronmental,Inc �1