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Quik Stop Station No 039 February 28,2000 Stockton,California Subsurface Site Assessment CONCLUSIONS Based on the data collected during this and previous investigations, Horizon concludes the following: Subsurface Soil • No detectable concentrations of gasoline hydrocarbons (TPHg, BTEX, MTBE), diesel hydrocarbons (TPHd), or motor oil hydrocarbons (TPHmo) appear to be present in the subsurface soils beneath the site A detectable concentration of 535 ppm of TOG was reported in soil sample S-20-MW1 However, since no detectable concentrations of TPHg, TPHd, or TPHmo were detected in soil sample S-20-MW1, the TOG analytical results likely represent a relatively immobile oil or grease hydrocarbon compound Groundwater • Since no detectable concentrations of TOG, TPHmo and TPHd were reported in the groundwater samples from well MW-1, the leaching potential of the TOG hydrocarbons . reported in soil sample S-20-MW1 is likely very low • Detectable concentrations of gasoline hydrocarbons were reported in the groundwater samples from wells MW-1 (ethylbenzene), MW-2 (TPHg and xylenes) and MW-3 (TPHg and xylenes) However, none of the concentrations of BTEX hydrocarbons have been detected in the groundwater above the primary or secondary Maximum Contaminant Levels (MCLS) recommended by the State of California (refer to Table 2) • The groundwater elevation data indicates that the groundwater beneath the site was flowing towards the northwest (away from Fremont Avenue) with a gradient magnitude of approximately 0 007 foot / foot on December 15, 1999 This groundwater flow direction may suggest an offsite upgradient source of the gasoline hydrocarbons detected in the water samples collected beneath the site 7039-12 rpt 8 HORIZON ENVIRONMENTAL INC