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I I ormer Discovery ChevrU January 27, 1998 Tracy.California AdditionaWsurface Investization Work Plan collected below the dispenser located inside the service bays. Benzene was detected at 0.88 ppm' in the sample collected from beneath the dispenser. Results are shown on the i GeoStrategies Table I included in Appendix C. li i In September 1994, three groundwater monitoring wells (MW-1 through MW-3) were drilled to approximately 20 feet below surface grade (bsg) to evaluate the presence of hydrocarbons in the soil and groundwater. TPHg was detected at concentrations of 340 and 240 ppm, respectively, in the soil samples collected at approximately 10 feet from the 1� borings for wells MW-1 and MW-2 near the former UST excavation. The groundwater from is wells MW-1 and MW-2 has been found to contain concentrations of TPHg up to 17,000 parts per billion (ppb), and benzene up to 3,100 ppb, but the recent trend has shown a significant decrease in these concentrations as shown in GeoStrategies Table 1 - Historical Groundwater Data (Appendix C). The groundwater gradient was found to be approximately 0.003 to the north-northwest and has remained constant. I. l The soil underlying the site was found to be predominantly clay and silt to the groundwater interface at about 10 to 11 feet bsg. Below this depth, sandy silt to silty sand was encountered. Gravel was encountered in the first 8 feet of the boring for MW-1, indicating Illi that this well was located at least partially in the UST excavation backfill. j On April 3, 1997, Horizon drilled two soil borings to further evaluate the presence of I hydrocarbons in the soil and groundwater, and completed the borings as monitoring wells I MW-4 and MW-5. The monitoring wells were completed to a depth of approximately 20 feet bsg. Groundwater was encountered in the borings at a depth of approximately 8'/z to 9 feet bsg. Soil samples were collected from the borings at 5-foot intervals from the ground surface to the total depth of the borings. No evidence of petroleum hydrocarbons was noted j during drilling of the monitoring wells. Ili I i 3001-13.wp 3 HORIZON ENVIRONMENTAL INC. j i