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Former Discovery Chevrolet March 20, 1998 <br /> Tracy,Cali f6mia Problem Assessment Report <br /> ppm in the sample collected from beneath the dispenser. Historical laboratory analytical <br /> results are included as Appendix B of this report. <br /> In September 1994, three groundwater monitoring wells (MW-1 through MW-3) were <br /> installed to approximately 20 feet below surface grade (bsg) to evaluate the presence of <br /> hydrocarbons in the soil and groundwater. TPHg was detected at concentrations of 340 and <br /> 240 ppm, respectively, in the soil samples collected at approximately 10 feet bsg from the <br /> borings for wells MW-1 and MW-2 near the former UST excavation. Results of laboratory <br /> analyses for a soil sample collected from 10 feet bsg in boring MW-3 indicated no detectable <br /> concentrations of TPHg, TPHd, the gasoline constituents benzene, toluene, ethylbenzene, <br /> and xylenes (BTEX), and total oil and grease (TOG) (see historical Table 3 in Appendix B). <br /> The groundwater from wells MW-1 and MW-2 has been found to contain concentrations of <br /> TPHg up to 17,000 parts per billion (ppb), and benzene up to 3,100 ppb, but the recent trend <br /> has shown a significant decrease in these concentrations. Results of laboratory analyses for <br /> the groundwater from well MW-3 have indicated no detectable concentrations of TPHg, <br /> TPHd, BTEX, and TOG as shown in GeoStrategies Table 1 - Historical Groundwater Data <br /> (in Appendix B). The groundwater gradient was found to be approximately 0.003 to the <br /> north-northwest and has remained constant. <br /> The soil underlying the site was found to be predominantly clay and silt to the groundwater <br /> interface at about 10 to 11 feet bsg. Below this depth, sandy silt to silty sand was <br /> encountered. Gravel was encountered in the first 8 feet of the boring for MW-1, indicating <br /> that this well was located at least partially in the UST excavation backfill. <br /> On April 3, 1997, Horizon drilled two soil borings to further evaluate the presence of <br /> hydrocarbons in the soil and groundwater, and completed the borings. as monitoring wells <br /> MW-4 and MW-5. The monitoring wells were completed to a depth of approximately 20 <br /> feet bsg. Groundwater was encountered in the borings at a depth of approximately 8% to 9 <br /> feet bsg. Soil samples were collected from the borings at 5-foot intervals from the ground <br /> surface to the total depth of the borings. No evidence of petroleum hydrocarbons was noted <br /> 3001-12.par 3 HORIZON ENVIRONMENTAL INC. <br />