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02 June 1997 AGE-NC Project No 95-0105 ' Page 6 of 11 2 10 SITE GEOLOGY Tan clay layers were encountered from the surface to depths of approximately 7 feet The upper clay ' layer has the qualities of an aquitard (a semipermeable lithology) Saturated, fine sand beds were encountered below approximately 7 feet bsg A dry, brown clay was also encountered at a depth of ' approximately 10 feet bsg Ground water in the sand layers is abundant, however it appears that the clay immediately above the sands, retards the'vertical migration of ground water ' The tank excavations at the site have been backfilled with sand, a permeable lithology, which may be creating a vertical conduit for ground water to the upper clay layer Currently, static ground water level is at approximately four feet bsg A copy of soil boring logs and a cross section are included in Appendix A ' 2 It GROUND WATER MONITORING AND SAMPLING Monitoring wells MW-1, MW-2 and MW-3 were sampled in August 1988, after their installation in February 1995, and have been sampled on a quarterly basis since July 1995 Over the course of sampling the monitoring wells, low concentrations of petroleum hydrocarbons have been detected in wells MW-1 or MW-2 Low concentration of TPH-g, benzene, toluene and xylenes have been � detected in samples collected from MW-3 ' Monitoring wells MW-4, MW-5 and MW-6 have been sampled on quarterly basis since their installation in May 1996 TPH-g and TPH-d were initially detected at concentrations as high as 43,000 ppb and 5,700 ppb, respectively Benzene was first detected at 320 ppb , However, ' hydrocarbon concentrations have dramatically decrease over the following three quarterly sampling events ' TPH-g and BTE&X have not been detected in the samples collected from MW-5, however TPH-d was detected in the ground water sample collected in August 1996 Petroleum hydrocarbons have not be detected in the ground water samples collected from MW-6 2 12 ESTIMATION OF HYDROCARBON MASS AND REDUCTION Soil impacted by the release of petroleum from the site extends north off the site under Grant Line Road, west off the site under the City of Tracy right of way in Parker Avenue, through out.the northwest portion of the site and under the canopy and building on the site Approximately 950 cubic yards of soil in the saturated and unsaturated zone were impacted by the unauthorized release of ` hydrocarbon from the site A total of 200 cubic yards of soil were removed during the 1995