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�../ Nwi MuuGERs 9ESIGNEFSFaW5ULi1N15 � 2.3 HYDROGEOL.OGY The site is covered by an asphalt surface and is underlain by approximately 17 to 20 ft of clay. The clay overlies silt and sandy silt with minor clay and sand lenses. The estimated high groundwater table depth is 25 to 30 ft bgs for this site (letter from Laurie A. Cotulla of the SJCEHD to John Stroh of the SJCMVCD, December 1991). However, in August 1994 and March 1995 groundwater was approximately 48 ft and 44 ft bgs, respectively (see Table 4). It should be noted that according to the Department of Water Resource ,,. Groundwater maps entitled "Lines of Equal Elevation of Water In Wells, Fall 1950", that groundwater at the site has dropped approximately 30 feet since 1950. Groundwater generally flows towards the north (see Figures 3 and 4). The three monitor wells installed at the site are screened from 43 ft to 63 ft bgs. 2.4 DATA SUMMARY AND RECOMMENDATIONS Tanks i and 2 Figure 5 shows a cross-section through the Tank 1 and 2 excavation. Soil contaminated with up to 5,500 mg/kg of TPHe was removed from the excavation down to a depth of 27 ft bgs in 1987. A sample collected from the bottom of the excavation at a depth of 27 ft contained 12 ftw mg/kg of TPHe. Soil samples collected above the estimated high groundwater table north and south of the excavation contained 2.1 mg/kg to 7.1 mg/kg of TPHe. Soil samples collected between the estimated high groundwater table and the August 1994 water table contained up to 11 mg/kg of TPHe. Benzene, toluene, ethylbenzene, and xylene (BTEX) were not detected in soil samples. Organic analytes have not been detected in groundwater samples. Since the soil petroleum impacted soil around the tank excavation is limited to low concentrations of TPHe, groundwater does not appear to be at risk of being contaminated by BTEX compounds. This is supported by two rounds of groundwater sampling results. ._ However, WESTON recommends that the quarterly groundwater sampling started in March 1995 be continued through the fourth quarter, 1995, to confirm that groundwater has not been impacted by BTEX compounds. At that time the necessity for continued monitoring at the site can be re-evaluated. No soil remediation is necessary in the vicinity of Tanks 1 and 2. LS1?R0JECUZANJ0QIMVC1WM.WP 4 12 May 1995 V