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Um • Mr Harlin Knoll Page 5 San Joaquin County Environmental Health Department May 19, 2003 encountered from the surface to 32 feet bgs and sand was present between 32 and 58 feet bgs Sandy clay was encountered in the bottom two feet of MW-7 Lithologic data from the current investigation and previous investigations were used to generate the geologic cross sections shown on Figure 3 and Figure 4 Cross section A-A' runs approximately perpendicular to the groundwater flow direction Cross section B-B' runs approximately parallel to the groundwater flow direction The aquifer in which the monitoring wells are screened, and in which contaminant migration occurs, is generally unconfined and consists of sand and silty sand with a saturated thickness ranging from 10 to 20 feet A gravel layer approximately 3-feet in thickness is present beneath the western portion of the site dust below the water table The thin clay layer located directly above the screen interval of MW-5 does not appear to be laterally continuous Groundwater was first encountered at approximately 51 feet bgs in MW-5, at 53 feet bgs in MW-6, and at 44 feet in MW-7 Groundwater stabilized at approximately 46 feet bgs in the monitoring wells As shown on Figure 5, groundwater flow is toward the southeast at a gradient of approximately 0 0014 ft/ft . Groundwater Analytical Data Groundwater analytical data from the second quarter 2003 monitoring event indicate that groundwater samples from the two newly installed downgradient wells (MW-6 and MW-7) did not contain concentrations of fuel-related compounds above reporting limits Based on these data, the down-gradient lateral extent of impacted groundwater is presently defined by wells MW-6 and MW-7 The groundwater sample collected from deep well MW-5 had reported concentrations of TPHg at 900 gg/L and MtBE at 1,900 µg/L MW-5 is screened in a thin 2-foot layer of silty sand, which is separated from the thicker, more heavily impacted sands above it by approximately 8 feet of clay (See Figure 4 and 5) The groundwater elevation measured in MW-5 is consistent with those in other site wells screened above it, suggesting that MW-5 is screened in the same water-bearing zone as the other site wells Based on the fact that low permeability clays and silts were encountered from 70 to 98 feet bgs, it is likely that the bottom of the well screen at MW-5 (70 feet bgs) represents the vertical extent of impacted groundwater Second quarter 2003 groundwater analytical data are presented in Table 1 and summarized on Figure 6 Laboratory analytical reports and chain-of-custody documentation are presented in Attachment C K 1WprocessV5 BP ARC01111911Well InslallatlonNwell installation report03 doe