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EXECUTIVE SUMMARY The objectives of this investigation were to assess the lateral extent of impacted groundwater and to establish a groundwater monitoring capability at the site To achieve these objectives, three on-site groundwater table monitoring wells were permitted, installed, surveyed, monitored and sampled The on-site water supply well was also sampled during the investigation Due to the unavailability of water well completion reports for San Joaquin County at the California Department of Water Resources (DWR), the proposed water well survey will be completed and documented in a subsequent quarterly groundwater monitoring report Soils observed during the drilling of the three monitoring well borings to a maximum depth of 87 feet bgs consisted mostly of massive intervals of clay, silt and fine sand mixtures interrupted by thin (i e, less than five feet thick) intervals of poorly-graded fine sand Laterally continuous sand sequences were observed at approximately 15 feet bgs and at 75 to 80 feet bgs Other sand intervals were observed as laterally discontinuous across the site During drilling, saturated soil and groundwater were first encountered between 79 5 and 80 1 feet bgs One week later, the static depth to the groundwater table, as measured in the three groundwater monitoring wells, ranged from 77 40 feet bgs in MW-1 to 78 23 feet bgs in MW-3 As such, groundwater does not appear to present under significant confining conditions The groundwater surface elevation ranged from a high of 22 64 feet (arbitrary datum) at MW-2 to a low of 22 42 feet (arbitrary datum) at MW-1 The resulting groundwater surface appears as a regular plane dipping to the north-northeast at a gradient of approximately 0 0037 foottfoot The 12 soil samples, three monitoring well groundwater samples, and the domestic water supply well sample collected during the investigation did not contain detectable concentrations of any of the gasoline constituent compounds analyzed Based on the results of previous subsurface investigation, soil and groundwater beneath the north end of the former middle UST are impacted by relatively low concentrations of gasoline hydrocarbons Most of the petroleum hydrocarbon mass remains concentrated in soil immediately below the north end of the former middle UST Vertically, the concentrations of gasoline hydrocarbons decrease rapidly with depth, and are defined between 65 and 80 feet bgs Based on the results from soil samples collected during the drilling of MW-1, MW-2 and MW-3, the lateral extent of impacted soil appears to be defined to the area of the former USTs Impacts to groundwater at the water table are defined to the northwest (MW-1), west (MW-2), and east (MW-3) The plume of impacted groundwater appears to be undefined to the south ' and north-northeast (downgradient) of the former USTs Based on these conclusions, Ramage Environmental recommends the following course of environmental corrective action 1 Continue to monitor and sample groundwater at the site on a quarterly basis 2 Complete the sensitive receptor survey for the site vicinity 3 Determine if an additional monitoring well is needed downgradient of the former USTs r 4