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Within the last five years, the depth to water has varied between 19 to 27 feet bgs in site wells (IT Corporation, 2002; and Shaw, 2006a, 2007). Fluctuation in water levels may be attributed to ' not only seasonal rainfall, but also the operation of the Port of Stockton and the pumping of groundwater supply wells in the Stockton vicinity. Groundwater elevations in the deeper wells were slightly lower than in the shallow wells (e.g., the well pair, MW-4RS and MW-4RD). This is an expected result of groundwater flow away from the Port of Stockton. The Port of Stockton is located to the north and west of the site and local groundwater is inferred to flow eastward from the port. Quarterly groundwater monitoring events have noted ' groundwater flow direction varying from northwest to northeast to east at an average gradient of 0.002 feet per feet. Slug tests were performed on wells MW-4RS and MW-4RD in May 2006. The resultant hydraulic conductivity for MW-4RS, screened from 35 to 50 feet bgs, was calculated to be 57 feet/day. The hydraulic conductivity at MW-4RD, screened from 75 to 80 feet bgs, was calculated to be 194 feet/day (Shaw, 2006b). 2.3 Investigation History Summary An environmental investigation of the site was initiated following the removal of the 550 -gallon UST in 1996. Soil samples collected from beneath the excavation indicated the presence of residual hydrocarbons. Subsequent site assessment revealed the presence of residual hydrocarbons in both the vadose zone and groundwater. SVE was conducted at the site from February 2000 until October 2004 to address the petroleum hydrocarbon contamination in the soil within the vadose zone. Over the treatment period, approximately 3,244 pounds of total petroleum hydrocarbons as gasoline (TPH-g) and 4.9 pounds of benzene were removed from the soil (CVRWQCB, 2007). In March 2006, Shaw performed a site characterization, including sampling soil and groundwater using direct -push and field screening techniques to assess the lateral and vertical distributions of residual hydrocarbons. Results indicated the presence of elevated petroleum hydrocarbon concentrations between 35 feet and 60 feet bgs and near the former UST location in the eastern - southeastern area of the site (Shaw 2006a). Quarterly groundwater monitoring has resumed at the site since September 2007. At the request of CVRWQCB and the San Joaquin County Environmental Health Department, further investigation of the vertical extent of the hydrocarbon contamination in the saturated zone is ' being conducted at the site. Results will be used to determine if groundwater treatment below 60 feet bgs is required. BiospargePilot Test Final doc 2-3 Shaw Environmental, Inc.