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were slightly lower than in the shallow wells (e.g., the well pair, MW -4R -S and MW -4R -D). 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 1 feet bgs, was calculated to be 194 feet/day (Shaw, 2006b). 2.3 Investigation History 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 characterization was conducted over a period of several years and results indicated the presence of residual hydrocarbons in both the vadose zone and groundwater. Petroleum hydrocarbons were found to depths of at least 60 feet bgs, or ' approximately 40 feet beneath the water table (Shaw, 2006a). Soil Vapor Extraction (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 several years, SVE treatment operations resulted in the removal of approximately 3,244 pounds of total petroleum hydrocarbons as gasoline (TPH-g) and 4.9 pounds of benzene from the soil (CVRWQCB, 2007a). In March 2006, Shaw performed a round of soil and groundwater sampling to assess the lateral and vertical distributions of residual hydrocarbons and evaluate conditions for potential treatment of the hydrocarbons within the saturated zone (Shaw 2006a). During the investigation, nine direct -push soil borings (MIP -1, -la, -lb, -2, -3, -4, -5, -6, and -7) were advanced, and the level of hydrocarbon contamination was screened using a real-time Membrane Interface Probe (MIP). The MIP investigation showed locally high hydrocarbon responses at select locations from 25 feet bgs down through 61 feet bgs, and approaching non -detect at 65 feet bgs. The ' maximum achievable direct -push depth was 65 feet bgs. Soil samples were also collected from direct -push soil borings (DP -4, -6, -8, and -9) for laboratory analyses during the March 2006 ' investigation. Soil benzene results ranged from non -detect to 5,500 micrograms per kilogram (µg/kg). Hydrocarbon contamination was generally noted from depths of 35 feet to 62 feet bgs and localized in locations at DP -4 and DP -9 (Shaw 2006a). I IFS_OnsdePetroleum Hydrocarbon Remediadon.doc 2-3 Shaw Environmental, Inc.