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Remedial Action Plan and Further Site Assessment 1 <br /> 7-Eleven Store No 19975 1399 N Main St Manteca CA May 30 2000 <br /> 1 0 GEOLOGY AND HYDROGEOLOGY <br /> tThe site is located in the middle of the northern San Joaquin Valley Shallow stratigraphic units in the <br /> vicinity of Manteca are mapped as Quaternary dune sand overlying the Quaternary Modesto Formation <br /> ' which consists predominantly of arkosic alluvium (California Division of Mines and Geology, 1991, <br /> Geologic Map of the San Francisco-San Jose Quadrangle, 1 250,000) The site is located in a light <br /> commercial area, and the immediate site vicinity is occupied by retail buildings (Figure 1) <br /> 1 <br /> Sensitive receptor surveys completed in 1999 (IT Corporation, April 1999) and March 2000 (Figure 2) <br /> ' indicate that there are no nearby municipal wells at likely risk due to hydrocarbons at this site, although <br /> there are several domestic wells within a 2,000-foot radius of the site <br /> ' First groundwater is encountered at depths of about 20 -25 feet below grade, equilibrated groundwater <br /> levels range between about 14 - 18 feet below grade Groundwater flow is generally to the north or <br /> northwest at a gradient of about 0 002 feet per foot (IT Corporation, 2000) (Figure 3) <br /> Subsurface soils generally consist of sand and silty sand to approximately 10 feet below grade, underlain <br /> by clayey silt and sift to approximately 20 feet below grade, underlain by fine-to coarse-grained sand and <br /> silty sand to about 35 or 40 feet, and interbedded clayey silt and clayey silty sand to the total depth of <br /> ' investigation (about 50 feet below grade)(Figure 4) <br /> Based on an assumed flowrate of about 10 gallons per minute (gpm), and an assumed average <br /> transmissivity of 1,000 gallons per day per foot(based on 10 foot thick saturated fine sand with hydraulic <br /> ' conductivity of 100 gpm), the groundwater extraction capture zone is estimated to be 380 feet on <br /> achievement of steady-state conditions The estimated capture limit is beyond the map area shown on <br /> Figure 3 Maintenance of an appropriate capture zone will be verified after installation of the extraction <br /> ' system based on actual conditions, and additional extraction wells will be installed if necessary to retain <br /> plume control The extraction well location and depth will be proposed under separate cover after results <br /> ' of additional site assessment <br />