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September 11, 1997 Harding Lawson Associates <br /> 38737 002 <br />' Ms Mary Ways <br /> San Joaquin County Public Health Services <br /> Environmental Health Division <br /> Page 2 <br /> 1 <br /> Hydrogeologic Framework <br />' The nearest major surface water body is the Mokelumne River, located approximately one-half mile <br /> northeast of the Site (Plate 2) The Mokelumne River flows to the northwest at this location and is subject <br /> to periodic flooding The regional groundwater flow direction is northeast, toward the Mokelumne River <br />' Locally the gradient is relatively flat and is subject to variable flow directions, most likely due to local <br /> pumping from nearby irrigation wells (WaterWork, 1991) <br /> SITE HISTORY AND PREVIOUS WORK <br />' Previous Site Investigations <br /> Previous work at the Site has been performed under the regulatory oversight of the PHS/EHD and the <br />' RWQCB and is briefly outlined below <br /> ♦ Removal of five underground storage tanks by TVG in 1987 <br />' ♦ Installation of six monitoring wells in three phases <br /> • Monitoring wells MW-1, MW-2, and MW-3 (Exceltech, 1987) <br /> • Monitoring well MW-4 (Exceltech, 1988) <br /> Morutoring wells MW-5 and MW-6 (WaterWork, 1989). <br /> o Quarterly monitoring activities from third quarter of 1989 through second quarter of 1994 (various <br /> reports by WaterWork and RESNA, 1989 to 1994) <br />' ♦ Groundwater pump and treat system second quarter of 1991 through second quarter of 1994 (various <br /> reports by WaterWork and RESNA, 1991 to 1994) <br /> ♦ Soil and groundwater investigation, in two phases, 1994 and 1995 (RAH Environmental 1995a &b) <br /> • Soil borings SB-1 to SB-7 soil and groundwater sampling <br /> • Hand augered borings HA1 to HA3 within the forklift shop soil and groundwater sampling <br /> ♦ Additional soil and groundwater investigation 1996 (Draft SECOR report) <br /> Soil borings SB-8 to SB-10 soil and groundwater sampling <br /> The locations of the monitoring wells and soil borings, the approximate location and dimensions of the <br /> former USTs and the excavation, the forklift shop and other relevant features are shown on Plate 3 <br /> i <br />' SITE CONDITIONS <br /> Soil Profile <br /> Based on HLA's review of the previous data, soils beneath the Site consist of a heterogeneous assemblage <br />' of mainly fine-grained sediments with minor amounts coarse-grained sediments The upper 12 to 15 feet <br /> consists mainly of fine-grained sediments including clay, silty clay, and clayey silt Beneath these fine <br /> grained sediments are discontinuous horizons of clayey fine grained sand and silty-clayey fine grained <br /> sand with occasional lenses of clayey fine- to medium-grained sand <br /> i <br /> 1 <br />