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2.2.2.1 Site Specific Lithology <br /> Based on information provided in the Site Investigation (SI) Report (PG&E, 1989), soil beneath <br /> the Site consists of highly interbedded clay, silt, silty sand, sand and gravel, which is consistent <br /> with the description of regional alluvial sediments. Low permeability clay and silt are <br /> predominant in the unsaturated zone (0 to 7 feet bgs) and the matrix of the uppermost aquifer <br /> consists of interbedded silty sand, sand and gravel. <br /> This information is consistent with the data generated during the RI/SSI for the Site in the <br /> unsaturated zone. However, within the capillary fringe and saturated zone, which is first <br /> encountered at approximately 7.5 feet bgs, high permeability sands and silty sands are <br /> predominant with interbedded clays, silts, and gravels as previously mentioned. One exception to <br /> this; however, was at location A3-2/TW-5, which consisted largely of low permeability, fine- <br /> grained sediments of silts and clays with trace interbedded sands to approximately 20 feet bgs. <br /> 2.2.3 Hydrogeology <br /> The Site is located within the Tracy Groundwater Subbasin of the San Joaquin Groundwater <br /> Basin. Locally, groundwater is recharged from the foothills of the Diablo Mountain Range to the <br /> west and through infiltration of precipitation. Groundwater production wells were the primary <br /> source of water in the Tracy area until approximately 1951 when the large scale importation of <br /> surface water from the Delta-Mendota Canal was initiated. Since 1951, the primary source of <br /> domestic water has been surface water (Hotchkiss and Balding, 1971). The Tracy Subbasin is <br /> drained by the San Joaquin River and one of its major west tributaries, Corral Hollow Creek <br /> (CDWR, 2003). <br /> Surface drainage at the Site is directed to existing stormwater catch basins strategically located <br /> within the Site and discharged to a drainage ditch south of the Site (Figure 2). Manmade culverts <br /> or drainage structures are not present within the Site; however, newer industrial developments <br /> and residential areas have upgraded drainage systems to manage stormwater. The Tom Paine <br /> Slough is the closest surface water body to the Site and is located approximately one mile north <br /> (downgradient). <br /> Regionally, groundwater is divided into three water bearing zones: a deep zone below the <br /> Corcoran Clay, an intermediate semi-confined zone above the Corcoran Clay and a shallow <br /> unconfined aquifer within approximately 25 feet of ground surface (CDWR, 1967). The two <br /> lower zones (deep and intermediate) were the primary source of groundwater pumping prior to <br /> 1951. Groundwater in the region near the Site has been degraded by overdrafting of production <br /> wells, which introduced saline/connate waters into the system. <br /> As depicted in Figure 4, groundwater at the Site is typically encountered within eight to 10 feet <br /> from ground surface and flows to the north-northeast at a gradient of approximately 0.003 feet <br /> per foot (ft/ft) (PG&E, 1989). Results from previous investigations show that shallow <br /> groundwater may be impacted from former Site operations. <br /> Based on data collected during this RI/SSI, the depth to groundwater, flow direction, and <br /> gradient are consistent with the historical data gathered at the Site. Water levels were measured <br /> to the nearest 0.01 foot using an electronic water-level indicator. Groundwater elevations were <br /> calculated by subtracting the depth-to-water measurements from the surveyed top of well casing <br /> elevation. Static groundwater was encountered during the RI at depths ranging from 8.55 to 9.55 <br /> PARSONS 11 FINAL RI REPORT-FORMER TRACY MGP <br /> APRIL 2010 <br />