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1.1 Regional Hydrogeologic Framework The MCT facility is located in the Great Valley geomorphic province of California on the ancestral delta ., of the San Joaquin River. Areas adjacent to the MCT facility are flat with little topographic relief Geologic data, water level measurements, aquifer test and water quality analyses of groundwater samples collected from monitor wells indicated that the sequence of alluvial sediments underlying the MCT facility from the water table to depths of at least 450 feet below land surface(bls)comprises one aquifer (Dudek, 1995a). The water table is approximately 55-60 feet bls. This aquifer is characterized by discontinuous lenses of sand, gravelly sand, silty sand, clayey silt, silty clay and clay. Resistivity and lithologic logs of exploratory boreholes indicated that sediments are generally more sandy from approximately 80 to 200 feet bls (Dudek, 1994a). 1.2 Site Hydrostratigraphic Units The aquifer underlying the MCT facility can be divided into four hydrostratigraphic subunits. Hydrostratigraphic subunits are defined as strata having similar hydraulic properties and lithology. These four hydrostratigraphic subunits are the: 1) shallow zone,2) 100-foot sand zone, 3) intermediate zone, and 4)deep zone. The shallow zone extends from the water table to approximately 80 feet bls. The 100-foot sand occurs between approximately 80 and 120 feet bls. The intermediate zone occurs between approximately 120 feet bls to depths ranging from 170 to 200 feet bls. The deep zone extends from below 170 to 200 feet his to approximately 450 feet bls. 1.3 Remedial Wellfield Design and Operation 1.3.1 Remedial Wellfield Remedial wellfield designs were proposed and summarized in previous reports(Hargis and Associates, Inc., 1989x, 1989b, 1991b and 1992). The purpose of the remedial wellfield was to capture and extract .. groundwater containing chromium concentrations greater than 50 ug/l(HA, 1991a and 1991b). Marley Cooling Technologies 3909-09 �- - Dudek&Associates,Inc- Semi-Annual Groundwater Monitoring Report October 2004/January 2005 -1- April 2005 _ r -