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Pilot Testing Report and Conceptual Corrective Action Plan July 15, 1994 Manna Pro, 4004 South EI Dorado, Stockton, CA 8 ' Below the first sand layer is a layer of gray clay approximately 3 to 10 feet thick The clay grades to silt, silty clay, and clayey silt near OBA and EW-1 This clay layer, which is apparently absent near ' MW-6, ranges from 10 feet thick at MP-11 to about 3 feet thick at MP-20 ' A water-bearing gray sand exists below the gray clay layer This water bearing sand exhibits semi- confined aquifer characteristics and ranges in thickness from approximately 20 feet thick at MW-6 to ' 10 feet thick elsewhere A thin discontinuous clay lens appears within the gray sand between MW-1 and MW-4 ' The water table elevation as shown on cross section A-A' (Figure 4) and on Figure 5, slopes to the north with a mound effect located between monitoring wells MW-2 and MW-6 The monitoring wells were monitored on April 14, 1994 Data from the monitoring event revealed a range of elevations of groundwater beneath the site from -23 79 feet MSL in MW-9 to -21 98 feet MSL in MWA Based on water elevation data (Figure 5), the groundwater flow direction is variable across the site For example, the apparent groundwater flow direction between the locations of MWA and MW-9 is generally northeast, while groundwater flow between MWA and MW-8 is generally toward the northwest The reason for the apparent groundwater mounding near MWA is unknown ' 33 Physical Characterization Summary The physical characterization of the site can be summarized as follows • The site is located on flat graded terrain between Duck Creek and the San Joaquin River The terrain is approximately 15 feet above sea level • Based on previous exploratory soil boring and well drilling soil logs, the site is underlain by several layers of sand and clay A predominantly clay zone exists beneath the facility from approximately 0 to 20 feet below grade, and 35 to 40 feet below grade Clay was also encountered at depths deeper than 50 feet below ' grade Sand layers were encountered approximately 20 to 35 feet below grade and 40 to 50 feet below grade ' The lower sand layer is apparently a semi-confined aquifer The groundwater gradient is generally to the north across the site However, groundwater mounding is apparent near the former UST pit ' 2819 R034 opo GROUNDWATER ' TECHNOLOGY, INC