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KEI-P88-1013 .R2 July 15, 1992 Page 8 HYDROLOGY AND GEOLOGY Ground water levels stabilized in the monitoring wells on June 3 , 1992 , at depths ranging from 64. 85 to 62 . 16 feet below the surface. The ground water flow direction appeared to be toward the north- northwest on June 3 , 1992 , with a hydraulic gradient of approxi- mately 4. 014 (based on water level data collected from the monitoring wells prior to purging) . The contours of equal ground water elevation on June 3 , 1992 , are shown on the attached Site Plan, Figure 1. The subject site is located adjacent to the Sacramento-San Joaquin delta in the Central valley geomorphic province. The Central Valley is a large, northwestward-trending, asymmetric structural trough that has been filled with as much as six vertical miles of sediment in the San Joaquin valley. Based on review of regional geologic maps (U. S. Geological Survey Miscellaneous Field Studies Map MF-1401 "Geologic Maps of the Sacramento-San Joaquin Delta, California" by Brian F. Atwater, 1982) , the subject site is underlain by Holocene and/or upper Pleistocene alluvium identified • as Alluvium of Calaveras River and Vicinity (Qcr) . The Calaveras River alluvium was deposited by the Calaveras River, Bear Creek and several lesser streams, and has not been described in detail. The results of our most recent subsurface study indicate that the site is underlain by a sequence composed predominantly of silty clays and clayey silts. Sand lenses up to 10 feet thick are also present in three of the wells (MW10, MW12 , and MW13) . The particle size analysis (sieve only) for the soil sample collected from MW13 at a depth of 65 feet below grade indicates that the sample is composed of 93 percent fine to medium sand. The sample is classified as sand (SP) . The results of the analysis are included as Plate 1. DISCUSSION AND RECOMMENDATIONS Based upon the analytical results for the ground water samples collected from wells MW10 through MW13 on June 3 , 1992 , and based upon the apparent ground water flow direction on this date, the upgradient wells at the Unocal site are showing greater concentra- tions of dissolved hvdrocarbon contaminants than the downgradient wells. Therefore, �in order to verify the flow direction and relative magnitude of contamination at the Unocal site, KEI recommends the implementation of a three or six-month monitoring and sampling program. The existing wells should be monitored on a monthly basis, and should be purged and sampled on a quarterly . basis . Once consistent ground water flow directions and contami- nant concentrations are established for the site, KEI will provide