ARCHIVED REPORTS_XR0008935 - Laserfiche WebLink

Home Browse Search

e KEI-J92-0205.R4 July 5, 1994 Page 5 A quarterly ground water sampling and monthly ground water monitoring program was reimplemented by KEI on December 30, 1992 , and continued until May 28 , 1993 (two consecutive quarters) . During this period, all seven Unocal monitoring wells were sampled twice and were monitored six times. The water samples were analyzed for TPH as gasoline, TPH as diesel, and BTEX. The analytical results of the water samples are summarized in the attached Table 3 . HYDROLOGY AND GEOLOGY The measured depth to ground water at the site on May 28 , 1993 (the last monitoring and sampling event at the former Unocal facility) ranged between 8.84 and 10. 36 feet below grade. Based on the water level data gathered during the quarter, the ground water flow direction appeared complex, as shown on the attached Figure 1. The average hydraulic gradient at the majority of the site on May 28, 1993 , was approximately 0. 001. A summary of historical ground water monitoring data that are available to KEI are presented in Table 1, including high and low ground water levels, ground water flow direction, and where possible, hydraulic gradient for each monitoring event. Regional and site specific geology have been described and presented on geologic cross-sections in Kleinfelder' s "Problem Assessment Report, Unocal Corporation, Unocal Stockton Terminal, " dated September 1, 1988 (Job No. 24-2059-01) . The following is an abbreviated summary of the general geology described in that report. The Unocal site is located in the west central portion of the northern San Joaquin Valley, which is part of the larger Great Central Valley Geomorphic Province. The Central Valley is a broad structural trough that has been filled with a thick succession of alluvial and lacustrine sediments from the Sierra Nevada Range on the east and the Coast Ranges on the west. A review of the Boring Logs for the seven monitoring wells and two exploratory borings drilled at the Unocal site indicate that the site is underlain by an interbedded sequence of clayey silt, silt, and silty clay to the maximum depth explored (20 feet below grade) . A fine to coarse-grained sand unit was also encountered in one of the two exploratory borings drilled on-site. This sand unit may be laterally discontinuous and appears to pinch out into several fine to medium grained stringers in at least one direction. The unsaturated zone underlying the site is approximately 9 to 11 feet thick (May 1993) and consists mainly of silty clay. The first water bearing unit beneath the site (first aquifer) is composed