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CLOSURE REQUEST 5-31-94 SB6, SB7, SB8, SB10 and SB12 further defined the limited extent of the soil contamination, see Table 4 and Figures 3B, 5A, 5B, 6, 7A, 7B, 7C, and 7D b) Vertical and lateral definition of free-product and dissolved constituents. Contaminated Ground Water Ground water samples beneath this site have been obtained since the initial sampling of soil boring SB2, on December 1, 1989 and from the soil borings, MW1 (12/4/89) , MW2 (12/6/89) , MW3 (12/11/89) and MW4 (12/11/89) . Qudterly ground water sampling of the monitoring wells was initiated on January 12, 1991 (MW1, MW2, MW3 , MW4 , MW5, MW6 and MW7) No floating product have ever been observed. The chemical analysis results of the water samples indicated the following 1 . The ground water contamination is associated with the UST area is of gasoline-range hydrocarbons . 2 . The contaminated ground water decreased rapidly once vapor extraction commenced on ground water monitoring wells MW1 and MW5 . 3 Since September 11, 1992 primary MCL' s have been obtained for this site 4 . The ground water plume was limited to the tank area and to • the area south and southeast of the tank excavation The plume is and always has been contained within the site boundaries, see Figure 4C IV LOCAL AND REGIONAL HYDROGEOLOGY Geology GEOLOGY AND GROUND WATER OCCURRANCE Regional Geomorphology And Hydrogeology The site lies within the northestern edge of the San Joaquin Valley, which is the southern half of the Great Central Valley geomorphic province of California. The Coastal Range province is the western boundary and the Sierra Nevada province is the eastern boundary of the Great Central Valley. All three of the above mentioned provinces trend in a northwesterly direction along with the mayor faults which are associated with the province boundaries . The Central Valley province represents the low position basin separating the Sierra Nevada and Coastal Range mountain masses Erosion of the mountain masses produce sediments which are carried into the valley and deposited as sequences of clay, silt, sand, and gravel formations, predominantly unconsolidated. The valley is continually subsiding, allowing the deposited sediments to build-up and become thicker, with little affect upon the surface elevations . The final result is a thick • sequence of interbedded sediments (gravels, sands, silts, and clays) along with the associated folds and faults . pagel4