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i 2 0 Site Description The site is located in the southern part of the City of Stockton, California at 370 55' 7 67" latitude and 1210 16' 43 45" longitude (Figures 1 and 2) The site is an essentially level commercial trucking facility,which contains an office building,two truck maintenance shops,a truck-washing facility, a small above-ground storage tank compound, a vehicle fueling facility and a large parking area for tank trailers 3 0 Geology The site is situated in the northern part of the San Joaquin Valley, in an area underlain by several thousand feet of terrestrial and marine sedimentary rocks The northern San Joaquin Valley has been a lowland area subject to marine intrusion, flooding and incremental sedimentation for millions of years The uppennost 1000 feet of sedimentary strata consist of Holocene, Pleistocene and Pliocene sediments, including the Laguna Formation, the Mehrten Formation and the Valley Spring Formation The sediments are interbedded mixed-fluvial and deltaic sediments, dominated by medium-grained sand and silts Generally, thin beds of unconsolidated clayey silt, silty clay and sandy clay are abundant in this upper zone,which are laterally discontinuous over distances of several tens of feet 4.0 Hydrogeology Depth to water during this quarter's sampling event vaned from 3153 feet below ground surface (bgs) in well MW-4 to 32 53 feet bgs in well MW-2 The elevation of groundwater in MW-1,MW-2,MW-3, and MW-4 was calculated by El to be at 18 99, 19 11, 19 07,and 19 18 feet below mean sea level (MSL) respectively, based upon casing elevation data from Kier & Wright Civil Engineers and Surveyors All groundwater elevation data including the historical data is summarized in Table 1 5 0 Results 5 1 Groundwater Level Measurements Figure 5 shows groundwater elevations at various dates The groundwater elevations have increased approximately two feet since last quarter and decreased over six feet since the corresponding season in 2000 The elevations fluctuate over very significant ranges (up to 5 feet in 9 months),which will affect contamination levels,flow direction,gradient,etc The elevations in wells MW-1, MW-3 and MW-4 move in tandem, with MW-2 generally following the same trend,but, moving higher or lower somewhat randomly This is due to the difficulty of measuring groundwater elevations in MW-2 because of free product in this well r . 2 I 4166