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ENSR AECCM 3.0 Conceptual model and extent of contaminant impact The result of site investigation activities performed between the time of tank removal in 1992 and the completion of February 2005 quarterly groundwater monitoring activities were comprehensively reviewed to update the CSM. The following sections provide a detailed description of the updated CSM. 3.1 Geology and hydrogeology 3.1.1 Geology The site is located in the San Joaquin River Basin within the Central Valley of California, a northwestward- trending trough between the Sierra Nevada and Coast Ranges. The sedimentary sequence is alluvium, which has a maximum thickness of 9,000 feet in the vicinity of the site. The surficial geology at the site is comprised ' principally of floodplain deposits from the San Joaquin River. Soil observed beneath the site are comprised primarily of silt and clay to-approximately 20 feet bgs, below which interbedded silt, clay, and sand extend to approximately 75 feet bgs; silt and clay again predominate between approximately 75 and 110 feet bgs, below which interbedded sand and silt were observed. Three zones of higher sand content have previously been identified between approximately 35 and 50 feet bgs, 60 and 75 feet bgs, and below 115 feet bgs, identified as Zone A, Zone B and Zone C, respectively. However, in some cases, these may be merely distinctions of convenience, as there is no contiguous low-permeability unit between Zones A and B. The depth and thickness of these zones are highly variable across the site, consistent with the alluvial depositional history of the region. Geologic cross-sections depicting subsurface stratigraphy with descriptions of soils encountered on and off site are illustrated in Figures 3-1 through 3-3. 3.1.2 Hydrogeology The depth to groundwater at the site has historically ranged from 25 to 48 feet bgs. Groundwater depths were at their recorded maximum (i.e., approximately 48 feet bgs) in 1994, when monitoring wells were first installed. Groundwater elevation then rose to approximately 25 feet bgs in 1999, 2000 and 2001, and have declined to their current depth of approximately 30 feet bgs. Interpreted potentiometric contour maps based on February 2005 data are presented for Zones A, B and C in Figures 3-4 through 3-6. The groundwater flow direction in all three zones is generally to the east, away from the San Juaquin River with a hydraulic gradient ranging from 0.004 to 0.007 feet per foot. The vertical to component the hydraulic p e y raullc gradient is consistently downward at the site. Poten#iometric surface monitoring data at multi-depth well clusters are presented in Table 3-1. These data indicate that the downward component to the hydraulic gradient between Zone A and Zone B is comparable to the horizontal, and is greater than the horizontal gradient between Zone B and Zone C, indicating that the primary direction of groundwater migration is downward between these units. iThe nearest water supply well to the site is a public supply well located approximately 1,600 feet east- northeast(downgradient)of the site. For public security reasons, information is not currently available for this well. However, based on information for other supply wells in the area, it is understood to be screened at a depth of approximately 300 and 400 feet bgs. No aquitard has been observed between Zone C and the estimated screened interval of the downgradient water supply well. However, given the depth of the regional productive interval and the predominance of clay deposits in the overlying stratigraphic units, it is believed that there is no strong connection between Zone C and the aquifer from which local water supply wells withdraw their water. i J:tiRem_Eng\Project Files1P60(6000- 6999)16940-UNOCAL Portfolio Clasure16940. 3-1 January 2006 UNOCAL Portfolio Closure\Site 0187-Stockton East Min0ACSM ReportlCSM Report_Final.dac i