ARCHIVED REPORTS_XR0012364 - Laserfiche WebLink

Home Browse Search

ENSR1 _ 3 3,0 Conceptual site model and extent of contaminant impact i The results of the remedial response action (E e , soil excavation in 1988) and subsequent site investigations performed since 1988, including periodic groundwater monitoring activities, were comprehensively reviewed to update the conceptual site model (CSM) The following sections provide a detailed description of the updated CSM 3.1 Geology and hydrogeology 311 Geology The site is within the Eastern San Joaquin County Ground-Water Basin, as defined by the California Department of Water Resources (1980) The area is underlain by flood basin and fluvial sediments consisting of clay, silt, and sand (Rogers, 1966) The topographic surface is nearly flat with a slight westward slope Three surface water bodies near the site include Yosemite Lake, the eastern terminus of Smith's Canal, approximately 2,000 feet west of the site, the eastward extension of the Stockton Deep Water Channel (including MacLeod Lake), approximately 1 mile south of the site, and the Calaveras River, approximately 6,000 feet northwest of the site All three bodies of water discharge to the San Joaquin River approximately 3 miles west of the site (AGS, 1990) rThe site elevation is approximately 10 feet above mean sea level The topography in the site vicinity is generally flat with a gentle slope to the west toward the San Joaquin River Soil encountered beneath the site is comprised primarily fine-grained deposits consistent with the alluvial depositional history of the region Inter-bedded clays and silts with some sand were observed in the soil borings The depth and thickness of these zones are variable across the site Site geologic cross-sections depicting subsurface stratigraphy are illustrated in Figures 3-1 (north—south) and 3-2 (east—west) 312 Hydrogeology The depth to groundwater at the site has ranged historically from approximately 21 feet bgs to greater than 50 feet bgs Groundwater depths were at their recorded lowest in 1988 at the site, when monitoring wells were first installed at the site From 1998 through 2001, groundwater elevations raised to approximately 25 feet bgs, but declined to their current depth of approximately 28 feet bgs Interpreted potentiometric contour maps based on the January 2006 well gauging data are presented for the water table wells (i e , Zone A wells) and the deeper Zone B wells (approximately 85 to 90 feet bgs) in Figures 3-3 and 3-4, respectively The groundwater flow direction at the groundwater table (i e , Zone A) continues to be in a northeasterly direction with a calculated hydraulic horizontal gradient of 0 003 feet per foot (ft/ft) The groundwater flow direction in the deeper zone (i e , Zone B) is also in a northeasterly direction with a calculated horizontal hydraulic gradient 0 006 ft/ft The vertical component to the hydraulic gradient varies at this site Groundwater elevations data from six well clusters are presented in Table 3-1 These well clusters include MW-13/ MW-14 (downward), MW-151 ' MW-16 (upward), MW-17/MW-18 (mixed), NP-1 /MW-10 (mixed), MW-11 /MW-12 (mixed), and MW-9/ MW-20 (downward) These data indicate that there is a downward component to the hydraulic gradient on the upgradient side of the site (MW-13/MW-14), while there is a mixed gradient (both upward and downward) immediately downgradient and crossgradient to the source area as noted at well couplets MW- 10/NP-1, MW-11/MW-12, and MW-17/MW-18 The primary direction of groundwater migration is either upward or downward between Zone A and Zone B, rather than horizontal 3-1