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2.0 HYDROGEOLOGIC CHARACTERIZATION The regional geologic setting in the vicinity of the SMS Briners facility is typical of the northern San Joaquin Valley and was discussed in the Interim Ground-Water Monitoring Network and Program, SMS Briners, Stockton, California (LSCE, 1985),the Interim ` Monitoring Report and Work Plan for Operational Revisions and Verification Monitoring (LSCE, 1989), the Monitoring Program Status Update Report and Work Plan for Source Control, Operational Revisions and Ongoing Monitoring(LSCE, 1990), and the Underground Injection Control Program, Permit Application for a CLASS I Injection Well (LSCE, 1991). These documents can be referenced to obtain a detailed discussion of the regional geologic setting. Informal,descriptive subdivisions of Quaternary alluvium,based on local lithostratigraphic characteristics, were presented in the Corrective Action Program (LSCE, 1992), and hydrostratigraphic designations were given. 2.1 Shallow Zone Groundwater Conditions Groundwater levels in shallow zone monitoring wells have fluctuated between depths of about 100 to 140 feet since 1992. Water level elevations in the shallow and deep zones, as measured in well clusters MW3A/313 and MW4A/4B,typically differ by less than a foot. Contours of equal groundwater elevations prepared for the shallow zone from August 1990 to present generally indicate that flow in the immediate vicinity of the facility predominantly occurs to the northwest (west-northwest to north-northwest)to northeast depending on the operation of local extraction wells. Since Spring 2002 when discharge from well P5 was discontinued,the general flow pattern in the shallow zone has been primarily to the northwest as shown on Figure 2-1 (LSCE, 2003a, 2004a, 2005a). r. Historical groundwater flow to the northeast, east, and east-southeast directions were primarily due to the operation of MW 1 as an extraction well for remedial purposes, Well A, and P5 (prior to Spring 2002). Flow to the northeast has most frequently been observed from 1996 to Spring 2001 after MWl started operating as an extraction well (LSCE, 2001a). Prior to 1996 the direction of flow exhibited a seasonal pattern, west and northwest during the winter and spring months (typically referred to as the wet season) and an east to northeast direction during the summer and early fall months (typically referred to as the dry season) as a result of seasonal pumping (LSCE, 1992 and 1995). Northwesterly gradients are typically on the order of 0.001 ft/ft and have ranged from about 0.0004 to 0.002 in the last three years. r 2.2 Deep Zone Groundwater Conditions Groundwater levels in the deep zone have fluctuated between depths of 100 to 125 feet since 1992. Contours of equal groundwater elevations prepared for the deep zone from March 1985 to present show that, in absence of obvious pumping impacts, the flow direction in the immediate vicinity of the facility ranges from westerly to northerly. However, stable or © LUHOORFF & SCALMANINI CONSULTING ENGINEERS 2