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Adjacent to the eastern side of the building is an unpaved, fenced yard. Previously, Colombo/Toscana operated a distribution depot for bakery products at the Site. Mr. Overton currently uses the warehouse building for storage. Three monitoring wells (MW-1, MW-7S, and MW-7D) and one vapor extraction well (VE-1) exist at the Site (Figure 3). 2.2 REGIONAL GEOLOGY AND HYDROGEOLOGY The Site is situated in the Great Valley geomorphic province, which is bounded by the Coast Ranges to the west and the Sierra Nevada province to the east. The site is also situated within the Eastern San Joaquin Subbasin of the San Joaquin Valley Groundwater Basin. The Eastern San Joaquin Subbasin is defined by the aeral extent of unconsolidated to semi-consolidated sedimentary deposits that are bounded by the Mokelumne River on the north and northwest, San Joaquin River on the west, Stanislaus River on the south, and consolidated bedrock on the east. Significant water-bearing formations in the Eastern San Joaquin Subbasin consist of the Alluvium and Modesto/Riverbank Formations, Flood Basin Deposits, Laguna Formation, and Mehrten Formation (DWR, 2003). The majority of the groundwater in the basin is characterized by calcium-magnesium bicarbonate or calcium-sodium bicarbonate types. Bicarbonate is the predominant anion in the eastern part of the basin. Reportedly, large areas of chloride type water occur along the western margin of the subbasin along the San Joaquin River. As a result of declining groundwater levels, poor-quality water has been moving east along a 16-mile front on the eastern side of the Delta. The degradation has been particularly evident in the Stockton area where the saline front was moving eastward at a rate of 140 to 150 feet per year (DWR, 2003). Each significant water-bearing formation in the Eastern San Joaquin Subbasin is described in the following subsections. Alluvium and Modesto/Riverbank Formations (Undifferentiated) These units are exposed within the subbasin along a band approximately 15 miles wide that extends from Stockton eastward. These units are Late Pleistocene to Recent in age and consist primarily of sand and gravel in the fan areas and clay, silt, and sand in the interfan areas. These units are thin on the eastern side of the basin and are 150 or more feet thick near the center of the basin. Groundwater is under unconfined conditions within these units. Well yields of approximately 650 gallons per minute (gpm) have been reported. Reportedly, because these units are limited in thickness, most wells penetrate them to tap deeper aquifers in the area (DWR, 2003). 2