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2.0 PHYSICAL CHARACTERISTICS OF THE SITE 2.1 Surface Features The HSA building occupies approximately 70 percent ofone cityblock in downtown Stockton,California. ` Another building occupies the northeast comer ofthe block. The block is defined by Sutter Street to the east,Washington Street to the south, San Joaquin Street to the west, and Market Street to the north (Figure 2-1). A parking lot is located south of Washington Street,under an elevated portion ofCalifomia State Highway 4. The site is relatively flat,with an average elevation of approximately 12 feet above mean sea level(msl). ` Mormon Slough and McLeod Lake are the surface water bodies nearest to the site. Mormon Slough is located approximately 1,700 feet south of the site and flows from the east towards the Stockton Deep ` Water Channel west ofthe site. McLeod Lake is located at the eastward end ofthe Stockton Deep Water Channel and is located approximately 1,900 feet west-northwest ofthe site. No surface water bodies are located within 2 miles downgradient of the site. 2.2 Site Geology and Hydrogeology 2.2.1 Lithology and Stratigraphic Relationships Four lithologic zones were identified during initial site activities. These lithologic zones were defined based on predominant grain size and are summarized in Table 2-1. Lithologic Zones 1 and 2 are located above the historical high water table and will not be discussed in detail. Lithologic Zone 3 varies in thickness from 10 to at least 25.5 feet and is composed of a relatively permeable sand. This sand contained the water table aquifer in 1989. Lithologic Zone 4 is composed ofa silt with clay and sand lenses. Lithologic Zone 3 is more permeable than Lithologic Zone 4 due to coarser sediments;Lithologic Zones 3 and 4 do not �- appear to be separated by a hydraulic barrier. 2.2.2 Hydrogeology and Groundwater Conditions Eleven monitoring wells have been installed at the site to date. Currently,eight wells remain in the regular monitoring well network. The majority of the monitoring wells are screened in Lithologic Zone 4. The water levels in the monitoring wells have been measured periodically since 1989. Historical and current groundwater elevations are presented in Table 2-2. In 1988,groundwater beneath the site was encountered at a depth of approximately 25 feet bgs(15 feet below msl)in Lithologic Zone 3. Monitoring wells MW-1,MW-2,and MW-3 were installed in this zone ` with screened intervals from approximately 21.5 to 45.5 feet bgs. Between 1989 and 1991,drought conditions resulted in water levels dropping by as much as 26 feet below the depths ofmonitoring wells MW-2 and MW-3. By 1991,the groundwater elevation reached a low point of approximately 37.8 feet 3