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Ms. Lori Duncan Page 2 of 6 2.0 BACKGROUND INFORMATION 2.1 Initial Investigation and Underground Storage Tank Removal The site currently operates as an automotive service center, and is owned by Mr. Ed Niemann et.al. One 550-gallon UST was previously utilized for storage of waste oil. The UST was removed under permit from PHS/EHD on May 26, 2000. A soil sample collected from beneath the tank contained elevated levels of petroleum and chlorinated hydrocarbons. Analytical results are summarized in Table 1. The location of the former UST is depicted on Figure 2. 2.2 Hydrogeological Setting The site is situated in the Lodi South quadrangle, Section 8, T2N, R6E, San Joaquin County, California. Regionally, the property is located in the San Joaquin Valley physiographic province. The valley is a topographic and structural basin bounded on the east by the Sierra Nevada and on the west by the Coast Ranges. Locally the topography slopes gently toward the southwest at approximately 6 feet per mile. The near surface geology of the region is comprised of unconsolidated alluvial deposits of Pleistocene to Holocene age. These deposits, sometimes referred to as the "older alluvium", consist of intercalated beds of gravel, sand, silt and clay. Underlying the older alluvium are Plio/Pleistocene continental deposits of similar derivation and lithology. The older alluvium functions as the most important aquifer in the site area (USGS Professional Paper 1401-C, 1986). Soils developed on the alluvium are generally well drained, differing from the parent material only in the increased volume of organic matter (DWR Bulletin No. 146, 1967). Significant surface water bodies in proximity to the site are Five Mile Creek, Mosher Slough, and the Calaveras River. Mosher Slough and Five Mile Creek, are located approximately 800 feet and 1 mile south of the site, respectively. These two waterways flow in a westerly direction. The southwest flowing Calaveras River is located approximately 4 miles west southwest of the site and is a tributary of the San Joaquin River. The Calaveras River generally exhibits moderate stream flow losses over portions of the year, which contributes to groundwater recharge (USGS Professional Paper 1401-D, 1989). The "older alluvium" is the most extensively developed geologic unit in the area and hosts both unconfined and confined zones. The unconsolidated, mostly coarse grained nature of the aquifer material results in high well yields. Specific capacities of wells completed in the older alluvium average 42 GPM/ft (USGS Professional Paper 1401-1), 1989). Drinking water wells in the region are generally completed at depths greater than 100 feet below ground surface due to the relatively poor quality of near surface groundwater. Regional groundwater flow in the site area is generally in an easterly direction toward a regional groundwater depression centered approximately 4 miles east of Stockton (San Joaquin County Flood Control and Water Conservation District, Fall 1998). Nsauto\reportslsummary_W P.doc