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' • March 31,2005 Mr.M. Scott Mansholt—CEMC Page 2 of 5 Site Investigation Workplan,Mountain House Sites 44 and#5 g REGIONAL/LOCAL GEOLOGY AND HYDROGEOLOGY Regionally,the study area is encompassed within the Tracy Sub-basin(Groundwater Basin Number 5-22.15) and is defined by the aerial extent of unconsolidated to semi-consolidated sedimentary deposits that are bounded by the Diablo Range on the west,the Mokelumne and San Joaquin rivers on the north,the San Joaquin River to the east,and the San Joaquin-Stanislaus County line on the south. The Tracy Sub-basin is located adjacent to the eastern San Joaquin Sub-basin on the east and the Delta-Mendota Sub-basin on the south(Department of Water ' Resources [DWR],Update 2003, Bulletin 118). The above-mentioned sub-basins are located within the larger San Joaquin Valley Groundwater Basin. ' The Tracy Sub-basin is comprised of continental deposits of Late Tertiary to Quaternary age, which include the Tulare Formation,Older Alluvium, Flood Basin Deposits, and Younger Alluvium. The thickness of these formations ranges from a few hundred feet in the western foothills to about 3,000 feet near the eastern margin of the basin(DWR, Update 2003, Bulletin ' 118). The Tulare Formation is comprised of semi-consolidated, poorly sorted, discontinuous deposits of clay, silt, and gravel. The Corcoran clay is situated near the top of the Tulare Formation,separating groundwater in the basin in what has been reported as two primary ' aquifers. The upper aquifer is reported to range in thickness from 15 to 250 feet below ground surface(bgs) and the lower aquifer is reported at a depth of 600 feet bgs. Generally, depth to groundwater in the Tracy Sub-basin varies from 5 to 10 feet bgs. Groundwater in the upper water-bearing zone flows in a north-northeast direction and is hydraulically controlled by low-permeable unconsolidated sediments. Soils encountered at this site and other sites in the vicinity appear to be fine-grained unconsolidated sediments, consisting ' mainly of silts and clays with minor amounts of poorly graded sand and gravel. Groundwater was first observed at the site at depths ranging from 8 to 17 feet bgs in 2001. PREVIOUS ENVIRONMENTAL ACTIVITIES ' Based on previous environmental investigations conducted by Condor Earth Technologies (Condor) during June and July 2001, six areas along ChevronTexaco's former OVP ROW were identified for further investigation in accordance with the CTA(Geomatrix 2002). This workplan focuses on petroleum-affected soil and groundwater identified at Mountain House Sites#4 and#5 (Figures 2, 3 and 4). Sites 94 and#5 are located approximately 3,000 and 1,900 feet southeast, respectively, of the intersection of Byron Road and Kelso Road,northwest of the City of Tracy. ' As discussed in Condor's Report Chevron Pipeline Easement Assessment Report dated November 26, 2001, petroleum-affected soil was observed at Site#4 from borings GP-53 and GP-180 at ' depths ranging from 8 to 16 feet below ground surface(bgs). Soil samples collected at depths of 13.5 and 15 feet bgs contained detectable total petroleum hydrocarbons as crude oil(TPHc) at concentrations of 13,000 milligrams per kilogram(mg/kg) and 3,600 mg/kg, respectively. During this investigation, groundwater was encountered beneath the site at depths ranging from 9 to 12 feet bgs. Grab groundwater samples were not collected during this investigation. On Site#5 petroleum-affected soil was observed in borings GP-64 and GP-183,at depths ranging ' from 4 to 15 feet bgs. Soil samples collected at depths of 16 and 12 feet bgs contained TPHc at concentrations of 7,600 and 65 mg/kg,respectively. Groundwater was encountered beneath Site #5 at depths ranging from 8 to 17 feet bgs. Grab groundwater samples were not collected during this investigation. Previous boring locations for Sites#4 and#5 advanced during the Condor investigation are shown on Figure 2.