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' • • Mr.M. Scott Mansholt—CEMC March 31,2005 Site Investigation Work Plan,Mountain House Site#6 Page 2 of 5 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 7 to 17 feet bgs in 2001. ' PREVIOUS ENVIRONMENTAL ACTIVITIES As discussed in Condor's Report Chevron Pipeline Easement Assessment Report(2001),petroleum- affected soil was observed at Site#6 from borings GP-152, GP-156 through GP-158, GP-160, and GP-161 at depths ranging from 4.5 to 20 bgs. Soil samples collected at depths of 6.5 to 12 feet bgs contained detectable total petroleum hydrocarbons quantified as crude oil(TPHc)at concentrations ranging between 880 milligrams per kilogram (mg/kg) and 29,000 mg/kg. The highest concentration of TPHc (29,000 mg/kg) was collected from soil boring GP-156 at a depth of 12 feet bgs. During this investigation, groundwater was encountered beneath the site at depths ranging from 7 to 17 ' feet bgs. Several grab groundwater samples were collected. TPH quantified as diesel(TPHd)were detected at concentrations ranging between 2,200 micrograms/liter(µg/L) and 260,000 µg/L. TPH quantified as gasoline(TPHg)were also detected in grab groundwater samples at concentrations ranging from 8,500 to 9,900 pg/L. The highest concentration of dissolved-petroleum constituents was detected ' from boring GP-161. Previous and proposed boring locations are shown on Figure 2. OBJECTIVE AND SCOPE OF WORK Based on previous environmental investigations conducted by Condor(2001),the objective of this work plan will focus on previously identified areas of petroleum-affected soil and groundwater at Mountain ' House Site 96 (Figure 2). The scope of work outlined below is being proposed to collect data required to fully characterize and define the lateral and vertical extent of residual petroleum identified beneath the subject site. SAIC proposes to advance 30 primary and 12 contingency soil borings to achieve the objectives of this work plan. The proposed soil boring locations for Mountain House Site#6 are presented on Figure 2 and the proposed boring objectives are presented in Table 1.