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16 April 1997 AGE -NC Project No 95-0121 Page 2 of 9 present in soil samples • The tank area was re -excavated in November 1988 Soil was removed from the former UST area to a depth of approximately 19 feet below site grade (bsg) Soil samples were collected from the excavation and screened for organic vapors using a photo -ionization detector (PID) Organic vapor readings indicated that petroleum hydrocarbons were present at depths in excess of 19 feet bsg As a result, the excavation was backfilled and a preliminary site assessment initiated • Between May 1989 and December 1994, a total of nineteen soil borings were advanced at the site, nine of the soil borings were completed as two-inch diameter monitoring wells (MW -1 through MW -9) Extensive petroleum hydrocarbon -impacted soil and ground water, including free product, was encountered during assessment of the site • In July 1995, one dual-purpose soil vapor extraction/ground water extraction well (EW -1), and two soil vapor extraction wells (VW -1, VW -2) were installed at the site In August 1995, an 8 -hour soil vapor extraction (SVE) pilot test was conducted at the site The pilot test was performed to provide vapor extraction data for proper system design of a remediation system • In November 1995, a Corrective Action Plan (CAP) was submitted to the PHS-EHD, and included details of the SVE pilot test Recommendations in the CAP included installation of an SVE system to remediate hydrocarbon -impacted soil and an in-situ air sparging (IAS) system to remediate hydrocarbon -impacted ground water • Between February 1991 and February 1997, eleven ground water monitoring/sampling events were perfonned at the site During this time, a trend in hydrocarbon concentrations suggested increasing concentrations from the "core" area wells, and decreasing concentrations from the perimeter wells 23 REGIONAL GEOLOGIC SETTING The subject site is located in the eastern portion of the Great Valley geomorphic province of California The valley is a nearly flat, elongate trough trending northwest and southeast for approximately 450 miles The Valley is enclosed by the igneous Sierra Nevada to the east and the sedimentary and metamorphic Coast Ranges to the west The surficial and upper several hundred feet of subsurface layers consists of a great thickness of predominantly unconsolidated alluvial and floodplain deposits (dominantly sands, silts, and clays) of Quaternary age, which are derived from the predominantly granitic mountains of the Sierra Nevada Beneath the upper sedimentary deposits lie a thick sequence of marine deposits of Mesozoic age These units are further underlain by a pre - Jurassic complex of igneous and metamorphic basement rock