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' well boring MW-12 Low to non-detected levels of petroleum hydrocarbon constituent concentrations . were detected in the remaining soil samples collected from well borings MW-9 through MW-12 Monitor ' well locations are illustrated on Figure 2 and data obtained during this assessment is summarized in Table 1 Details of this assessment are presented in the Clearwater Site Investigation and Well Installation Report, dated August 28, 2001 4 3 Previous Site Characterizations—Groundwater Investigations ' In June and July 1995, TPE installed five monitor wells (MW-1 through MW-5) In December 1993, HET installed three groundwater monitor wells, designated as MW-6 through MW-8 In December 1994 ' and January 1995, BC installed one dual air sparge/soil vapor extraction well (AS-1/SVE-1) and two single completion soil vapor extraction wells (SVE-2 and SVE-3) Since 1994, the depth to groundwater has risen beneath the site from 80 to 50 feet bgs, significantly above the screened interval of wells MW-1 ' through MW-8 Therefore, in an effort to replace MW-1 through MW-8, Clearwater installed four groundwater monitor wells with shallower screened intervals, designated as MW-9 through MW-12 Monitor well construction details are presented in Table 2, Monitor Well Construction Details ATC ' Associates Inc (ATC) performed the most recent groundwater gauging and sampling event on November 6, 2001 During this gauging event the depth to groundwater beneath the site ranged from 55 01 (MW-6) ' to 55 80 (MW-7) feet bgs During the most recent groundwater monitor event a hydraulic gradient of approximately 0 007 vertical feet to horizontal foot (ft/ft) trending towards the north-northeast was • calculated Groundwater conditions are illustrated in Figure 3, Groundwater Summary Map - (ATC, ` November 7, 2001) Analytical results for past and current groundwater sampling events are summarized in Table 3,Historical Summary of Groundwater Data ' 4 4 Previous Feasibility Studies ' In March 1995, BC conducted vapor extraction and air sparge pilot studies at the site Soil vapors were extracted from SVE-2 at flow rates ranging from 20 to 40 standard cubic feet per minute (scfm) at induced vacuums ranging from 64 to 110 inches of water Soil vapors were extracted from SVE-1 at flow rates ranging from 38 to 40 standard cubic feet per minute (scfm) at induced vacuums ranging from 108 to 116 inches of water Air was injected into AS-1 at a flow rate of 8 scfm under pressures ranging from 26 to 38 ' pounds per square inch (psi) Based on the results of the vapor extraction/air sparge pilot studies, it was estimated that each vapor extraction/air sparge well would have an approximate radius of influence of 40 feet Details of this assessment are presented in the BC Corrective Action Plan, dated May 1995 4.5 Site and Area Lithology According to BC, the subject site is located in the San Joaquin Valley which is a north-south trending basin bounded by the Sierra Nevada Mountain Range to the east and the Coast Ranges to the West ' Within the Stockton area the valley floor consists of Quaternary age alluvial sediments deposited by the ' EZ 100966SAFSWP ATC Associates Inc S 1EZ Serve1EZ 1009661Reports1100966SAFSWP ATC Project No 43 25827 0026 ' 4