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I 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 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/soll vapor extraction well (AS-1/SVE-1) and two single completion soil vapor extraction wells (SVT--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 ' 20, 2002 During this gauging event the depth to groundwater beneath the site ranged from 57 97 (1\4W-6) to 58 75 (MW-7) feet bgs During the most recent groundwater monitor event a hydraulic gradient of ' approximately 0 003 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 20, 2002) Analytical results for past and current groundwater sampling events are summarized m Table 3,Historical Summary of Groundwater Data 4.4 Previous Pilot Testing and Site Remediation Activities 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 Au was infected 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.fiction 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 San Joaquin River and its tributaries Soil lithology in the Stockton area includes sands, silty sands, slits, clayey silts and clays The site lies at an elevation of approximately 25 feet above mean sea level Sediment deposition in the area has produced nearly horizontal topographic features in the vicinity of the site Soils encountered beneath the site during the previous assessment activities consist of inter-bedded alluvial deposits, including clays, silts, sands, and fine gravels EZ 100966ASAR rpt ATC Associates Inc I\San DiegolEZ Serve11009661Reports1100966SAR001 rpt ATC Project No 43 25827 0026 4