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it <br /> • Ms Lori Duncan Project 20805-140 010 <br /> May 13, 1999 <br /> Page 3 <br /> In May 1994, RESNA submitted a problem assessment report (RESNA, May 11, 1994, <br /> Report No 30004-8) to the San Joaquin County Public Health Services (SJCPHS) and <br /> the California Regional Water Quality Control Board (CRWQCB) <br /> The February 1998 UST removal and confirmation soil sample results were presented in <br /> the Pinnacle April 30, 1998 UST Removal Report Soils beneath the former UST <br /> complex contained residual petroleum hydrocarbons Highest detected TPHG <br /> concentrations exceeded 1,000 mg/kg Petroleum hydrocarbon impact to soils beneath the <br /> former piping trenches and dispensers were detected in five of 21 samples analyzed <br /> TPHG concentrations did not exceed 12 mg/kg and benzene concentrations ranged from <br /> 0 0054 to 0 03 mg/kg <br /> GEOLOGY AND HYDROGEOLOGY <br /> The site is underlain by the Modesto Formation, which is composed of alluvial sediments <br /> that range in size from silt to gravel (Marchand and Allwardt,1977) The Modesto <br /> Formation dips gently to the west(Wagner, et al , 1981) The site is located in the Eastern <br /> San Joaquin County Ground Water Basin (California Department of Water Resources, <br /> 1980) Ground water levels have risen from approximately fifty feet below ground <br /> surface in 1992 to current levels of approximately thirty five feet below ground surface <br /> The ground water basin has been developed for intensive irrigation, domestic, industrial, <br /> municipal, and stock use <br /> Sediments underlying the site consist primarily of silt with some clay and some silt with <br /> fine to medium grained sand The sedimentary units are generally discontinuous From <br /> ground surface to approximately 10 to 20 feet below grade, sediments consist of silts with <br /> some clay with sand and sandy silt lenses From approximately 20 to 28 feet below grade, <br /> sediments consist of silts with some sand in the northwest portion of the site and laterally <br /> grading southeast into a sandy silt Sediments from 28 to 65 feet (total depth explored) <br /> consisted of silt with some clay in the western portion of the site and laterally grade <br /> eastward to a sandy silt This unit also contains sand and silt lenses Ground waLu was <br /> first encountered at approximately thirty-five feet below ground surface The subsurface <br /> conditions are displayed on Figures 3 and 4 <br /> SCOPE OF WORK <br /> Pinnacle supervised installation of three on-site air sparge wells (AS-1, AS-2, and AS-3) <br /> on October 26 and October 27, 1998 In addition, one dual completion vapor extraction <br /> well (VW-5S and VW-5D) was installed on October 28, 1998 Woodward Drilling, a <br /> CP 1B 1ARCO\05481REPORTS10548P201 DOC Mlsg 1 <br /> A-3 <br />