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n � Soil and Groundwater Investigation Report ARCO Facility No 6100 Tracy, California • Page 2 Originally one 550 gallon waste oil underground storage tank (UST), one 10,000-gallon gasoline UST and four 6,000 gallon UST's were present at the site The waste oil tank was removed in 1987 From November 1990 to February 1991, the five UST's and associated product delivery lines were removed and replaced with four 10,000-gallon UST's Soil investigations were conducted between 1991 and 1995 Concentrations of total petroleum hydrocarbons as gasoline (TPHG) and benzene, toluene, ethylbenzene, and xylenes (BTEX) were detected in soil from 6 to 115 feet bgs, with the highest concentrations identified at approximately 26 to 45 feet below ground surface (bgs) A total of 31 soil borings, ranging in depth from 26 to 116 feet bgs, were drilled, 20 of the borings were completed as vapor extraction wells Groundwater was not encountered during any of the investigations A water supply well was installed onsite in 1969 in the southwestern corner of the property and currently supplies water to the service station The well was constructed of 8-inch diameter casing to 540 feet bgs, screened from 396 to 416 and from 496 to 536 feet bgs, and gravel packed from 35 to 540 feet bgs (The IT Group, September 10, 1999) Reportedly a sanitary seal was not installed A water sample from this well was collected and analyzed in August 1999 All analyzed constituents were reported below laboratory detection limits Depth to water was recorded in the well at the time of sampling to be approximately 40 feet bgs In October 1998, Pinnacle Environmental Solutions, Inc submitted Hydrocarbon Mass Evaluation, presenting the estimated mass of TPHG and BTEX constituents An addendum to this report was prepared by the IT Group and submitted in September 1999 San Joaquin County PHS/EHD submitted a Case Closure Summary to the California Regional Water Quality Control Board (CRWQCB) on February 1, 2000 requesting "no further action" of environmental work at the site CRWQCB declined the request for no further action (March 14, 2000) and required that a groundwater investigation be conducted at the site On June 5, 2000, Delta submitted a work plan for groundwater assessment on behalf of ARCO The work plan was subsequently approved with comments by CRWQCB on June 9, 2000 SITE INVESTIGATION The purpose of the work was to investigate the possible impact of petroleum hydrocarbons, including fuel oxygenates, to groundwater beneath the site To accomplish this objective, the following field activities were conducted a Advanced one soil boring to 200 feet bgs, • Collected soil samples at designated depths for soil classification and laboratory analysis, and