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Subsurface Investigation and Well Installation Report ARCO Facility No 6100 Tracy, California Page 2 Soil investigations were conducted between 1991 and 1995 Concentrations of total petroleum hydrocarbons as gasoline (TPHG), and benzene, toluene, ethylbenzene, and total xylenes (BTEX) were detected in soil from 6 to 115 feet below ground surface (bgs), with the highest concentrations identified at approximately 26 to 45 feet 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 VW-1 through VW-20 Groundwater was not encountered during any of these investigations A water supply well was installed in the southwestern corner of the property in 1969 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 Water samples were collected and analyzed from this well in August 1999, February 2001, and November 2001 All petroleum hydrocarbon constituents and oxygenates were reported below laboratory detection limits Depth to water in the water supply well was approximately 40 feet bgs at the time of the August 1999 sampling event 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 Public Health Services (SJCPHS) 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 The work plan was subsequently approved with comments by CRWQCB in a letter dated June 9, 2000 Delta contracted Water Development Corporation of Woodland, California, to drill and install monitoring well MW-1 in August 2000 The boring was drilled to a depth of 202 feet bgs and the monitoring well was completed to a depth of 115 feet bgs The screen interval is from 95 to 115 feet bgs Results of laboratory analyses on soil samples collected from the boring indicate that petroleum hydrocarbons are predominantly present at depths between approximately 105 to 125 feet bgs Liquid-phase hydrocarbons have been observed in monitoring well MW-1 since September 2000 A free-phase product skimmer was installed in the well to remove the product The skimmer was removed from the well in January 2001, and product has been recovered by manual bailing on a weekly basis Product thickness has ranged from 0 39 to 1 89 feet Depth to water in well MW-1 has consistently been measured at approximately 106 feet bgs i P 1ARCO\61001REPORTS16100-well install report doe