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Work Plan for Additional Site7Cssessment ARCO Facility No. 6100 Tracy,Califomia Page 2 Soil investigations were conducted between 1991 and 1995. Concentrations of TPHG and 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. 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 pack 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 and again in February 2001. Depth to water was recorded in the well at the time of the 1999 sampling to be approximately 40 feet bgs. All petroleum hydrocarbon constituents and oxygenates were reported below laboratory detection limits. 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 fixrther 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. 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. A status report was submitted on April 6, 2001 documenting monitoring activities conducted during the first quarter 2001. In addition to MW-1, selected onsite vapor extraction wells containing rechargeable amounts of water were included in the C:\,kRCO\6100\REPORTS\6100 workplan 05-22-Ol.doc