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Mr.Nuel Henderson,Jr. • • October 15,2002 ARCO Station 6100 Project No.U 6100 Work Plan Page 2 SITE DESCRIPTION The site is currently an active ARCO service station located at the northwestern comer of the intersection of Patterson Pass Road and Interstate 580 near Tracy, California. The facility configuration is shown on Figure 2. The site is approximately 300 feet above mean sea level (msl)with topography sloping towards the southeast. Adjacent land parcels are undeveloped. SITE BACKGROUND Originally, one 550-gallon waste oil underground storage tank (UST), and one 10,000-gallon and four 6,000-gallon gasoline USTs were present at the site. The waste oil tank was removed in 1987. From November 1990 to February 1991, the five gasoline USTs and associated product delivery lines were removed. The five USTs were replaced with four 10,000-gallon gasoline USTs. 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. SJC/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. 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 P:\Bp-Arco for URS\6100\Work Plans\6100 workplan-l.doc