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Mr.Nuel Henderson,P.G., SJCEHD September 21,2005 ARCO Station 6100,Tracy,California Project No.U6100-15-1 Additional Site Assessment Work Plan Page 2 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 Site Assessment and Remedial Activities 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. 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. SJCEHD 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 Environmental Consultants, Inc. (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-I 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 (free product) have been observed in monitoring well MW-I 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 regular basis. Product thickness has ranged from K:\Bp-Arco for URS\6100\Work P1ans\arco6100-workp1an 09.2005.doc