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a f 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 3 a sheen to 1.89 feet. Approximately 201 gallons of free product and groundwater mixture have been removed from well MW-1 as of June 21, 2005. Depth to water in well MW-1 has been measured at depths ranging from approximately 104.5 to 106 feet bgs. Free product has also been observed in wells MW-15, VW-1, and VW-4. Delta conducted an additional on-site site assessment in August and September 2001. Delta oversaw the advancement of six soil borings (B-33 through B-38)to depths ranging from approximately 135 to 140 feet bgs, and the installation of eight wells (MW-2 through MW-9). A report documenting the findings of the investigation was submitted on February 28, 2002. The report concluded that the petroleum hydrocarbon impact to soil decreased significantly below approximately 65 feet on-site, and that the vertical extent of petroleum hydrocarbon impact to soil was adequately assessed on-site. Stratus directed the completion of an off-site subsurface investigation between February and June 2004. The investigation consisted of advancing four exploratory soil borings (B-39 through B-42), and completion of five monitoring wells (MW-10, and MW-12 through MW-15). Well VW-15 was also destroyed at this time. Results of the investigation were presented in a document titled Subsurface Assessment Report and Site Conceptual Model Update (September 3, 2004). Given the findings of this investigation, the lateral extent of petroleum hydrocarbon impact to soil and groundwater is not fully characterized south-southeast of the site. Information regarding historical soil borings completed at the site, and details regarding the well installations, are summarized in Table 1 and Table 2. Location of all soil borings and monitoring wells are included on Figure 2. ARCO upgraded the product lines and product dispensers in January and February 2002. Approximately 315 tons of petroleum hydrocarbon impacted soil was excavated and removed from the site at this time. Lateral conveyance piping for future use in soil vapor extraction was connected to wells VW-1, VW-3, VW-4, VW-6, VW-7, VW-8, VW-11 through VW-14, VW-16, and MW-2 through MW-9 at the time of the site renovation. The conveyance piping was stubbed up at the northwest corner of the property to be manifolded for connection to a soil vapor extraction (SVE) system at a later date, if warranted. Continuous SVE was completed at the site between March and November 2004, using two 200 cubic feet per minute (cfm) King, Buck & Associates, Inc. electric catalytic oxidizers equipped with 7.5 horsepower blowers. An estimated mass of 9,625 pounds of TPHG was removed from the subsurface using the electric catalytic oxidizers. The SVE system was upgraded in May 2005, with a Solleco 500 cfm thermal catalytic oxidizer. This Solleco SVE system is currently extracting soil vapors from wells VW-1, VW-3, VW-4, VW-8, VW-14, MW-2, MW-8, and MW-9. As of June 14, 2005, an estimated KABp-Arco for URS\6100\Work P1=s\arco6100-workp1an 09-2005.doc