SITE INFORMATION AND CORRESPONDENCE

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ARCO Station #0960 • - 2 24 November 2009 225 S. Cherokee Lane • Lodi, San Joaquin County vapor extraction was the preferred soil remediation. The three onsite monitoring wells (MW-1 through MW-3) were not destroyed prior to SJCEHD closure of the Site in 1997, but were retained for future monitoring by ARCO. Case #2: Subsequently ARCO terminated their business at the Site, and in August 2002 the remaining four 10,000-gallon gasoline USTs were removed. Approximately 45 tons of soil was disposed offsite at an unnamed location. Confirmation soil samples obtained from the UST excavation pit contained detectable concentrations of TPHg, TPHd, ethylbenzene, xylenes, and methyl tert-butyl ether (MTBE). The maximum soil concentrations in 2002 were: TPHg, 400 mg/kg; Ethylbenzene, 1.2 mg/kg; Xylenes, 3 mg/kg; and MTBE, 1.4 mg/kg. In October 2002; the maximum groundwater monitoring results (MW-2) were: TPHg, 410 pg/L; benzene, 3.2 pg/L; toluene, 8.5 pg/L; ethylbenzene, 6 pg/L; xylenes, 9.5 pg/L; MTBE, 3.7 pg/L; and 1,2-DCA, 26 pg/L. The file indicates that SJCEHD reopened the case due to 1,2-DCA in November 2003. In March 2004 and November 2004 soil borings B-25 through B-28 were advanced to a maximum depth of 104' below ground surface (bgs). In December 2004, two additional monitoring wells (MW-4 and MW-5) were installed. A Site Closure Report was submitted in February 2006 to the SJCEHD; however, Regional Board staff did not concur to the requested closure, and the case was transferred to the Regional Board on 6 October 2006. On 31 October 2006, Regional Board staff requested an additional investigation to delineate lateral extent of the groundwater plume and a soil vapor intrusion investigation. In January 2007, four monitoring wells (MW-6 through MW-9) and three soil vapor probes (VP-2 through VP-4) were installed and sampled. Maximum groundwater concentrations in October 2008, the last monitoring event, were TPHg, 300 ug/L; MTBE, 11 ug/L; and 1,2-DCA, 1 .1 ug/L. Risk Assessment Investigations to date show the bulk of onsite soil contamination existed from 6' bgs down to 17' bgs, where the consultant determined residual soil contamination poses minimal risk to human health from direct exposure. The May 2007 soil vapor Investigation compared soil vapor sample results from three sample locations onsite with the San Francisco Bay Regional Water Quality Control Board Environmental Screening Levels (ESLs). All soil vapor results were below the commercial ESLs, and the consultant determined there was no risk from vapor intrusion. The consultant determined that residual petroleum hydrocarbons in groundwater did not pose a significant threat to human health or the environment. Land use (commercial) is not expected to change in the foreseeable future. Sensitive Receptor Survey The No Further Action Report (NFAR) identified one active municipal well located 812' east of the site. The nearest surface water body, the Mokelumne River, is approximately 1.2 miles north (upgradient) of the Site. The consultant determined that sensitive receptors are not threatened by residual groundwater contamination. Site Geology and Hydrogeology The NFAR listed Site soils as inter-fingered beds of gravels, sands, silts and clays down to 104' bgs, the total depth investigated. From 2004 to 2008, depth to water varied from