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t URS • Ms. Victoria McCartney Page 3 San Joaquin County Environmental Health Department �. ARCO 5450 December 22,2005 Site Investigation(February 2002 On February 21, 2002, three soil borings (SB-1, S13-2, and SB-3) were completed at the site in response to a letter sent Oby the San Joaquin County Public Health Services (SJCPHS) dated August 31, 2001. The soil borings were advanced to a depth of approximately 20 feet bgs to evaluate the horizontal and vertical extent of hydrocarbons in soil and to evaluate whether groundwater had been impacted. Grab groundwater samples were collected from each of the borings. TPH-g was not detected in any of the samples; however, MtBE was detected in all three samples at concentrations ranging from 3.7 gg/L in SB-2 to 68 gg/L in S13-1. Analytical data for the February 2002 investigation are presented in Attachment D. Additional site assessment was recommended, including the installation of groundwater monitoring wells to verify that groundwater was impacted by petroleum hydrocarbons, and to determine groundwater flow direction. Groundwater Monitoring(August 2002 through May 2005) On August 28, 2002, three groundwater monitoring wells were installed to a depth of approximately 25 feet bgs at the site to monitor MtBE in groundwater. Initial groundwater samples, collected on i September 11, 2002, indicated that TPHg and MtBE were detected in well MW-2 at concentrations of 98 gg/L and 62 gg/L, respectively. MtBE was also reported in the sample from MW-3 at 0.77 l.tg/L. Soil samples taken at the capillary fringe at each boring (approximately 11 feet bgs) were found to be free of fuel-related contaminants. e- Quarterly groundwater monitoring was conducted, beginning in November 2002 and extending to May 2005. Analytical results (Table 3) indicate a general decrease of MtBE concentration in all three wells. No analytes were observed above detection limits in MW-3 or MW-4 since the fourth quarter sampling event of 2004. Groundwater analytical results are presented in Tables 1 and 3. REMEDIATION Approximately 35 cubic yards of soil were removed during over excavation activities of impacted soil in October 1995 from a previous product piping upgrade. Current activities consist of remediation via natural attenuation. CONTAMINANT MASS REMAINING AND DISTRIBUTION Mass estimates for baseline soil and groundwater contamination are based on historic data collected in 1999 and 2002. Present-day mass estimates are based on current groundwater monitoring results (Tables 1 and 3). Figures showing the plume dimensions used for the historic and present-day mass K:1Wprocess\15 BP ARC01054501Soil Closure 051Draft of 5450 Closure Report.doc