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L Mr. Harlin Knoll �1 San Joaquin County Environmental Health Division 4PRION 1 May 2001 Page 7 attenuation rates for TPHg in vadose zone soils. If one assumes a conservative 5 percent per year rate for mass reduction, the remaining mass after 4 years of natural degradation would be estimated at 207 pounds or 32 gallons of weathered gasoline. The site data were evaluated against the State Water Resources Control Board (SWRCB) guidance document, Guidelines for Investigation and Cleanup of MTBE and Other Ether- based Oxygenates, dated 27 March 2000. The site's MTBE concentrations (below 50 µg/I in groundwater) and its distance (greater than 2,000 feet) from a drinking water well indicate that a low risk site classification should be assigned. Therefore, BFS believes that V the site assessment objectives have been met and the site should be formally approved as closed. L Following written acceptance of site closure, BFS will close the monitoring wells and restore the surfaces around the well boxes to their previous condition. Rationale for Site Closure r , BFS is requesting site closure based on the following factors: f 0 Future and/or ongoing sources of gasoline and its constituents, specifically L MTBE, including USTs, related piping, and impacted soil in the vicinity of Lthe USTs have been removed or properly closed in place at the site. ❑ Analytical data collected during well installation activities indicate 1 concentrations of TPHg, BTEX, and MTBE in the site soils that would fall into L the state category of low risk. Four wells were drilled and selected soil samples from each boring were collected and analyzed. TPHg, BTEX, and LMTBE were not detected in soil samples from wells MW-2, MW-3, and MW-4. The soil sample analyzed from MW-1 yielded MTBE at a concentration of 99 µg/kg and nondetectable concentrations of TPHg and BTEX. This concentration is below published Preliminary Remediation Goals (PBGs) by the Environmental Protection Agency Region 9. L ❑ Groundwater samples collected during the five rounds of groundwater sampling indicate low impact to groundwater at the site. The cross-gradient L well MW-3 did not contain detectable concentrations of constituents associated with gasoline. In monitoring well MW-1, TPHg and BTEX were not detected and only small concentrations of MTBE were detected during L five quarterly rounds of sampling (5.0, 42.0, 7.7, 4.0, and 8.6 µg/I of MTBE, Lrespectively). Well MW-2 has only trace amounts of MTBE. The MTBE r SAS FS\5TOCKTONATRATRO08.DOC 5/1/01 L