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1 1 1 REQUEST FOR SITE CLOSURE REPORT -7- Former Beacon Station #510 • 3300 Waterloo Road, Stockton, California AMV Project No 19042 04 5.0 ATTAINABLE CLEANUP LEVELS Remedial activities, including excavation and removal of soil containing petroleum hydrocarbon constituents, have removed approximately 286 pounds of TPHg and 2,464 pounds of TPHd from the subsurface beneath the site Remedial actions by excavation methods were limited to the maximum vertical extent of the excavation equipment and presence of aboveground site structures such as the canopy, pump islands, and the station building. Based on the analytical results of soil samples obtained from soil bonngs advanced at the site in September 1994, less than 35 pounds of TPHg and less than 330 pounds of TPHd remain in soil beneath the site Calculations presented in this report indicate that approximately 88 percent of the mass of petroleum hydrocarbon constituents initially present in soil underlying the site has been removed by excavation Because of the isolated and limited extent of petroleum hydrocarbon constituents in soil underlying the site, it is apparent that further expenditures to perform additional soil remediation are not warranted at this site Historical analytical results of ground water samples collected from monitoring wells at the site are compiled in Table 4 Ground water samples have been collected once from monitoring wells MW -i through MW -4 in April 1987 These monitoring wells are currently dry In April 1987, only the ground water sample collected from monitoring well MW -4 contained a detectable concentration of benzene (1.2 µg11). Each ground water sample collected in April 1987 contained detectable concentrations of TPHg (Table 4), the highest TPHg concentration was reported in the sample collected from MW -4 (59 9 µg/1) In April 1993, monitoring wells MW -5, MW -6, and MW -7 were installed These wells were installed at locations near dry monitoring wells MW -1, MW -4, and MW -2, respectively Neither TPHg nor benzene have ever been reported at detectable concentrations in ground water samples collected from monitoring wells MW -5, MW -6, or MW -7 in monitoring events since April 1993 (Table 4) The most recent sampling results (October 1994) continue to indicate that petroleum constituents are not present in ground water samples collected from these monitoring wells 6.0 RATIONALE FOR CLOSURE The California Regional Water Quality Control Boards (Regional Boards), and their representatives, were granted authority to require water quality investigations at any locations (within or beyond the boundaries of California) by the Porter -Cologne Water Quality Control Act (now updated and incorporated in Division 7 (Water Quality) of the California Water Code) Under Section 13307 of that Act, the Regional Boards are authorized to establish policies and • procedures for oversight and supervision of persons performing "investigation of, and cleaning rptl 14 mj BQ011527