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L (RWQCB,2004) and the laboratory detection limit of 50 ug/L for the last four quarters of groundwater monitoring. Groundwater samples collected from well MW-5 have been reported L below the TPHD WQO of 100 ug/L(RWQCB,2004) and the laboratory detection limit of 50 ug/L for the last four quarters of groundwater monitoring without subjecting the samples to silica gel cleanup. 7.0 Discussion and Summary of Closure Rationale LBased on the information presented here and in previous reports,SHN concludes that results from source removal,site investigations,and groundwater monitoring support the request for closure at the WOS site. In summary,SHN requests that the WOS site be closed based on the following site information: • The USTs have been removed and the soil containing petroleum hydrocarbons is t isolated under a cap. L • The site has been adequately characterized. Soil and groundwater containing petroleum hydrocarbons are well defined,and site geology and hydrogeology have L been characterized through historic boring installation and quarterly monitoring activities. • No sensitive receptors have been impacted. • LNAPL has never been observed in site groundwater monitoring wells • All petroleum hydrocarbon concentrations have been below the laboratory detection limits for groundwater samples collected in all site wells during the last four quarters of groundwater monitoring when samples were subjected to the silica gel i cleanup procedure. L • Trend analyses of historic groundwater analytical results from wells MW-1,MW-2, MW-3,MW-5,and MW-6 indicate that concentrations of TPHD in these wells should reach the WQO of 100 ug/L (RWQCB,2004)within a reasonable time. The trends predict a conservative estimate of time to WQO achievement because only results not subjected to the silica gel cleanup procedure were used when applicable. • Groundwater containing petroleum hydrocarbons does not appear to be migrating, based on the absence of significant petroleum hydrocarbon concentrations observed in downgradient hydropunch groundwater samples and well MW-7. Well MW-7 has only been reported with one detection of petroleum hydrocarbons in groundwater exceeding the laboratory detection limits (47 ug/L TPHD;March 30, 2007). The detection of TPHD detected in well MW-7 on March 30,2007,was below L the TPHD WQO of 100 ug/L(RWQCB,2004). No petroleum hydrocarbons in groundwater have been detected in well MW-7 since the March 30,2007,monitoring Levent. • The remaining petroleum hydrocarbons in soil at the site will not pose a long term threat to groundwater quality as evidenced by the decreasing concentrations of petroleum hydrocarbons detected in site monitoring wells,the vertical separation of identified soil impacts and the highest groundwater elevations recorded in site wells, L —( v \\Eureka\projects\2007\007152-woS\PUBS\rpt\20100324-1stHalf2010GWMR&RFC.doc C'_.ny7 y 15