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' • Monitored Natural Attenuation. As resented in the site conceptual model Section 2 9), • p p { natural attenuation generally describes a range of physical and biological processes, which ' when unaided by deliberate human intervention, reduce the mass and toxicity of petroleum hydrocarbons and limit the extent of their migration in groundwater These in-situ processes ' include biodegradation, dispersion, dilution, sorption, and volatilization, producing the nontoxic end-products of carbon dioxide and water Each of these processes has varying effects on mass/volume reduction, minimizing mobility, and reducing toxicity ' Biodegradation is effective in reducing the hydrocarbon mass by converting hydrocarbons into nontoxic end-products of water and carbon dioxide, but has limited effect on the mobility of the hydrocarbons The process of adsorption does not reduce the hydrocarbon ' mass, but greatly reduces the mobility of hydrocarbons Though quantifying the extent to which each of these various processes are affecting the mass, toxicity, and mobility of hydrocarbons at the site requires additional analysis, empirical site-specific data supports the ' conclusion that natural attenuation is reducing the concentrations of hydrocarbons in groundwater and reducing the length of the hydrocarbon-impacted groundwater in the downgradient direction (Figure 9) This trend of reducing concentrations of hydrocarbons ' over time is also demonstrated in the hydrographs prepared for wells MW 10, RW 1, RW4, and MW8 (Appendix Q Of particular interest is the hydrograph generated for MW8, which was installed downgradient of the Shell service station As presented in Section 2 6, the USTs were removed from the upgradient Shell service station in June 1990 TPH-g and benzene were detected in soil samples collected from beneath the former Shell USTs at concentrations of 4,300 and 10 mg/kg, respectively ExxonMobil well MW8 was installed in February 1988 upgradient of the ExxonMobil site and downgradient of the Shell service station During the installation of MW8, a soil sample ' collected at 28 5 feet bgs contained 3,200 mg/kg TPH-g and 12 mg/kg benzene Groundwater samples collected from ExxonMobil well MW8, as presented in Table 1, have contained up to 120,000 gg/L TPH-g (April 1988) and 1,100 µg/L benzene (February 1993) ' As presented in the hydrograph for MW8 (Appendix C) and in Table 1, the analytical results of the groundwater sample collected from MW8 in March 2002 were 215 µg/L TPH-g and 0 60 µg/L benzene This reduction in concentrations of hydrocarbons in MWS over time immediately downgradient of the Shell service station is attributable to either natural attenuation or ' migration of the mass associated with these hydrocarbons downgradient A review of the concentrations of wells installed downgradient of MW8 and upgradient of the Exxon former tank field (IWI, MW1, MW1, MW2, MW4, MW7, P1 and P2) does not exhibit the level of ' concentrations detected in MW8 As such, the reduction in concentration of hydrocarbons in MW8 is most likely attributable to natural attenuation Though the analytical results of samples collected from MW8 were not included in the closure documents (Appendix D), the ' SJCEHD considered the environmental investigation of this Shell service station complete and issued a"letter of no further action"regarding the Shell site Based on a review of the boring logs and groundwater data generated during the investigations of the Shell and ExxonMobil sites, the hydrogeologic properties at the two sites are similar In addition to similarities to rising groundwater and declining groundwater IG1Pojects1739421MASTERSWP�V;M3D31WPtxdoc 17