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! concentration trends, and general hydrogeologic conditions, another similarity can be drawn <br /> • between the data generated at the Shell and ExxonMobil sites in support of natural <br />! attenuation <br />! SHELL SERVICE STATION: <br /> The highest historical soil concentrations and highest historical groundwater concentrations <br /> of hydrocarbons detected at or downgradient of the Shell service station are presented below <br /> IConstituent Cone Units Sample Location/ Date Notes <br /> Name De th <br />' TPH-g 4,300 mg/kg T2-S UST 06/05/90 Soil <br /> Excavation Historical <br /> Hi h <br /> Benzene 12 mg/kg MW8 28 5 feet 02/23/88 Soil <br />' Histoncal <br /> High <br /> TPH-g 120,000 µg/L MW8 GW Sample 04/11/88 GW <br /> Historical <br />' <br /> High <br /> Benzene 1,100 pg/L MW8 GW Sample 02/23/93 GW <br /> Historical <br />' Hi h <br /> EXXONMOBIL SITE: <br />' The highest historical soil concentrations and highest concentrations of hydrocarbons <br /> detected in groundwater during 2002 at the Exxon Mobil site are presented below <br />' Constituent Cone Units Sample Location/ Date Notes L <br /> Name Depth <br /> PH-g 12,000 mg/kg V4 24 feet 02/18/91 Soil Historical <br />' <br /> High <br /> Benzene 170 mg/kg V4 24 feet 02/1§8/91-, Soil Historical l%g� <br /> High <br /> PH- --122,000 L UVB-MWI GW Sample 12/18/02 GW Hi h2002 <br />' enzene 11,600 B <br /> L UV -MWI GW Sample 12/18/02 GW Hi h 2002 <br /> E] <br /> Though LPH has not been identified on the Shell site, a comparison ese two data sets <br />' indicates (1) that the magnitude of highest concentrations of TPH-g and benzene detected in <br /> soil at the two sites are within an order of magnitude of each other, and (2) that the current <br /> concentrations of hydrocarbons in groundwater at the ExxonMobil site approximate the <br />' concentrations of hydrocarbons detected at the Shell site in 1988 and 1993 As the <br /> historically high concentrations of hydrocarbons in groundwater at the Shell site have <br /> naturally attenuated to levels acceptable for site closure, natural attenuation is a viable <br />' remedial option for the ExxonMobil site <br /> Although natural attenuation has been shown to reduce the mass and toxicity of petroleum <br />' hydrocarbons and limit the extent of migration in groundwater, the time required to <br /> accomplish the remedial goals and objectives for this project would require monitoring for <br /> changes in geochemical parameters as well as hydrocarbon concentrations The feasibility of <br />' this alternative is, in part, dependent on determining if the mass degradation rate is sufficient <br />' G lNDjcctSN73942UVIA5TER\WP\VM3031WPtx dm 18 <br /> i <br />