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1 ARRA V Ac <br /> Mineralized iron (Few} within the aquifer becomes a terminal donor under anaerobic conditions. <br /> Ferric iron reduces to soluble ferrous iron (Fe++). Consequently ferrous iron concentrations <br /> i increase in groundwater containing detectable levels of benzene (Table 2). Chart 5 illustrates the <br /> relationship of benzene to ferrous iron in groundwater within the study area. The illustrated <br /> benzene to ferrous iron trend is consistent with an anaerobic system undergoing biodegradation. <br /> Sulfate availability, as with nitrate, tends to decrease with increased benzene concentrations <br /> (Table 2). This is confirmed as illustrated in Chart 6. Lower concentrations of sulfate were <br /> reported in wells containing detectable concentrations of benzene. <br /> Based upon the consistent results produced by available data, Terra Vac concludes that the <br /> benzene plume is undergoing biodegradation within the study area. Biodegradation rates are <br /> oxygen limited and appear to have reached equilibrium with existing site hydrogeologic <br /> conditions. However, Terra Vac believes that there may still be a benzene sources in the area of <br /> the former UST complex. Benzene plume regression will be observed subsequent to the removal <br /> of source residuals. <br /> h <br />` a t Project 30-0212 <br /> April 17, 1997 <br /> 6 <br />