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1 <br /> 1 <br /> ARCADIS GERAGHTY&MILLER <br /> In groundwater at the Site on February 5, 1999 Upgradient and cross-gradient concentrations ,h- <br /> of sulfate are greater than those found in and downgradient from the plume center The shape ,� V4'-) <br /> of the sulfate contours generally correlate to the shape of the hydrocarbon plumes shown in , ► = <br /> ' Figures 6-1 and 6-2, although more dispersed Sulfate appears to acting as an electron .A <br /> acceptor for the biologically mediated degradation of the Site petroleum hydrocarbons <br /> r- <br /> 6 5 3 5 Methanogenesis <br /> r <br /> During methanogenesis, carbon dioxide (CO2) is used as an electron acceptor, is <br />' reduced by methanogemc bacteria and methane (CI-4) is produced When methanogenesis <br /> occurs, CH4 concentrations increase above background The presence of methane in <br /> groundwater is indicative of strongly reducing conditions Methane is not present in fuel, so its <br /> presence in groundwater above background at a petroleum hydrocarbon groundwater plume is <br /> strongly indicative of microbial degradation of fuel hydrocarbons Concentrations of CHS at <br /> the Site are summarized in Table 4 of Appendix D Dissolved methane was detected in <br />' samples collected from monitoring wells U-1, U-2, U-3, U-8, U-9, and well nests NP-1 and <br /> NP-2 All other samples were nondetect for dissolved methane The methane data indicates <br /> that strong reducing conditions exist in the vicinity of U-3 and at the 60-65 foot depth in well <br />' nests NP-1 and NP-2 and therefore that rrucrobial biodegradation of the plume is occurring <br />' 46 <br />