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V ARCADIS GERAGHTY&MILLER and conservatively extrapolated from, Site data The method of contour extrapolation is ' consistent with the findings of Rice et al (1995) in which over 270 benzene plumes, in similar geological environments, were examined Based on the conceptual model depicted in Figure ' 6-3, it is calculated that the leading edge of the benzene plume, as defined by the 1 ug/L isoconcentration contour interval, in the mid level zone occurs approximately 230 feet down ' gradient from the source area The leading edge of the similarly defined benzene plume in the deep zone is calculated to occur approximately 185 feet down gradient from the source area 1 6 5 3 Electron Acceptor Utilization Biodegradation of petroleum hydrocarbons by microorganisms ultimately results in the ' production of carbon dioxide, methane, and water During biodegradation, microorganisms transform available nutrients into forms useful for energy and cell reproduction by facilitating the transfer of electrons from donors to acceptors This results in oxidation of the electron ' donor (natural organic material and petroleum hydrocarbons) and reduction of an electron ' acceptor The more important electron acceptors in groundwater include dissolved oxygen, ' nitrate, iron (III), sulfate, and carbon dioxide Biodegradation causes measurable changes in groundwater geochemistry During aerobic respiration, oxygen is reduced to water, and ' dissolved oxygen concentrations decrease Once the dissolved oxygen is depleted, microbes will increasing utilize less energy-efficient alternative electron acceptors nitrate is reduced to 1 NO2',N2O,NO,NW, or N2,iron(III)is reduced to iron(II), sulfate is reduced to H2S, CO2 is reduced by methanogentc bacteria and CHL is produced The following discussions evaluate ' spatial trends m the February 5, 1999 monitoring data to ascertain the presence of microbial biodegradation processes ' 43