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Complete conversion of benzene to carbon dioxide and water requires about 3.1 gg/L oxygen for each gg/L of hydrocarbon degraded. As a result, a decrease in dissolved oxygen (DO) and an increase in carbon dioxide levels within a hydrocarbon plume are indicators that aerobic microorganisms are present and degrading the petroleum hydrocarbons(Buscheck and others, 1993; Borden and others, 1995). Following the depletion of oxygen, some microorganisms can utilize other compounds as electron acceptors. These include nitrate, ferric iron, sulfate, and carbon dioxide (in order of decreasing free energy exchange). The oxidation-reduction reactions for these electron acceptors under anaerobic conditions are shown below: Nitrate(Denitrification): C6H6+ 7.2 NO3+7.2 H+—+ 7 CO2(g)+3.6 N2+7.6 H2O Ferric Iron(Iron Reduction): C6116+36 Fe(OH)3+72 H+—> 7 CO2(g)+36 Fe+2+94 H2O Sulfate(Sulfate Reduction): C6116+4.5 SO4 2+3112O—>2.25 H2S+2.25 HS-+ 7 HCO3"+0.25 H+ Carbon Dioxide(Methanogenesis): C6116+CO2(g)+26W—> 7 C144(g)+2 H2O Therefore, an increase in biodegradation byproducts (i.e., ferrous iron, carbon dioxide and/or methane), and a decrease in biodegradation electron acceptors (i.e., dissolved oxygen, nitrate and/or sulfate) within a contaminant plume are indicators that biodegradation of petroleum hydrocarbon compounds is occurring (Buscheck and others, 1993; Borden and others, 1995). Total alkalinity and redox potential (Eh) of the groundwater can also be used as indicators of anaerobic biodegradation (Buscheck and others, 1993; Buscheck and O'Reilly, 1995). The total alkalinity of a groundwater system is indicative of its ability to neutralize acid. It results from the presence of hydroxides, carbonates, and bicarbonates of elements such as calcium, magnesium, sodium, potassium or ammonia. These compounds result from the dissolution of rock(particularly limestone and other carbonate rocks), the transfer of carbon dioxide from the atmosphere, and the respiration of microorganisms. An increase in alkalinity across a dissolved hydrocarbon plume is potentially an indicator of biodegradation. The redox potential of groundwater generally ranges from -400 mV to +800 mV. Under oxidizing conditions, the Eh of groundwater is positive; reducing conditions yield negative Eh readings. Therefore, a decrease in redox potential within a contaminant plume is an indicator that anaerobic biodegradation is occurring. In summary, the following trends observed within a petroleum hydrocarbon plume in groundwater would indicate the potential occurrence of intrinsic bioremediation: Relative Increase In Concentration Relative Decrease In Concentration Carbon Dioxide' Dissolved Oxygen Ferrous Iron' Nitrate Alkalinity Sulfate Redox Potential (Eh) Notes: ' Biodegradation Byproducts, 2 Biodegradation Electron Acceptors `� s:\Environmenta1117861\reports\3gmr2005.doc 5 ATC Associates Inc.