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• 5.1 PROPOSED FIELD WORK ' Althoughnatural attenuation has been shown to reduce the mass and toxicity of petroleum Y hydrocarbons and limit the extent of migration in groundwater, the time required to accomplish ' the remedial goals and objectives for this project requires monitoring for changes in geochemical parameters as well as hydrocarbon concentrations To further quantify the feasibility of this ' alternative, field data is required to determine if biodegradation, which reduces mass, is the significant process of the natural attenuation mechanisms, and to assess if the mass degradation rate is sufficient for site conditions ' Intrinsic biodegradation is thought to be the most dominant of the natural attenuation processes reducing hydrocarbon concentrations at the site, which include dispersion, dilution, sorption, and volatilization Typically, evidence of biodegradation, such as the decrease in concentrations of biodegradation reactants or the increase in concentrations of biodegradation products, is strongest within areas of the highest concentration of hydrocarbons in groundwater As presented in Figure 3, wells have been installed upgradient of the site, within the area of hydrocarbon-impacted groundwater, and downgradient This allows for a comparison of natural attenuation indicators both within and outside of hydrocarbon-impacted groundwater Wells with the highest concentrations and widest range of hydrocarbons would be expected to exhibit the strongest evidence of biodegradation Wells with progressively lower hydrocarbon concentrations would be expected to exhibit evidence of biodegradation to a progressively lesser extent These general expected trends can be complicated by (1) other chemical reactions which compete for the same biodegradation reactants and products, and (2) concentrations of biodegradation reactants and products which are below detectable concentrations or have been ' diluted to below detectable concentrations Microorganisms obtain carbon and energy for survival, growth, and reproduction from organic molecules, such as BTEX compounds, through a series of enzyme-catalyzed oxidative reactions The resulting metabolic intermediates can be completely oxidized into carbon dioxide and water The oxidative reactions yield electrons which, through a series of enzyme-catalyzed electron transport steps, produce the energy microbes need for maintenance and growth In order to pass through these energy generating steps, an electron sink or acceptor is required The most thermodynamically favored electron acceptor used in biodegradation of petroleum hydrocarbons is dissolved oxygen (DO) When DO is used as the electron acceptor, the process is called aerobic respiration During aerobic biodegradation, DO levels are reduced as aerobic respiration occurs, and carbon dioxide (CO2) and water are then produced Aerobic biodegradation typically occurs when DO concentrations are >3 mg/L In the absence or near absence of DO, the biodegradation process 1s called anaerobic respiration In general, DO measurements of less than 1 or 2 mg/L suggest that anaerobic conditions may exist Thermodynamically, the energy yield for microorganisms from hydrocarbon metabolism I varies with electron acceptors After dissolved oxygen, which is the most thermodynamically favored electron acceptor, has been depleted, anaerobic respiration uses nitrate, manganese, ferric iron, sulfate, and then carbon dioxide as electron sinks (terminal electron acceptors) G 1Projects\739425MASTE21WP1WP03031WPtx doc 22