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- - - <br /> pCLEARWATEK <br /> C; It Q EJ I' <br /> Enviranmenta!Services <br /> :........................................................................................................................... <br /> .................................................................. ......... <br /> Method of Metabolism ORP Range in rmlhv0lts4 <br /> Aerobic Degradation +150 to +500 <br /> Denitrification -100 to +750 <br /> :.......................................................................................................................................................... <br /> ......................................... <br /> ........... <br /> ........... <br /> ............................. <br /> ..: <br /> Sulfate Reduction -770 to -220 <br /> i= 7 Iron reduction -470 to +100 <br /> r <br /> :...............................................................................................................................:............................................................................................................................ <br /> : <br /> =} Alkalinity in the groundwater reacts with organic acid byproducts of biodegradation and thus <br /> acts as a buffering agent to maintain pH levels suitable for microbes. This reaction causes <br /> alkalinity to decrease in the presence of biodegradation. With sufficient alkalinity present to <br /> buffer the products of biodegradation, pH values remain constant inside and outside of the <br /> plume. If the groundwater does not contain sufficient alkalinity, the organic acids may build up, <br /> reducing the pH and eventually creating an environment inhospitable to the hydrocarbon- <br /> utilizing microbes. <br /> 2.3.4 Results of Monitored Natural Attenuation Evaluation <br /> The monitored natural attenuation (MNA) study focused on aerobic and anaerobic <br /> biodegradation processes. Positive results for some or all natural attenuation pathways were <br /> anticipated because contaminant concentrations appear to be declining based on determination of <br /> first-order decay rates. The results of this study suggest that both aerobic and anaerobic <br /> biodegradation processes are occurring within the contaminant plume in the upper water bearing <br /> a ' zone. <br /> 2.3.5 Dissolved Oxygen Distribution <br /> There appears to be an areal correlation between lowered DO concentrations and the center of <br /> the dissolved TPHg plume, thus suggesting that aerobic biodegradation processes within the core <br /> of the plume are occurring. Monitoring data suggest that the oxidation-reduction potential in the <br /> center of the plume appears to be at the low end of optimum range to support aerobic <br /> biodegradation. The more-elevated DO readings at the fringe of the TPHg plume suggest that <br /> 4Cookson, J.T.,Jr., 1995. Bioremediation Engineering: Design and Application: (Ed.Nalven, G.). McGraw-Hill, <br /> Inc.,New York,NY, <br /> ZB 178E iSQC System Installation/Start-up. 7 Nov.24,2004 <br />