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CLEARWATER C R 0 U P Env"Onmental Services buffer the products of biodegradation, pH values remain constant inside and outside of the plume If the groundwater does not contain sufficient alkalinity, the organic acids may build up, reducing the pH and eventually creating an environment inhospitable to the hydrocarbon- utilizing microbes Results of Monitored Natural Attenuation Evaluation The monitored natural attenuation (NINA) study focused on aerobic and anaerobic biodegradation processes Positive results for some or all natural attenuation pathways were anticipated because contaminant concentrations appear to be declining based on determination of first-order decay rates The results of this study suggest that both aerobic and anaerobic biodegradation processes are occurring within the contaminant plume in the upper water bearing zone Dissolved Oxygen Distribution Dissolved oxygen concentrations ranged from 177 mg/L (MW-2) to 5 30 mg/L (MW-7) in field, down-well measurements, and 3 70 mg/L (MW-4) to 5 90 mg/L(MW--1) in samples tested at the laboratory (these discrepancies are discussed below) The distribution of DO was examined with respect to the distribution of the known TPHg in the upper water bearing zone (Figure 5) There • appears to be an areal correlation between low DO concentrations and the center of the dissolved TPHg plume, thus suggesting that aerobic biodegradation processes within the core of the plume have already gone through an aerobic stage and exhausted DO in this location Monitoring data suggest that the reduction-oxidation potential in the center of the plume appears to be at the low end of optimum range to support aerobic biodegradation The more--elevated DO readings at the fringe of the TPHg plume suggest that aerobic biodegradation processes are likely occurring in the outer limits of the plume The spatial correlation between low DO and elevated contaminant concentrations at the plume core is typical of many sites with older fuel releases (i e >5 years), where intrinsic bioremediation has been confirmed Reduction-Oxidation Potential Distribution Reduction-oxidation potential data ranges from 131 millivolts (MW-1) to 540 millivolts (MW- 3), and has a similar distribution with respect to the contaminant plume as DO This style of Eh distribution suggests that more reducing conditions are present within the center of the plume, corroborating the interpretation that anaerobic biodegradation processes are likely occurring in this area Based on previous monitoring data and published Eh ranges for anaerobic biodegradation processes, it appears that denitrification and iron reduction, as evidenced by depressed DO, nitrate and ortho-Phosphate concentrations and increased concentrations of dissolved—phase ferrous iron within the core (MW-5B) of the plume, may be the key anaerobic pathways at the plume's core Eh values at the site are too great to accommodate sulfate reduction Interpretations regarding anaerobic pathways are further examined using anaerobic electron receptor analytical data below ZB178C/1Q03 Monitoring Rpt 8 April 23,2003