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I CLEARWATER ' c R o u P Environmental Serceces ' products of biodegradation, H values remain constant inside and outside of the plume If the P g > P ' 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 2 3 5 Results of Monitored Natural Attenuation Evaluation ' The monitored natural attenuation (MNA) 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 2 3 6 Dissolved Oxygen Distribution Dissolved oxygen concentrations ranged from 3 7 mg/L (MW-5B) to 4 2 mg/L (MW-8) The distnbution of DO was examined with respect to the distribution of the known TPHg in the upper water bearing zone There appears to be an areal correlation between lowered DO concentrations ' and the center of the dissolved TPHg plume, thus suggesting that aerobic biodegradation processes within the core of the plume are occurring Monitoring data suggest that the oxidation- reduction 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 too The spatial correlation between reduced 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 I Dr Randal von Wedel, biochemist for CytoCulture laboratories in Point Richmond, CA recently compared the historical MNA data for Clearwater and hypothesized to the variability of the MNA data " intrinsic bioremediation appears to be most active in the down-gradient portion ZB178C RAP 11 May 18,2004