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CLEARWATER G R Q U P Envirnnmentul Serwces MONITORED NATURAL ATTENUATION TESTING As suggested in Clearwater's Corrective Action Plan and subsequently approved by the SJC/SJC/EHD, MNA testing was continued at the site this quarter The testing included field measurement of dissolved oxygen (DO) and oxidation-reduction potential (Eh), and the collection and submittal of additional groundwater samples to CytoCulture for bacterial enumeration, and inorganic chemical and nutrient assay Pu ose of Monitored Natural Attenuation Testin The purpose of MNA testing was to evaluate the possible occurrence and rates of intrinsic bioremediation of residual dissolved-phase hydrocarbons at the site The work was incorporated with the regular scheduled quarterly groundwater monitoring for contaminant concentrations Monitored Natural Attenuation Natural attenuation of dissolved hydrocarbon plumes is a function of several factors including aerobic and anaerobic biodegradation, volatilization, dispersion/advection, and sorptionl Although all of these factors contribute to actual or apparent removal of contaminant mass from the dissolved-phase, only biodegradation processes were examined in this study as they, particularly aerobic biodegradation processes, tend to be the most rapid processes, and thus have the greatest potential for ongoing remediation through natural attenuation However, all of the processes are described briefly to provide a general background The MNA monitoring results follow A detailed discussion of MNA is included with Clearwater's guidelines in Appendix B Empirical Determination of Contaminant First-Order Decay Rates If all or some of the aforementioned natural attenuation processes are present within a plume, a reduction of contaminant concentrations and mass is usually observed over time This usually occurs at a site which has experienced source removal and/or some active remediation, so that natural attenuation processes rates actually overtake the rate at which contaminants partition into the dissolved-phase The rate at which contaminants concentrations or mass appears to be declining is referred to here as the "first-order decay rate" First-order decay rates can be calculated using either historical contaminant concentrations from individual wells or historical plume mass calculations, if the plume has been delineated for an extended period First-order decay rates for the contaminants beneath this site were calculated using historical monitoring data for wells MW-1, MW-4, MW-SB, and MW-8 Concentrations of TPHg and benzene for the wells were plotted against time, and an exponential curve was fitted to each plot (Figures 6A, 613, 6C, and 6D) Preliminary first-order decay rates were taken from, the equation for each curve The preliminary first-order decay rates for TPHg and benzene in MW-1 were calculated to be 0 08%/day and 0 220/o/day respectively (Figure 6A) The first-order decay rates for TPHg and benzene concentrations in well MW-4 were calculated 'McAllister,P M and Chiang,C Y, 1994 "A Practical Approach to Evaluating Natural Attenuation of Contaminants in Ground Water" In Ground Water Monitoring and Remediation, Spring 1994 ZB178C/2Q03 Momtormg Rpt 5 September 15,2003