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CLEARWATER <br /> G R D U P <br /> Environmental Services <br /> Purpose of Monitored Natural Attenuation Testing <br /> The purpose of MNA testing was to evaluate the possible occurrence and rates of intrinsic <br /> bioremediation of residual dissolved-phase hydrocarbons at the site The work was incorporated <br /> with the regular scheduled quarterly groundwater monitoring for contaminant concentrations <br /> Monitored Natural Attenuation <br /> Natural attenuation of dissolved hydrocarbon plumes is a function of several factors including <br /> aerobic and anaerobic biodegradation, volatilization, dispersion/advection, and sorptionl <br /> Although all of these factors contribute to actual or apparent removal of contaminant mass from <br /> the dissolved-phase, only biodegradation processes were examined in this study as they, <br /> particularly aerobic biodegradation processes, tend to be the most rapid processes, and thus have <br /> the greatest potential for ongoing remediation through natural attenuation However, all of the <br /> processes are described briefly to provide a general background The MNA monitoring results <br /> • follow A detailed discussion of MNA is included with Clearwater's guidelines in Appendix B <br /> Empirical Determination of Contaminant First-Order Decay Rates <br /> If all or some of the aforementioned natural attenuation processes are present within a plume, a <br /> reduction of contaminant concentrations and mass is usually observed over time This usually <br /> occurs at a site which has experienced source removal and/or some active remediation, so that <br /> natural attenuation processes rates actually overtake the rate at which contaminants partition into <br /> the dissolved-phase The rate at which contaminants concentrations or mass appears to be <br /> declining is referred to here as the "first-order decay rate" Fust-order decay rates can be <br /> calculated using either historical contaminant concentrations from individual wells or historical <br /> plume mass calculations, if the plume has been delineated for an extended period First-order <br /> decay rates for the contaminants beneath this site were calculated using historical monitoring <br /> data for wells MW-1, MW-4, MW-5B, and MW-8 <br /> I <br /> • iMcAllister,P M and Chiang,C Y , 1994 "A Practical Approach to Evaluating Natural Attenuation of <br /> ZB178C/4Q03 Monitoring Rpt 7 January 29,2004 <br />