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CLEARWATER <br /> G R O U P <br /> Enw-onmenta[ Servicss <br />' <br /> included field measurement of dissolved oxygen {DO}, and the collection and submittal of <br />' additional groundwater samples to CytoCulture for bacterial enumeration, and inorganic <br /> chemical and nutrient assay <br /> The purpose of MNA testing was to evaluate the occurrence and rates of intrinsic bioremediation <br />' of residual dissolved-phase hydrocarbons at the site The work was incorporated with the regular <br /> scheduled quarterly groundwater monitoring for contaminant concentrations <br />' 2 3 1 Monitored Natural Attenuation <br /> Natural attenuation of dissolved hydrocarbon plumes is a function of several factors including <br /> Iaerobic 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 /> Ithe greatest potential for ongoing remediation through natural attenuation However, all of the <br /> processes are described briefly to provide a general background The NINA monitoring results <br /> Ifollow A detailed discussion of NINA is included with Clearwater's guidelines in Appendix A <br /> I2 3 2 Empirical Determination of Contaminant First-Order Decal <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 /> i <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 /> Ideclining is referred to here as the "first-order decay rate" First-order decay rates can be <br /> calculated using either historical contaminant concentrations from individual wells or historical <br /> • 1McAllister, PM and Chiang, C Y , 1994 "A Practical Approach to Evaluating Natural Attenuation of <br /> I <br /> ZS 178C RAP 7 May 18,2004 <br /> I <br />