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EAR <br /> C-LWA'T-ER <br /> C. R t7 U 1' <br /> • Envrronmenlc! Services <br /> The groundwater flow direction and gradient as determined by this sampling event is <br /> substantially the same as was determined by previous monitoring events Groundwater elevation <br /> and analytical data are included in Table 1 A groundwater elevation map based on the <br /> groundwater elevation data from all monitoring wells was prepared and is presented as Figure 3 <br /> A revised dissolved-phase hydrocarbon distribution map including iso-concentration contours of <br /> benzene in groundwater is presented as Figure 4 Analytical reports for this groundwater <br /> monitoring event are included as Appendix C <br /> MONITORED NATURAL ATTENUATION TESTING <br /> As suggested in Clearwater's Corrective Action Plan and subsequently approved by the <br /> SJC/SJC/EHD, MNA testing was begun at the site this quarter The testing included field <br /> measurement of dissolved oxygen (DO) and oxidation-reduction potential (Eh), and the <br /> collection and submittal of additional groundwater samples to CytoCulture for bacterial <br /> enumeration, and inorganic chemical and nutrient assay <br /> PuLpose of Monitored Natural Attenuation Testin <br /> The purpose of NINA 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 sorptioni <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 NINA is included with Clearwater's guidelines in Appendix A <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" First-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 /> iiMcAllister,P M and Chiang,C Y , 1994 "A Practical Approach to Evaluating Natural Attenuation of <br /> Contaminants in Ground Water " In Ground Water Monitoring and Remediation, Sprang 1994 <br /> ZB178C/IQ03 Monitoring Rpt 5 April 23,2003 <br />