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dal <br /> CLEARWATER <br /> G R O U P <br /> Environmental Services <br /> GROUNDWATER MONITORING AND SAMPLING RESULTS <br /> Groundwater sampling results for gasoline range hydrocarbons are summarized as follows <br /> Depth to groundwater range 1143 (MW-4) to 13 19 (MW-8) feet below <br /> top-of-casing (TOC) <br /> Flow direction and gradient Easterly at approximately 0 006 feet/foot <br /> Benzene concentration range From 15 µg/L (MW-2) to 300 µg/L (MW- <br /> 5B) <br /> Toluene concentration range From 10 µ (MW-2) to 70 (MW-1) <br /> Ethylbenzene concentration range From <0 50 µglf, (MW-7) to 79 µg/L (MW- <br /> 1) <br /> Xylenes concentration range From 16 µg/L (MW-7) to 230 µg/L (MW- <br /> 1) <br /> TPH-g concentration range From <50 µg/L (MW-2, -4) to 3,900 µg/L <br /> (MW-1) <br /> MTBE concentration range Not detected in concentrations exceeding <br /> indicated laboratory method reporting limits <br /> (<0 50 to <5 0 ) <br /> The groundwater flow direction and gradient as determined by this sampling event is <br /> substantially similar to previous monitoring events Groundwater elevation and analytical data <br /> are included in Tables 1 and 2 Well construction data are provided in Table 3 A groundwater <br /> elevation map based on the groundwater elevation data from all monitoring wells was prepared <br /> and is presented as Figure 3 Revised dissolved-phase hydrocarbon distribution maps including <br /> iso-concentration contours of benzene and TPHg in groundwater are presented as Figure 4 and <br /> Figure 5 respectively Analytical reports for this groundwater monitoring event are included as <br /> 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 /> ZB178C/3rdQtr 2004 Monitoring Rpt 6 October 8,2004 <br />