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4 CLEARWATER • G R Q U I' I N C Enviromnenta! Serv.ces GROUNDWATER MONITORING AND SAMPLING RESULTS Groundwater sampling results for gasoline range hydrocarbons are summarized as follows Depth to groundwater range 9 40 (MW-2) to 1132 (MW-8) feet below top-of-casing (TOC) Flow direction and gradient Southeasterly at approximately 0 004 feet/foot Benzene concentration range From not detectable (MW-2, MW-3, MW-4, MW-6) to 3,200 micrograms per liter (µg/L) in MW-5B, and at 17 µ in MW-7 Toluene concentration range From <0 50 µg/L (MW-1, MW-2, -3, -4, -6, -7)to 560 (MW-5B) Ethylbenzene concentration range From <0 50 µg/L (MW-1, 2, -3, -4, -6, -7, &MW-8) to 620 (MW-5B) Xylenes concentration range From <0 50 µg/L (MW-1, -2, -3, -4, & MW- 6) to 1,400 µ (MW-5B) TPH-g concentration range From <50 µg/L (MW-2, -3, -4 & -6) to 21,000 µg/L (MW-5B), and at 1,200 µg/L in MW-7 MTBE concentration range Not detected in concentrations exceeding indicated laboratory method reporting limits (<0 50 µg/L to <5 0 µg/L) The groundwater flow direction and gradient as determined by this sampling event is substantially similar to previous monitoring events Groundwater elevation and analytical data are included in Tables 1 and 2 Well construction data are provided in Table 3 A groundwater elevation map based on the groundwater elevation data from all monitoring wells was prepared and is presented as Figure 3 Revised dissolved-phase hydrocarbon distribution maps including iso-concentration contours of benzene and TPHg in groundwater are presented as Figure 4 and Figure 5 respectively Analytical reports for this groundwater monitoring event are included as Appendix B Empirical Determination of Contaminant First-Order Decal 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 Z$178C/2ndQtr2004 Monitoring Rpt 6 August 9,2004