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CLEARWATER ' c R o u P Environmental Services 2.3 Groundwater Elevations and Gradient ' On September 30, 2002, the static water levels ranged from 10 75 feet below the top of casing (MW-2) to 12 08 feet below the top of casing (MW-8) The groundwater flow beneath the site ' during this event was calculated to be generally southeast at a gradient of approximately 0 0025 feet/foot(Figure 5) Groundwater elevation data are summarized on Table 3 ' Groundwater levels have fallen approximately 1 5 feet since the last monitoring event in May, 2002 The general groundwater flow direction and gradient are similar to those observed in ' previous quarterly events Groundwater appears to be at or near its seasonal low As previously reported, the core of the dissolved-phase plume has migrated in the direction of groundwater ' flow to the southeast, and now the core of the plume appears to reside in the area of MW-5 where both TPHg and benzene concentrations are the highest ' 2.4 Groundwater Sample Analytical Results ' Results of samples collected during this monitoring event confirm that the extent of the dissolved-phase plume has been substantially defined Concentrations of dissolved-phase hydrocarbon contaminants were detected primarily in samples from wells MW-1 ( immediately fdown-gradient of the former UST area) and MW-5B, MW-6, MW-7 and MW-8 (all down- gradient wells) The sample from MW-5B again contained the maximum TPHg (15,000 µg/L) ' and benzene concentrations (1,300 gg/L) hi general, dissolved-phase contaminant concentrations this quarter were as anticipated, and appear to follow seasonal trends over the ' project history, which is consistent with the decreasing dissolved-phase contaminant concentration trend observed in up-gradient and perimeter wells and the increasing contaminant concentration trend in down-gradient wells since initiation of monitoring T�" Pk Nt 15 MI '�1 7 1 Samples collected from wells MW-2 and MW-3 and MW-4 were essentially free of detectable ' contaminant concentrations, except for 140 pg/L of TPHg in well MW-3 While MTBE has been detected at the site by EPA Method 8020M, it has never been confirmed by EPA Method ' 8260 in the same sample (Table 3) 1t should be noted that MTBE has only been detected at the site once during the history of groundwater monitoring at the site, in MW-1 February, 2002 at a ' concentration of 1 1 pg/L (Table 3) Therefore, MTBE is not a contaminant of concern at the . site, and is discussed in greater detail below ' ZS 178C CAP 5 November 20,2002