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eeologfea(hcklucsinc Page 5 Groundwater Monitoring Report Project No 724 2 August 11, 2003 • Intennediate wells MW-106 and MW-107 contained trace BTEX concentrations and low TPH-G and MTBE concentrations ' • TPH-G was non-detected in water table wells MW-4, MW-5, MW-6 or MW-7 « A contour diagram illustrating the extent of the TPH-G plume in the shallow aquifer was not ptepared for this season ' • TPH-G was detected in intermediate wells MW-2, MW-3, MW-106 and MW-107 Figure 5 — Intermediate Well TPH-G Concentrations - illustrates the extent of the TPH- ' G plume in the intermediate aquifer Benzene was non-detected in water table wells MW-4, MW-5, MW-6 or MW-7 so a contour ' diagram illustrating the extent of the benzene plume in the shallow aquifer was not prepared MTBE has been present in well MW-7 (5 2 ug/1), which continues to be the only water table well that contains MTBE - see Figure 7 — Water Table Well MTBE Concentrations Figure 6 -- Intennedtate Well Benzene Concentrations & Figure 8 — Intermediate Well MTBE Concentrations suggest that a significant plume does not exist below the water table The presence of TPH-G contamination in well MW-106 and MW-107 suggests that a submerged plume is being sourced by residual soil contamination beneath the present water table It is noted that the screened intervals of these wells (55-60 feet bgs) are placed near the elevation of the groundwater table in 1992 — approximately 54 feet bgs As indicated in Figure 9 — MW-2 TPH-G vs GW Elevation, with the exception of June 20, 1997, contaminant concentrations have generally mimicked the pattern of groundwater elevation changes for the past 10 years in down gradient well MW-2 The June 20, 1997 test result appears to be an anomalous data point or a point at which the rising groundwater table encountered residual soil contamination Oxidation/Reduction Potential (ORP) measurements (Table 5) show that eight of ten wells have negative values The negative ORP values obseived in the wells show that they are ' either within the core of the contamination plume or aie in the reaction/depletion shadow down gradient of the plume Well MW-4's ORP values fluctuate between positive and negative values suggesting that the well is on the up gradient edge of the plume Dissolved ' oxygen values corroborate with the ORP values measure in these wells 3.0 CONCLUSIONS & RECOMMENDATIONS The contamination plume as observed from these wells is stable and does not appear to be ' sourcing a significant off site plume In GTI's opinion, the levels of contamination present do not warrant an engineered clean up 1