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""410, Page 5 Groundwater Monitoring Report Project No 7242 November 17, 2003 ' • Wells MW-4 and MW-6 did not containeum etrolhydrocarbon contaminants above the p Y laboratory reporting limits ' • Deep well MW-104 remains free of detectable contamination Wells MW-2, MW-3 and MW-5 have trace amounts to low concentrations of the BTEX ' and/or TPH-G 0 MW-7 contained low levels of MTBE • Intermediate wells MW-106 and MW-107 contained trace BTEX concentrations and ' low TPH-G and MTBE concentrations • TPH-G was non-detect in water table wells MW-4, MW-5, MW-6 or MW-7. A contour diagrain illustrating the extent of the TPH-G plume in the shallow aquifer was not prepared for this reason • TPH-G was detected in intermediate wells MW-1, 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 piepared MTBE has been present in well MW-7 (5 1 ug/1), which continues to be the only water table well that contains MTBE - see Figure 7 -- Water Table Well MTBE Concentrations ' Figure 6 -- Intermediate Well Benzene Concentrations & Figure S — Intermediate Well MTBE Concentrations suggest that a significant plume does not exist below the water table However, the presence of TPH-G containination in well MW-106 and MW-107 suggests that residual soil contamination beneath the present water table may be the source of the submerged groundwater plume 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 foi the past 10 years in down gradient well MW-2 The June 20, 1997, ' test iesults appear to be an anomalous data point or a point at which the rising groundwater table encountered residual soil contamination ' Oxidation/Reduction Potential (ORP) and dissolved oxygen (DO) measurements (Table 5) show that all ten wells have negative values The negative ORP values observed in the wells show that they are either within the core of the contamination plume or are in the react to n/dep leti on 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 1