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feologualTecluucsI/M Page 5 Groundwater Momtonng Report Project No 724 2 May 15,2003 • 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-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 prepared for this reason • 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 The shallow groundwater plume is evident in well MW-4 as trace benzene (0 3 ug/1) and defined laterally to the east (down gradient) by MW-5 and MW-6 This situation is illustrated in Figure 6 — Water Table Well Benzene Concentrations MTBE has been ' present in well MW-7, which continues to be the only water table well that contains MTBE - see Figure 8 —Water Table Well MTBE Concentrations ' Figure 7 — Intermediate Well Benzene Concentrations & Figure 9 — 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 10 — MW-2 TPH-G vs GW Elevation, with the exception of June ' 20, 1997, contaminant concentrations have been stable 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 nine of 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 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