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JCeofggu'a17Tecfurres Inc Page 5 Groundwater Monitonng Report Project No 724 2 March 7,2002 ' • Intermediate wells MW-106 and MW-107 contained low BTEX concentrations and low to moderate TPH-G • Deep well MW-104 did not contain contaminant concentrations above the laboratory reporting limits for the fifth consecutive event ' • TPH-G was not detected in any of the water table wells (MW-4 through MW-7) • 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 * MTBE was detected in intermediate well MW-2, MW-106 and MW-107 utilizing EPA Method 602 The levels detected are consistent with those detected in past events utilizing EPA Method 8260 • MTBE was detected in water table well MW-7 This is the first time MTBE has been ' detected in the well but it has not been confirmed with EPA Method 8260 The shallow groundwater plume is evident in well MW-4 and defined laterally to the east ' (down gradient) by MW-5, MW-6 and MW-7 This situation is illustrated in Figure 6 — Water Table Well Benzene Concentrations Figure 7 — Intermediate 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 been stable in down gradient well MW-2 Oxidation/Reduction Potential (ORP) measurements (Table 5) corroborate the lab analysis and show that only wells MW-5, MW-6 & MW-7 have nonoral ORP The negative ORP values observed in the rest of 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 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 The levels of contamination present do not warrant an engineered clean up GTI recommends performing fate and transport modeling to determine risk to off site l receptors and proceeding with site closure