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C7e0l0gtcal Tec(mcs Inc Page 5 Groundwater Monitoring Report Project No 724 2 March 11, 2003 • TPH-G was detected in intennediate wells MW-2, MW-106 and MW-107 Figure 6 — ' Intenriediate Well TPH-G Concentrations - illustrates the extent of the TPH-G plume in the intermediate aquifer The shallow groundwater plume is now evident in well MW-7 and defined laterally to the east (down gradient) by MW-5 and MW-6 This situation is illustrated in Figure 5 (discussed above), and Figure 7 — 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 9 —Water Table Well MTBE Concentrations ' Figure 8 — Intermediate Well Benzene Concentrations & Figure 10 — 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 11 — MW-2 TPH-G vs GW Elevation with the exception ption 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 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 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 ' In their correspondence dated April 24 2002 the San Joaquin Count Environmental p p � q Y ' Health Department (SJC/EHD) directed the submittal of a work plan for additional lateral plume definition GTI prepaied this work plan on June 27, 2002 On September 3, 2002, Mr Nuel Henderson of SJC/EHD issued a letter requesting modifications to the work plan ' On September 25, 2002, GTI submitted a letter modifying the June 27, 2002 work plan The modifications have been approved by SJC/EHD and the work will be scheduled pending cost pre-approval from the USTCFP The goal of the revised plan is to target certain sand horizons for discrete screen monitoring and detennine if preferred paths of 1