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Gealogual Technics tmc Page 5 ' V Quarter 2002 Groundwater Monitoring Report Project No 507 2 March 7,2003 • Deep well MW-109 continues to intennittently contain trace amounts of (at or dust ' above the laboratory reporting limits) BTEX contamination The reduction in contaminant concentrations from the first sampling event is noted, however sufficient monitoring has been performed to conclude that it is probably a drilling artifact or the bottom of the groundwater plume 0 Figures 4 and 5 indicate a previously inverse relationship between groundwater elevation and contaminant concentrations in wells MW-9 and MW-5 respectively ' These two parameters have demonstrated parallel slopes for recent events In Figure 4, the inverse trend was consistent for well MW-9 for the first four monitoring events, but since the March 2001 event the relationship has been predominantly parallel In Figure r5, the inverse trend was constant for MW-5 for the first five monitoring events, but since the January 2001 event the relationship has been parallel Trend lines added to the data in Figures 4 & 5 show that as the groundwater elevation has lowered since 1999 that the concentration of TAH-G has increased Suggesting that concentration is directly related to groundwater level ' 0 Figure 6 indicates an inconsistent relationship between groundwater elevation and contaminant concentrations in well MW-6 No data is included for the December event due to the well's inaccessibility o The groundwater gradient is relatively flat and the direction of groundwater flow has varied during the course of the investigation ' C Oxidation-Reduction Potential (ORP) was measured (Table 4) and the results suggest that with the exception of MW-2, all of the wells sampled are either in the core of the contamination plume or are immediately down gradient from the plume and within the ' halo of reaction depleted water caused by biodegradation of the contaminants s Figure 7 is a contour map indicating the TPH-G concentrations on December 20, 2002 The groundwater plume is localized in the vicinity of the former UST excavation The bulls-eye pattern to the TPH-G contour map in Figure 7 shows that the center of the plume has not moved from its source and suggests that contaminant migration may be occurring primarily by diffusion with a slight influence by advection r ' 3.0 CONCLUSIONS AND RECOMMENDATIONS _— Conclusions ' 1 Elevated concentrations of BTEX, TPH-G and MTBE are present in a laterally limited groundwater plume that is centered on MW-9 2 The MTBE and previously monitored 1,2-DCA plumes are insignificant in comparison ' to the BTEX and TPH-G plumes 3 The lateral extent of the contamination plume is defined to the east, northeast and southeast by wells MW-3, MW-2 and MW-1 respectively