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Geologcal TC&AWs Inc Page 5 2°d Quarter 2003 Groundwater Monitormg Report Project No 507 2 September 24,2003 ' • Deep well MW-109 has intermittently contained trace amounts of(at or dust above the ' laboratory reporting limits) BTEX contamination The well was free of contamination for the .lune 2003 event The reduction in contaminant concentrations from the first sampling event on June 1, 2001 is noted, however sufficient monitoring has been ' performed to conclude that it was probably a drilling artifact or the bottom of the groundwater plume • Figure 4 illustrates the relationship between groundwater elevation and TPH-G concentrations in well MW-9 (located in the core of the plume) An inverse trend was consistent for the first four monitoring events, but since the March 2001 event the relationship has been predominantly parallel The March 2003 event recorded a return ' to an inverse relationship between the two parameters that continued for the June event • Figure 5 illustrates the relationship between groundwater elevation and TPH-G concentrations in well MW-5 (located up/cross gradient of the core of the plume) An inverse trend was consistent for the first four monitoring events, but since the March 2001 event the relationship has been predominantly 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 TPH-G has increased This suggests that concentration is directly related to groundwater level at the site Mo Figure 6 indicates an inconsistent relationship between groundwater elevation and contaminant concentrations in well MW-6 No data is included for the December 2002 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 ' a 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 Dissolved oxygen concentrations do not routinely corroborate with the ORP values, ' suggesting that there may be a problem with the DO measurements • Figure 7 is a contour map indicating the TPH-G concentrations on June 24, 2003 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 • 1