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' Geotoglut re mics/a Page 5 4"'Quarter 2003 Groundwater Mon tormg Report Project No 507 2 Apn1 12,2004 • Deep well MW-109 did not contain contaminants above the laboratory reporting limits for the third consecutive event The reduction in contaminant concentrations from the ' first sampling event on June 1, 2001 is noted and 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, and then it was parallel between March 2001 and December 2002, but since has had a variable relationship • Trend lines added to the data in Figure 4 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 • 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, and then it was parallel ' between March 2001 and December 2002, but since has had a variable relationship • 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 ® Figures 4, 5, & 6 illustrate that the concentrations of TPH-G increases as the water table decreases, suggesting that the plume is stable and not being degraded by natural biodegradation causes It also suggests that the vadose zone soils contain abundant mobile phase petroleum product • The groundwater gradient is relatively flat and the direction of groundwater flow has varied during the course of the investigation • Oxidation-Reduction Potential (ORP) was measured (Table 4) and the results suggest ' that 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 • Figure 7 is a contour map indicating the TPH-G concentrations on December 12, 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 rplume has not moved from its source and suggests that contaminant migration may be occurring primarily by diffusion with a slight influence by advection 1 1