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1 - Geotog"t Te hau lac. I Page 5 4'h Quarter 2004 Groundwater Monitoring Report Project No 507 2 February 22,2005 contained MTBE since its installation in May 2001 The source of this contamination may be surface infiltration from unpaved areas and from the open storm water drainage ' ditch traversing the front of the parcel • Water table well MW-10 did not contain contaminant concentrations above the laboratory reporting limits since February 2002 The well has previously contained xylene at levels ' just above the laboratory reporting limit of 0 3 µg/1 • Deep well MW-109 did not contain contaminants above the laboratory reporting limits for the fifth 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 relationslup 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 probably indicates 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) Trend lines added to the data in Figure 5 show that as the groundwater elevation has lowered ' since 1999 that the concentration of TPH-G has also decreased 0 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 (Since MW-6 was not sampled during the 4t' quarter monitoring Figure 6 consists of the historical data) • The groundwater gradient is relatively flat and the direction of groundwater flow has varied during the course of the investigation • Oxidation-Reduction Potential (ORP) and Dissolved Oxygen (DO) were measured (Table ' 4) and the results suggest that all eleven 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 The ORP ' values are slightly more positive for wells MW-7 &MW-10 for this event o Figure 7 is a contour map indicating the TPH-G concentrations on December 29, 2004 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 -