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""["glut rech&a Inc Page 5 3rd Quarter 2002 Groundwater Monitonng Report Project No 507 2 January 17,2003 ' 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 parallel For the September 2002 event, the relationship has returned to an inverse relationship In Figure 5, 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 ' O Figure 6 indicates an inconsistent relationship between groundwater elevation and contaminant concentrations in well MW-6 ' . The groundwater gradient is relatively flat and the direction of groundwater flow has vaned during the course of the investigation The site is quite close to the San Joaquin River and tidal fluctuations might be responsible for the observed changes in ' groundwater direction s 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 M contamination plume or are immediately down gradient from the plume and within the halo of reaction depleted water caused by biodegradation of the contaminants D Figure 7 is a contour map indicating the TPH-G concentrations on September 16, 2002 ' The groundwater plume is localized in the vicinity of the former UST excavation ' 3.0 CONCLUSIONS AND RECOMN MNDATIONS ' 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-I respectively 4 The plume is defined to the northwest and southwest by wells MW-10 and MW-7 respectively 5 The BTEX/TPH-G groundwater contamination plume is intermittently defined by MW- 109 in the vertical plane 6 The groundwater gradients flow to the northeast or east with relatively flat slopes 7 The site exhibits a predominantly downward component of vertical groundwater flow 1