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GeologicalTecWcsInc Page 5 ' 2"a Quarter 2002 Groundwater Monitoring Report Project No 507 2 October 15,2002 since the March 2001 event the relationship has been parallel In Figure 5, the inverse trend was constant for MW-5 for the first five monitonng events, but since the January 1 2001 event the relationship has been parallel • 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 varied 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 • Oxidation-Reduction Potential (ORP) was measured (Table 4) and the results suggest ' 7 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 June 20, 2002 The groundwater plume is localized in the vicinity of the former UST excavation 1 3.0 CONCLUSIONS AND RECOMMENDATIONS Conclusions 1 Elevated concentrations of BTEX, TPH-G and MTBE are present in a laterally limited ' groundwater plume 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 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 may be defined by MW-109 with additional monitoring, if the concentrations remain non-detect 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 Recommendations • Maintain the modified quarterly monitoring schedule • Monitor the site wells for indicators of remediation by natural attenuation j • A work plan for "In Situ Remediation Feasibility Study" dated September 20, 2002 has been submitted to the San Joaquin County PHS/EHD for consideration We have talked with Mike Infurna of the County and will provide an addendum to the plan that will address his concerns on in situ remediation ASAP