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GeologKa,Teclutacs 144 Page 6 1 1St Quarter 2005 Groundwater Monitoring Report Project No 507 2 April 18,2005 1 ' 3.0 CONCLUSIONS AND RECOMMENDATIONS Conclusions ' 1 Elevated concentrations of BTEX, TPH-G and MTBE are present in a laterally lirruted (less then 100 ft radius) 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 TPH-G plume is defined to the southeast, northeast and east by wells MW-1, MW-2 and MW-3 respectively ' 4 The lateral extent of the TPH-G plume is defined to the southwest and northwest by wells MW-7 and MW-10 respectively 5 The BTEX/TPH-G groundwater contamination plume is defined vertically by MW-109, ' which is screened 40 feet below MW-9 This suggests that the vertical gradient has not moved the contaminants very deep into the subsurface 6 The groundwater gradient flows to the northeast or east with relatively flat slopes, which ' appear not to significantly influence contaminant migration 7 The groundwater contaminant concentrations have increased in the core of the plume (well MW-9) after a temporary decrease (after the injection pilot test started on March 19, 2004) in the first quarter 2004 8 The center of the plume has not migrated beyond the source area giving evidence that the plume is not migrating laterally or vertically by advective flow 9 The data shows that the plume is stable and is not being degraded by natural biodegradation causes Recommendations • Maintain the modified quarterly monitoring schedule ' • GTI submitted our June 10, 2003 "In-Situ Remediation Pilot Test" work plan for an in- situ hydrogen peroxide infection pilot project The SJC EHD issued approval of the work plan in their September 10, 2003 letter and the work was completed in July 2004 ' A report on the pilot test was submitted to SJC EHD for review They have since commented on the field work and requested that a site conceptual model (SCM) be submitted • GTI proposes advancing several continuous core borings in the core contaminant area There is uncertainty regarding the interbedded nature of the fine grained silts/clays with the coarser grained sand units in the vertical profile of the soil at the site The purpose of ' the borings is to provide detailed understanding of the thinner geologic units at this site GTI developed our March 7, 2005 "Site Characterization Work Plan" that includes continuous core borings to determine the presence of sand/silt layers in the core ' contaminant area The data generated by this effort will then be used to formulate a site conceptual model required by SJC EHD After the site conceptual model issue is resolved GTI will propose further corrective action work