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4 e044241 Teckws&C ' 2nd Quarter 2005 Groundwater Monitoring Report Page 6 . Project No 507 2 June 30,2005 ' 3.v CONCLUSIONS AND RECOMMENDATIONS ' Conclusions 1 Elevated concentrations of BTEX, TPH-G and MTBE are present in a laterally limited ' (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 infection 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 Lower the sampling frequency of historically low concentration and non-detect wells to a semi-annual basis to achieve cost ' savings (MW-1, MW-2, MW-3, MW-7, MW-10 and MW-109) • GTI submitted our June 10, 2003 "In-Situ Remediation Pilot Test' work plan for an m- 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 SIC 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 GTT 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 SIC PHS/EHD issued an April 15, 2005 correspondence that requested additional information and denied approval of the work plan We submitted a