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47wfog4ut Teclrxlcs titc Page 6 4'h Quarter 2004 Groundwater Morutonng Report Project No 507 2 February 22,2005 3.0 CONCLUSIONS AND RECOMAUNDATIONS ' 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 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 appears not to significantly influence contaminant nugration ' 7 The groundwater contaminant concentrations have increased in the core 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 iTugrating 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 A work plan for "In Situ Remediation Feasibility Study" dated September 20, 2002 and an addendum to the plan were submitted to San Joaquin County Environmental Health Department (SJC EHD) SJC EHD approved the study in their March 20, 2003 letter and requested a detailed work plan for evaluation GTI submitted our June 10, 2003 "In-Situ ' Remediation Pilot Test" work plan for an in-situ hydrogen peroxide infection pilot project and SJC EHD issued approval of the work plan in their September 10, 2003 letter The installation of three, 3/4" diameter monitoring wells was completed on ' February 5, 2004 and the injections began in mid March 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 ' 0 Since there is uncertainty regarding the intrerbedded nature of the fine grained silts/clays with the coarser grained sand units in a vertical profile of the soil at the site due to the 5- foot sampling interval, we propose to perform several continuous core boreholes The ' purpose of this work is to provide detailed understanding of the thinner geologic units at this site The data generated by this effort will then be used to formulate a site conceptual model required by SJC EHD