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GeolagtealTcckxteslnc Page 6 Yd Quarter 2004 Groundwater Monitoring Report Protect No 507 2 r November 4,2004 3.0 CONCLUSIONS AND RECOMNWNDATIONS ' Conclusions I 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 �-Not trv�c_ I>C s ku lvd a ,' A,d g6� p 1 3 The lateral extent of the contamination plume is defined to the southeast, northeast and ' east by wells MW-1, MW-2 and MW-3 respectively 4 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 by MW-109 in the vertical plane, which is screened 40 feet below MW-9 This suggests that the vertical ' gradient is not moving the contaminants 6 The groundwater gradient flows to the northeast or east with relatively flat slopes, which appears not to significantly influence contaminant migration ' 7 The groundwater contaminant concentrations have increased in the core plume (well MW-9) after a temporarily decrease (after the injection pilot test started on March 19, 2004) in the last monitoring event 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 8 The data shows that the plume is stable and is not being degraded by natural ' biodegradation causes Recommendations ' ® Maintain the modified quarterly monitoring schedule o 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 (SIC 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 pian for an in-situ hydrogen peroxide injection pilot ' project and SIC EHD issued approval of the work plan in their September 10, 2003 letter The installation of three, 314" diameter monitoring wells was completed on February 5, 2004 and the injections began in mid March 2004 A report on the pilot test ' is under development and will provide recommendations for interim remedial action ' 4.0 Ll MITATIONS ' This report was prepared in accordance with the generally accepted standard of care and practice in effect at the time Services were rendered It should be recognized that definition and evaluation of environmental conditions is an inexact science and that the state or practice