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S E C 0 R 76 Station No. 11193 August 13, 2004 Page 5 interim groundwater batch extraction performed from August 2000 through May 2003 has resulted in the reduction of MtBE within the source area, which is evidenced by decreasing MtBE trends in wells MW-1 R, MW-2R, and VW-1. During this time period, concentrations of MtBE in downgradient well MW-6 steadily increased since the initiation of interim remedial activities until August 2002, which suggests that this interim remedial alternative is limited in its effectiveness in controlling the migration of residual MtBE beneath the site. However, since August 2002, concentrations of MtBE have declined, which may suggest that interim groundwater batch extraction has resulted in reduced concentrations within the source area, and subsequent reductions in concentrations of MtBE that have potentially migrated off-site. Given the relatively flat east to northeast hydraulic gradient historically reported beneath the site, decreased concentrations of MtBE in the source area, interim groundwater batch extraction activities appear to have significantly influenced the extent and magnitude of the dissolved plume beneath and downgradient of the site. Though interim remediation via groundwater batch extraction appears to have effectively reduced concentrations of MtBE in the source area, MtBE and a significant amount TBA mass still remains within the source area. During interim groundwater batch extraction activities, concentrations of TBA have remained at elevated, yet stable levels. Since MtBE concentration trends appear to have become asymptotic as a result of groundwater batch extraction activities and elevated concentrations of TBA are present in source area wells MIA R, MW-21, and 1-1, SECOR proposed an alternate interim remedial action plan in the Revised Interim Remedial Action Plan and Work Plan for Ozone Sparge Well Installation and Additional Off-site Assessment dated September 15, 2003. The completed scope of work outlined in that work plan is presented below. COMPLETED SCOPE OF WORK To further expedite remediation of the dissolved plume beneath the site, SECOR discontinued the interim remediation via groundwater batch extraction on June 8, 2004. To prevent the further off-site migration of MtBE and TBA, SECOR installed 10 ozone sparge (OS) wells between March 31 and April 2, 2004 and an OS system was installed between April 26 and May 7, 2004 (Figure 2). With subsurface soils consisting predominantly of mixtures of silt and sand within the upper portion of the saturated zone (approximately 12 to 14 feet bgs), the introduction of ozone through several OS points will directly destroy dissolved petroleum hydrocarbons, MtBE, and TBA, and. stimulate in-situ aerobic biodegradation of dissolved petroleum hydrocarbons by increasing subsurface oxygen concentrations. Though concentrations may initially increase due to the desorption of petroleum hydrocarbons from soil caused by the aggressive mechanical scrubbing action of the ozone microbubbles, OS is capable of facilitating subsequent rapid degradation of the dissolved plume beneath the site. The installation of OS wells along the easternlnortheastern property boundary will act as a barrier, inhibiting further migration of dissolved petroleum hydrocarbons, MtBE, and TBA downgradient of the site. The off-Site monitoring wells (MW-11 and MWA2) that were proposed in the September 15, 2003 work plan will be installed later this summer and a separate report describing that work will be submitted upon completion. I:1ConocoPhillipslTosco Marketing1111931ReportslOzone Well install Rpt 6-04 recovered.doc