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Ifl . S E `lir ® R Former BP Station# 11193 September 22, 2004 Page 5 Interim groundwater batch extraction performed from August 2000 through June 2004 has resulted in the reduction of MtBE within the source area, which is evidenced by decreasing MtBE trends in wells MW-11R, MW-2R, and VW-1 Between August 2000 and August 2002, concentrations of MtBE in downgradient well MW-6 steadily increased suggesting that this interim remedial alternative was 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 resulted in reduced concentrations within the source area, and subsequent reductions in concentrations of MtBE that potentially migrated off-site Given the relatively flat east to northeast hydraulic gradient historically reported beneath the site and 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 appeared to have effectively reduced concentrations of MtBE in the source area, MtBE and a significant amount TBA mass still remained within the source area During interim groundwater batch extraction activities, concentrations of TBA remained at elevated, yet stable levels Since MtBE concentration trends appeared to have become asymptotic as a result of groundwater batch extraction activities and elevated concentrations of TBA were present in source area wells MW-1 R, MW-2R, and VW-1, SECOR proposed an alternate interim remedial action . plan in the Revised Intenm Remedial Action Plan and Work Plan for Ozone Sparge Well Installation and Additional Off-site Assessment dated September 15, 2003 CURRENT REMEDIAL ACTION In order to begin ozone sparge well and system installation activities, interim remediation via groundwater batch extraction was discontinued on June 8, 2004 To prevent the further off-site migration of MtBE and TBA, SECOR supervised the installation of 10 ozone sparge (OS) wells between March 31 and April 2, 2004 and an OS system between April 26 and May 7, 2004 (Figure 1) 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 has the potential to 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 eastern/northeastern property boundary was designed to act as a barrier, inhibiting further migration of dissolved petroleum hydrocarbons, MtBE, and TBA downgradient of the site From March 31 through April 2, 2004, SECOR field staff supervised W C Exp orate nand Wells (WDC) during the installation of OS wells OS-1 through OS-10 The borings were drilled using hollow stem auger drilling equipment The locations of the wells were selected ' based on an assumed 30-foot radius of influence to maximize the remedial coverage in the vicinity of the USTs and along the eastern/northeastern perimeter of the site CPT Report 9-04 doc