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subsurface, and potentially cause temporary increases in the mobility of hydrocarbons Air quality issues during excavation would need to be addressed Additionally, areas of Isuspected residual hydrocarbons beneath Rosemarie Lane and the property north of the site would be left in place • Enhanced Bioremediation. Due to the predominance of low permeability zones under the site, substances used to enhance intrinsic biodegradation such as oxygen and nutrients will be Idifficult to deliver to the appropriate locations 0 Steam Stripping. Although this technology is aggressive for source remediation, it involves an extremely high cost, especially for source areas which are primarily trapped below the water table In addition, the need for removing residual LPH in low permeability soils greatly reduces the efficiency of this technology, and increases the energy demand To infect steam or hot water into a low permeability zone 35-45 feet bgs, multiple temporary injection wells would be required The addition of heat and/or liquids below the water level in areas that contain LPH and residual phase hydrocarbons can cause downgradient mobilization due to changes in pore pressure These changes in conditions alter the saturated zone Soil vapor extraction wells and water/vapor treatment would also be required Thus, both cost and the logistics of implementation make this technology less acceptable under the current conditions 0 Air Sparging and Soil Vapor Extraction (SVE). SVE is only applicable for the vadose zone Although it is a useful technology for vapor control, it cannot be used alone for removal of hydrocarbons below the water table Air sparging in conjunction with SVE was implemented at the site from 1994 to 1996 Two air spargmg wells and three SVE wells were installed at and near the downgradient edge of the former tank field area As described 1n Section 2 7 and shown in Figure 10, SVE influent concentrations and the cumulative estimated pounds of TPH-g removed became asymptotic over time The water table has increased by approximately 4 feet since 1996 Because the saturated zone is heterogeneous and largely silty, infected air is not uniformly distributed around the air Sparging well and instead flows in preferential channels in the saturated zone, making hydrocarbon removal from fine-grained materials and hydrocarbon vapor recovery difficult Based on current site conditions and former operational experience, further use of this alternative is not expected to be effective • Groundwater Extraction with In-well Air Stripping (UVB). Groundwater recovery well RW3 was converted to a UVB well in 1996 A UVB system was operated in this well from February 1997 to September 2000 Extracted groundwater was pumped from the lower screened interval of the well, which is screened from 35 5 and 39 5 feet bgs The upper screened interval where the lifted and VOC-stripped groundwater was discharged from is screened from 24 8 and 32 feet bgs UVB systems have the same inherent limitations of a groundwater pump-and-treat system, which are the lack of solubility of hydrocarbons in groundwater and partitioning of hydrocarbons between groundwater and soil Heterogeneities within hydrocarbon-impacted soils result in preferential groundwater • pathways through more permeable soils, which typically contain less hydrocarbon mass than finer-grained soil The concentrations of TPH-g and benzene over time in well UVB-MW 1 ' G1Projects1739421MA5TER\WP\%W3031WPtxdoc 15