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i Biodegradation - Soil and Groundwater Biodegradation refers to an in-situ remedial process that uses natural attenuation to reduce or degrade organic constituents of concern(COCl) over time The process uses natural degradation to reduce the concentrations of contaminants to below regulatory standards Biodegradation as a remedial option is usually coupled with institutional controls and long-term monitoring The principal COCl at the site are fuel-related constituents (e g ,benzene, toluene, ethylbenzene, xylenes, and MTBE) BTEX and MTBE are readily degradable in aerobic environments, and intrinsic bioremediation is frequently approved as part of remedial programs for fuel spill sites The dissolved-phase hydrocarbon concentrations within groundwater have exhibited a significant decrease in all wells, except MW-13, since groundwater monitoring and sampling was initiated in 1993 Natural attenuation appears to be occurring at the site, and would probably be part of the mass removal mechanism even if an active remediation system were installed To implement this approach would require a demonstration showing that the rate of degradation offsets the potential for off-site migration Also, the risk for potential exposure to contaminants must either be controlled or demonstrated to be within acceptable limits Soil samples collected from site borings were previously submitted for physicochemical and microbiological analyses The analyses revealed very low viable recoveries for heterotrophic and selective microorganisms at the dilutions tested The results suggested the possibility that some level of biotoxicity may exist at the site hampering biodegradation IBiodegradation of the groundwater may be an active process at this site However, soil analyses indicate that current subsurface soil conditions are not conducive to biodegradation The use of biodegradation alone would not be a viable option for this site Physical Containment Physical containment usually involves the construction of a slurry or cut-off wall across the flow path of impacted groundwater Cut-off wails can be used in conjunction with a treatment filter to create a funnel and gate system, in which the cutoff walls (the fiznnel) channel the CDCs to the high conductivity treatment filter(the gate) The filter can be a zone of enhanced bioactivity or other treatment system such as in-situ air stripping Physical containment of the dissolved-phase hydrocarbon plume does not appear to be warranted at this time Groundwater monitoring data at the site, which began in June 1993, indicates that the hydrocarbon plume is generally stable Chemical Neutralization Chemical neutralization is commonly used for acidic or caustic wastes There are currently no feasible chemical neutralization technologies for fuel-related organic compounds R U�'0 I U �u �cEs`