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Table 1 Potential Nabaal Attenuation Mechanisms Mechanism Description Potential for BTEX Attenuation Bioiogiew Aerobic Microbes utilize oxygen as an electron Most significant attenuation mechanism if acceptor to convert contaminant to sufficient D O is present (D O ?: 1-2 ppm) CO2,water,and biomass Anaerobic Alternative electron acceptors (e g, Rates are typically much slower than for Derutrification NO3, SO:, Fe", CO2 are utilized by aerobic biodegradation,and toluene is the Sulfate Reducing microbes to degrade contaminant-, only component of BTEX shown to Methanogenic consistently degrade Fe Reducing Hypoxic At low D O (01 to 2 ppm), oxygen Has not been demonstrated in the field for and a secondary electron acceptor are BTEX used to degrade contaminants Physical Volatilization Contaminants are removed from Normally minor contribution relative to ground water by volatilization to the biodegradation More significant for shallow vapor phase in the unsaturated zone or highly fluctuating water table Dispersion Mechanical and molecular mixing Decreases concentrations but does not processes reduce concentrations result in a net loss of mass Sorption Contaminants partition between the Sorption retards plume migration but does aqueous please and the soil matrix not permanently remove BTEX from ground Sorption is controlled by the organic water as desorption may occur carbon content of the soil Cheinical Hydrolysis,dehydro- Hydrolysis and/or other chemical Significant for many halogenated halogenation, and other reactions occur in ground water to compounds, but not observed for BTEX chemical reactions reduce the mass of contaminant present tive electron acceptor such as nitrate,carbonate,or iron physical Phenomena (111) must be available In addition, microbes capable Ira addition to biodegradation, physical processes of utilizing the alternative electron acceptor to degrade such as dispersion, sorption, and volatilization result in the contaminants must be present Typically, anaerobic plume attenuation The effects of dispersion and sorp- biodegradation rates are much stover than aerobic bio- tion on contaminant migration dre understood relatively degradation rates and benzene has not been shown to well and are described in Table 1 and the literature degrade consistently under anaerobic conditions (Wil- (gear 1979,Karickhoff 1981) Another possible physical sfln 1993,Barbaro et al 1992) Further study is required mechanism for natural attenuation is volatilization of to aemonstrate that anaerobic biodegradation occurs to contaminants from ground water Volatilization is the a significant extent in the field process by which compounds are transported from the soluble ground water plume, through the capillary HYpoxic Biodegradation fringe, and into the soil gas of the vadose zone To Between aerobic and anaerobic conditions (i-e 0 1 determine the rate of mass loss from volatilization rela- te 2 ppm), there is a region sometimes labeled the tive to the total rate of mass loss, Chung et al (1989) hypoxic zone Studies by Mtkesell et al (1993) in the estimated the mass disappearance ratio of volatilization hypoxic zone have indicated that biodegradation of relative to total mass degradation By applying the anal- BTEX may occur at relatively low dissolved oxygen ysis to a Michigan gas plant facility,where the total rate levels provided a secondary electron acceptor is avail- of mass ions had been determined rigorously by a mass We While the biological pathways for degrading balance approach (see following section),volatilization 8TEX have not been determined under these condi- was calculated to account for no more than 5 percent 401is bacteria capable of uttliz.ng BTEX under hypoxic, of the total mass loss of benzene The actual volatiliza- denitrifying conditions have been isolated from aquifer tion at this site is expected to be less than this method Soils Additional study is required to demonstrate bio- estimates due to the mass transfer limitations imposed degradation under hypoxic conditions in the field. by the capillary fridge At other sites Where the water SPRING 1994 GYVNIR ■ 163 �i