Abiotic attenuation

Hydrolysis and dehydrohalogenation reactions are the most thoroughly studied abiotic attenuation mechanisms. In general, the rates of these reactions are often quite slow within the range of normal grotmd-water temperatures, with half-lives of days to centuries Hydro-... 

Hydrolysis, dehydrohalogenation, hydrogenolysis, and dihaloelimination are the main abiotic attenuation reaction mechanisms. Abiotic reactions are very dependent on the environmental circumstances, and in general, temperature has the most significant t effect... 

Natural attenuation refers to processes that naturally transform contaminants to less harmful forms or hnmobihze contaminants so that they are less of a threat to the environment see Natural Attenuation for Groundwater Remediation by the National Research Council (2000). This includes dispersion/dilution, sorphon, volatihzahon, and degradation (abiotic, biotic). 

Photolysis is another abiotic process that could contribute to the attenuation of nitramines in natural media. Because HMX is more resistant to degradation than RDX [49,52,53], very few reports pertain to HMX photolysis. However, several reports regarding the photodegradation of RDX [48,56-58] suggested that photolysis was accomplished through the initial homolysis of the N-N02 bond. Such... 

Fellows RJ et al., Biotic and Abiotic Transformations of Munitions Materials (TNT, RDX) by Plants and Soils, Potentials for Attenuation and Remediation of Contaminants, International Business Communications Symposium on Phytoremediation [Abstract], May 8-10, Arlington, VA, 1996. 

Natural attenuation (NA) refers to the reduction of contaminant concentrations in environmental media by processes such as dilution, dispersion, sorption, volatilization, and biotic or abiotic transformations. As a passive remediation approach, NA is mostly applied in connection with organic contaminants which migrate from the source in groundwater. Naturally attenuating contaminant plumes show a decline in the dissolved contaminant mass as a function of time, and a decline in contaminant concentrations downgradient from the source. Before potential receptor exposure points are reached, natural attenuation is expected to reduce dissolved contaminant concentrations below regulatory standards (Wiedeme-ier et al. 1999). 

This chapter is focused on the natural attenuation behavior of CS at the field scale. The first part of the chapter reviews many of the physical, chemical and abiotic natural attenuation processes that attenuate CS concentrations in ground water. Some of these processes have been described in more detail in previous chapters in the handbook and are therefore only reviewed in brief In the second part of this chapter, we will review the biological processes that bring about the degradation of the most common chlorinated solvents, present conceptual models of chlorinated solvent plumes, and summarize data from field studies with chlorinated solvent contamination. 

Figure 23.1.4. Abiotic and biological transformation pathways for selected chlorinated solvents. (From reference 1 after references 29, 53). [From T.H. Wiedemeier, H. S. Rifai, C. J. Newell and J.T. Wilson, Natural Attenuation of Fuels and Chlorinated Solvents in the Subsurface. Copyright 1999 John Wiley Sorts, Inc. Reprinted by...

Two conceptual models are provided for environments in which Type 3 behavior occurs. For sources with PCE and TCE, the major natural attenuation processes are dilution and dispersion alone (no biodegradation). As shown in 23.1.7, the PCE and TCE plumes extend from the source zone and concentrations are slowly reduced by abiotic processes. Chloride concentrations and oxidation-reduction potential will not change as groundwater passes through the source zone and forms die chlorinated ethene plume. If TCA is the solvent of interest, significant abiotic hydrolysis may occur, resulting in a more rapid decrease in TCA concentrations and an increase in chloride concentrations. 

In plants, phenolic metabolites can stimulate cellular protective response coupled to antioxidant function in the presence of biotic and abiotic stress (Briskin 2000). Among abiotic stress, UV light induces phenolic phytochemicals through the phenylpropanoid and flavonoid glycoside pathways as a protective of metabolic response (Logemann et al. 2000). This UV inducible phenolic phytochemical response can help to counter intracellular ROS produced in response to UV. This UV-inducible phenolic response ean be coupled to antioxidant enzyme response (Rao 1996) to attenuate damage from UV radiation. 

Both biological and chemical (abiotic) pathways exist for the reduction of chromate and uranyl however, the reaction kinetics for the two broad classes differ appreciably. Reduction of chromate will probably occur through chemical means, albeit that the chemical reactant may result from microbial processes, in anaerobic environments ferrous Fe will dominate the reduction of chromate at mildly acidic to alkaline pH values while sulfide will dominate at lower pH values given equal availability. The microbial reduction of Fe (hydr)oxides promotes reduction of Cr(Vl) to Cr(Ill)— the result of a coupled, two-step, biotic-abiotic reaction pathway in which Fe(ll) produced during Fe respiration catalyzes the reduction of Cr(Vl). Thus, attenuation of chromate in saturated soil environments may be in large part attributable to dissimilatory Fe reduction. Enhancing Fe reduction may therefore promote the reductive stabilization of Cr. 

The phrase natural attenuation as understood within the context of subsurface remediation of contaminated soil and water evolved over a period of several years. It has largely been accepted as the politically and scientifically appropriate term, however it is still frequently used synonymously with Intrinsic remediation , natural recovery , and natural assimilation . The phrases intrinsic bioremediation and passive bioremediation refer strictly to biologically mediated attenuation processes, which are obviously a less broad definition, not including all the physical and abiotically mediated processes that... 

Recall from Section 1.3.1 that the mass balance equations (Eqs. 1.1a and 1.1b) contain internal source and sink terms which are due to chemical reactions. Chemical reactions result in the transformation of one chemical substance into another. In environmental media, i.e., sinface waters, the subsurface, and the atmosphere, chemical reactions may occur abiotically or be biologically mediated by organisms. Examples of abiotic chemical reactions include dissolved limestone (CaCOs) precipitating from dripping water to form a solid-phase stalactite or stalagmite in a cave, the attenuation of nuclear fallout over time, the degradation into dust of... 

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