Contaminant attenuation

The problem of diffuse-source agricultural and mbano-industrial pollution should command special attention, in part because of the complex role played by the unsaturated zone, which can encompass a mix of both contaminant attenuation and contaminant accumulation, with periodic release into the aquifer system. River beds can play a similar role in the case of many industrial pollutions. 

TABLE VI Chemical and Microbial Processes Influencing Contaminant Attenuation... 

For contaminant attenuation by retention mechanisms, the contaminants must be retained by specific adsorption. Other mechanisms for retention include chemisorption, i.e. high affinity specific adsorption in the inner Helmholtz layer through covalent bonding, via hydroxyl groups... 

Chemical mass transfer responsible for partitioning of contaminants constitutes a significant part of the processes involved in the transport and fate of contaminants. In the longer term, the redox environment (pE) and subsequent reduction-oxidation reactions will ultimately determine the final fate of the contaminants. The assessment of whether retention or retardation processes are responsible for the observed partitioning and hence the attenuation of contaminants within the soil matrix, is vital and critical in the evaluation of the natural attenuation capability of the soil barrier system. The dilemma facing both regulatory agencies and practitioners is obvious If potential pollution hazards and threats to public health and the environment are to be minimized or avoided, How can one ensure that the processes for contaminant attenuation in the substrate are the result of (irreversible sorption) retention... 

Farquhar, G. J. Constable, T. W. 1978. Leachate contaminant attenuation in soil. Waterloo Research Institute, Project No. 2123, University of Waterloo, Waterloo, Canada. 

Contaminant attenuation mechanisms involved in phytoremediation are complex and not limited only to the direct metabolism of contaminants by plants. Certain indirect attenuation mechanisms are implicated in phytoremediation, such as the metabolism of contaminants by plant-associated microorganisms, and plant-induced changes in the contaminated environment. In terrestrial species, transport of contaminants to the plant is dominated by uptaking water by roots, and distribution within the plant relies on xylem or phloem transport (Macek et ak, 2000). Various terms, reflecting each specific attenuation mechanism, have been extensively used to better describe specific applications of phytoremediation. These include phytoextraction, phytodegradation, phytotransformation, phytovolatilization and rhizodegradation (Burken and Ma, 2006). 

Standardization and Testing. Potency is determined by titrating the amount of Hve vims using a suitable cell substrate. Safety testing is also performed on seed lots to assure proper attenuation and on vaccine to assure absence of unwanted contaminants. 

Ex situ bioremediation may use various biological wastewater treatment processes, soil piles, or land appHcation. With in situ bioremediation, the basic process is the same microbes, soil, and water working together as a bioreactor. Where the in situ techniques differ are in how contaminants and microbes are brought in contact and how oxygen, nutrients, and other chemical supplements ate distributed in the soil—water—air matrix. Typical in situ bioremediation techniques include natural or intrinsic attenuation, air sparging, and bioventing. 

Van der Meer JR, C Werlen, SF Nishino, JC Spain (1998) Evolution of a pathway for chlorobenzene metabolism leads to natural attenuation in contaminated groundwater. Appl Environ Microbiol 64 4185-4193. 

The U.S. EPA publishes natural attenuation reports94 96 that provide a general description on approaches to clean up contaminated waste sites. One U.S. EPA report lists five questions about each cleanup approach ... 

Other U.S. EPA reports explain what MNA means when the term is used to describe a potential strategy to remediate a contaminated site. They also describe the various physical, chemical, and biological processes of natural attenuation that may occur at a site. Other informational materials are in preparation and will provide more specific details and scientific depth for the evaluation of MNA as a remedy at specific sites. 

The primary action in this alte mative is capping of one or more contaminated areas (such as Areas 1 and 2 of Figure 16.21) and then natural attenuation of the contaminated groundwater. The cap would be consistent with the state RCRA (which is more stringent than the Federal requirement) landfill closure requirements. 

Two assumptions about the surface have been made to determine the effect of natural attenuation on the contaminated groundwater. First, despite the fractured nature of the bedrock, it has been assumed that the subsurface is homogeneous so as to facilitate the evaluation. Second, the potential for reduction in TCE concentrations has been assessed using a hydrogeologic model in which the fact that the cap would reduce existing leachate production by 75% is taken into account. This model is assumed to predict that the concentration of TCE in the groundwater would be reduced to an excess cancer risk level of 28 pg/L in 60 yr and an excess cancer risk level of 5 pg/L, approximately equal to the MCL, in approximately 100 yr. 

Alternative 2 achieves protection by preventing exposure through capping and natural attenuation of the contaminated groundwater. Alternative 3 combines treatment to reduce the risk from the TCE-contaminated soil and groundwater and capping of the lead area. Alternatives 4 and 5 reduce risks posed by all portions of the site through treatment. 

Environmental impacts Continued impact from existing conditions Would be some migration of contaminant pluma as part of attenuation process Vapor extraction may affect air quality and odors although it will meet emission standards would be aquifer drawdown during groundwater extraction See Alternative 3 fixation may also affect air quality and produce odors Incineration may affect air quality by producing odors, although if will meet emission standards... 

Alternative 2 uses no treatment technologies. All contaminated soil and groundwater would remain however, contaminates will in time attenuate naturally. 

In some cases, such as areas of lower contaminant concentrations or in remote locations, contaminants stripped from the groundwater may be allowed to attenuate naturally in the vadose zone. 

U.S. EPA defines MNA as the reliance on natural processes, within the context of a carefully controlled and monitored site cleanup approach, to achieve site-specific remediation objectives within a time frame that is reasonable compared to that offered by other more active methods. The natural processes include biodegradation, dispersion, dilution, sorption, volatilization, stabilization, and transformation. These processes reduce site risk by transforming contaminants to less toxic forms, reducing contaminant concentrations, and reducing contaminant mobility and bioavailability. Other terms for natural attenuation in the literature include intrinsic remediation, intrinsic bio-remediation, passive bioremediation natural recovery, and natural assimilation. 30... 

U.S. EPA published a guide about the steps needed to understand the rate and extent to which natural processes are reducing contaminant concentrations.56 Although this guide is directed at sites contaminated by chlorinated solvents, some of the steps would also have relevance for sites contaminated by oxygenates like MTBE. The guide identifies parameters that are useful in the evaluation of natural attenuation and provides recommendations on how to analyze and interpret the data... 

Romdhane M, Gadri A, Contamine F, Gourdon C, Casamatta G (1997) Experimental study of the ultrasound attenuation in chemical reactors. Ultrason Sonochem 4 235-243... 

Boyd SA, Lee J-F, Mortland MM (1988) Attenuating organic contaminant mobility by soil modification. Nature 333 345-347... 

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