Contaminant Pollutant Attenuation

It has become clear over the past ten years that the 1980s demonstration of anaerobic microbial dechlorination of PCBs is probably the most important discovery in the field of PCB biotransformations since Ahmed and Focht first demonstrated in 1973 that PCBs were biodegradable. Many new anaerobic microbial activities have been enriched and characterized from anaerobic fresh water and marine environments and heavily polluted industrial sediments. These anaerobic cultures are capable ofdechlorinating PCBs, thereby transforming highly chlorinated Aroclors to lower-chlorinated mixtures. This natural attenuation process is an important contributor to PCB degradation and detoxification in the environment and can form the basis for intrinsic remediation of many PCB-contaminated sites. 

A pollution-control enthusiast claims that a certain metropolitan area has such a high concentration of atmospheric contaminants that the solar insolation is attenuated by SO percent. How do you evaluate this claim Assuming a turbidity factor of 4.5, what solar altitude would be necessary to produce the 50 percent attenuation factor ... 

Public perception of a risk often becomes heightened when there is a perceived lack of voluntary control on exposure scenarios and exposure levels. It follows that human exposure to chemicals via the environment as a result of pollution or contamination tends to be a particular concern and a useful lobbying focus for many environmental NGO. Social mobilisation can result in risk level attenuation rather than amplification [505]. For instance, when a chemical is perceived as being of particular value to society, the level of risk may be more acceptable to many members of the public [506]. 

Natural attenuation by itself, however, often is not sufficient to achieve a desired extent or rate of contaminant removal from an aquifer. In these instances, one remedial option may be to enhance the natural rate of biodegradation of pollutant chemicals in the aquifer. This strategy, called in situ bioremediation, is considered to be one of the most attractive remedial techniques from a cost perspective, because many of the high costs associated with pumping and treating groundwater or excavating contaminated aquifer material are avoided. Furthermore, the potential exposure of cleanup workers to pollutant chemicals is reduced if many of the contaminants are mineralized while still in the aquifer. 

Surfactants and microemulsion systems can be used for ex situ treatment of contaminated soil or in situ soil decontamination. In situ remediation is usually preferred if excavation of the contaminated soil is not possible or expensive, e.g. beneath buildings or for contaminations at great depth. Often bioremediation or natural attenuation is used for decontamination. In most cases, these techniques only permit the effective degradation of contaminants in the plume formed by dissolved pollutants which may be very large. However, for the remediation of a contaminated site, it is also necessary to remove the source where the pollutants maybe adsorbed in large quantities or may be present as solid or liquid phases. The latter are called NAPL (non-aqueous phase liquids) and a differentiation is made between LNAPL (light non-aqueous phase liquids) with a lower density than water and DNAPL (dense non-aqueous phase liquids) with a higher density than water (see Fig. 10.1). 

Inherent in each hydrogeologic setting are the physical characteristics which affect the ground-water pollution potential. Many different biological, physical and chemical mechanisms may actively affect the attenuation of a contaminant and, thus, the pollution potential of that system. Because it is neither practical nor feasible to obtain quantitative evaluations of intrinsic mechanisms from a regional perspective, it is necessary to look at the broader parameters which incorporate the many processes. After a complete evaluation of many characteristics and the mappability of the data, the most important mappable factors that control the ground-water pollution potential were determined to be ... 

A second study characterizing a huge groundwater contamination caused by chemicals used for wood impregnation revealed mono- and dichlorinated naphthalenes, chlorobenzo(b)thiophene, acenaphthene and methylated naphthalenes as the main pollutants. Anaerobic microbial degradation of the main pollutant 1-chloronaphthalene was evident from identification of 1 -chloro-4-naphthol and 1-chloronaphthoic acid as biotic transformation products. In contrast inorganic and compound specific stable carbon isotope analyses indicated only a minor degree of microbial transformation. Thus, sorption seemed to be the main attenuation process within the aquifer affected by the coal tar based contamination described above. 

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. 

Risks from contaminated land arise from pollutant linkages that present unacceptable risk to receptors. The purpose of risk management it to make changes that remove these unacceptable risks. This can be done in different ways the source of contamination may be removed or reduced the pathway by which contaminants reach the receptor may be broken or attenuated or the receptor can be removed or modified. 

The Spittelwasser example (see Section 8.3.2) indicated that, unlike problems related to conventional polluted sites, the hazards here are primarily connected with the transport and deposition of contaminated solids in a catchment area, especially in downstream regions. Any problem solution strategy for such sites, therefore, has to consider both the chemical stabilization - for example, by processes of (enhanced) natural attenuation - and an increase in mechanical stability (reduced erodibility). 

Eor most organic pollutants the key mechanism of NA is mass reduction by microbial decomposition. Most inorganic pollutants cannot be reduced in mass and are mainly attenuated by sorption processes. For this group of contaminants, NA aims at a reten-... 

Christensen TH, Bjerg PL, Kjeldsen P. (2001). Natural attenuation as an approach to remediation of groundwater pollution at landfills. In Treatment of Contaminated Soil... 

However, phytoremediation does have certain disadvantages and limitations. This technology is limited by depth (roots) and also by the solubility and the availability of the pollutant. Although it is faster than natural attenuation, phytoremediation requires long time periods and is restricted to sites with low contaminant concentrations. The plant biomass obtained from phytoextraction requires proper disposal as hazardous waste. Phytoremediation depends on the climate and season. It can also lose its effectiveness when damage occurs to vegetation from disease or pests. The introduction of inappropriate or invasive plant species should be avoided (non-native species may affect biodiversity). Contaminants may be transferred to another medium, the environment, and/or the food chain. Amendments and cultivation practices may have negative consequences on contaminant mobiUty. 

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