Equilibrium models, assessments environment

Weber W, McGinley P, Katz L (1992) A distributed reactivity model for sorption by soils and sediments. 1. Conceptual basis and equilibrium assessments. Environ Sci Technol 26 1955-1962... 

Equilibrium models are used to assess the environmental impact of power plant siting. The use of a single, maximum concentration factor for bivalve molluscs as input into the model in this situation is appropriate for screening purposes, i.e., to determine whether the maximum credible value would impact the environment. However, when more realistic estimates are required, selection of concentration factors applicable to the site, species, and situation is necessary. 

Equilibrium models are widely used in assessments of trace metal bioavailability, toxicity, and transport through the environment. Properly applied, equilibrium models are powerful tools in such assessments. Due to a variety of factors, however, equilibrium modeling often falls short of its full potential. One problem, of special importance in equilibrium characterizations, is simplistic modeling. The use of simplistic chemical models is particularly important because it affects not only the modeling of complex natural systems, but also modeling of relatively simple chemical media used to generate primary thermodynamic data. 

Concentrations are useful for assessing trends, but do not adequately describe the fate of a contaminant in the environment. The rates of movement of contaminants from one compartment to another are necessary to assess fate, and to construct models that can predict fate under different environmental conditions. Models that describe contaminant transport and fate can range from simple equilibrium box models to highly complex dynamic models. For modelling PCB fate, accuracy and precision are limited by our ability to describe the processes involved, and the availability of actual field data for calibrating and validating the models. 

Indeed, OCPs, once released into the environment, are distributed into various environmental compartments (e.g., water, soil, and biota) as a result of complex physical, chemical, and biological processes. In order to perform appropriate exposure and risk assessment analyses, multimedia models of pollutant partitioning in the environment have been developed. Properties which are at the base of such a partitioning are water solubility (WS), octanol-water partition coefficient (Ko ), soil adsorption (K ), and bioconcentration factors (BCFs) in aquatic organisms, following these four equilibriums ... 

Two categories of numerical models have been developed to assess the initial stability and movement of sediments. The majority of these models were initially developed for terrestrial applications (i.e., design of earth dams) but have the potential to be employed to advantage in the marine environment. These models are (1) limit equilibrium and (2) finite elemen and are discussed in Sections 11.4.2 and 11.4.3. Solutions involving earthquake loading are included for completeness. 

The discovery and characterization of the thiyl/thiolate conjugation equilibrium (3) [14] is one of the most significant contributions of the pulse radiolysis technique to the chemistry of cellular oxidative stress. Without such measurements, including the direct observation of the reduction of thiyl radicals by ascorbate [13] and electron transfer from disulphide radical anions to oxygen [ 15], it would be indeed difficult to prepare a reasoned argument for the competing reaction pathways from a quantitative viewpoint. Further work is needed to develop these concepts further. Not least, the dimension of space (as well as an appreciation of interfacial phenomena) needs to be added to the dimension of time if we are to model realistically the cellular environment and assess quantitatively, for example, the importance of the synergy between the lipophilic phenolic antioxidant, vitamin E and the hydrophilic electron-donor, vitamin C (ascorbate) [134]. 

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