Models of Environmental Fate

Multimedia models can describe the distribution of a chemical between environmental compartments in a state of equilibrium. Equilibrium concentrations in different environmental compartments following the release of defined quantities of pollutant may be estimated by using distribution coefficients such as and H s (see Section 3.1). An alternative approach is to use fugacity (f) as a descriptor of chemical quantity (Mackay 1991). Fugacity has been defined as fhe fendency of a chemical to escape from one phase to another, and has the same units as pressure. When a chemical reaches equilibrium in a multimedia system, all phases should have the same fugacity. It is usually linearly related to concentration (C) as follows  

Examples of models of the environmental fate of chemicals, utilizing partition coefficients and fngacities, are given in the works cited earlier, and also in Chapter 3 in Walker et al. (2000, 2006). 

3 INFLUENCE OF THE PROPERTIES OF CHEMICALS ON THEIR METABOLISM AND DISPOSITION 

In Chapter 2, the fate of organic pollutants in living organisms and the processes involved were described. In this section, the relationship between the properties of chemicals and the operation of those processes will be briefly reviewed. Many examples of the influence of properties of a chemical on its own metabolic fate will be encountered in Part 2 of this book. 

Organic Pollutants An Ecotoxicological Perspective, Second Edition 

---

Uncertainty on tlie other hand, represents lack of knowledge about factors such as adverse effects or contaminant levels which may be reduced with additional study. Generally, risk assessments carry several categories of uncertainly, and each merits consideration. Measurement micertainty refers to tlie usual eiTor tliat accompanies scientific measurements—standard statistical teclmiques can often be used to express measurement micertainty. A substantial aniomit of uncertainty is often inlierent in enviromiiental sampling, and assessments should address tliese micertainties. There are likewise uncertainties associated with tlie use of scientific models, e.g., dose-response models, and models of environmental fate and transport. Evaluation of model uncertainty would consider tlie scientific basis for the model and available empirical validation. 

Sabljic, A. and Piver, W.T., Quantitative modeling of environmental fate and impact of commercial chemicals, Environ. Toxicol. Chem., 11, 961-972, 1992. 

Although LCA contains elements of the tools discussed earlier in this chapter—mass balance, multimedia modeling of environmental fate and transport, and risk characterization— it is a distinct discipline with its own jargon, precepts, and limitations. 

When more in-depth analysis of environmental fate is required, the analyst must select the modeling procedure that is most appropriate to the circumstances. In general, the more sophisticated models are more data, time, and resource intensive. 

As probabilistic exposure and risk assessment methods are developed and become more frequently used for environmental fate and effects assessment, OPP increasingly needs distributions of environmental fate values rather than single point estimates, and quantitation of error and uncertainty in measurements. Probabilistic models currently being developed by the OPP require distributions of environmental fate and effects parameters either by measurement, extrapolation or a combination of the two. The models predictions will allow regulators to base decisions on the likelihood and magnitude of exposure and effects for a range of conditions which vary both spatially and temporally, rather than in a specific environment under static conditions. This increased need for basic data on environmental fate may increase data collection and drive development of less costly and more precise analytical methods. 

The need to balance costs against benefits both in the public and private sectors resulted in a search for methods of predicting the fate and effects of chemicals in the environment. Actual field testing of all cases of interest is both too costly and too dangerous to perform. Mathematical models, therefore, have been developed to provide descriptive tools and predictive approaches to this problem. At the symposium on which this book is based, a collection of user-oriented information was presented and covered the following aspects of environmental fate modeling ... 

Simple models are used to Identify the dominant fate or transport path of a material near the terrestrial-atmospheric Interface. The models are based on partitioning and fugacity concepts as well as first-order transformation kinetics and second-order transport kinetics. Along with a consideration of the chemical and biological transformations, this approach determines if the material is likely to volatilize rapidly, leach downward, or move up and down in the soil profile in response to precipitation and evapotranspiration. This determination can be useful for preliminary risk assessments or for choosing the appropriate more complete terrestrial and atmospheric models for a study of environmental fate. The models are illustrated using a set of pesticides with widely different behavior patterns. 

Human Exposure and Health Risk Assessments Using Outputs of Environmental Fate Models... 

Zhang L, Dai S (2007) Application of Markov Model to environmental fate of phenanthrene in Lanzhou Reach of Yellow River. Chemosphere 67 1296—1299... 

Dazhi S, Xuqian L (2010) Application of Markov chain model on environmental fate of phenanthrene in soil and groundwater. Procedia Environ Sci 2 814—823... 

MASS BALANCE MODELS OF CHEMICAL FATE 1.5.1 Evaluative Environmental Calculations... 

Gerba.C. P., Yates, M. V. Yates, S. R. (1991). Quantitation of factors controlling viral and microbial transport in the subsurface. In Modeling the Environmental Fate of Microorganisms, ed. C.J. Hurst, pp. 77-88. Washington, D.C. American Society for Microbiology. 

The ability to predict the behavior of a chemical substance in a biological or environmental system largely depends on knowledge of the physical-chemical properties and reactivity of that compound or closely related compounds. Chemical properties frequently used in environmental assessment include melting/boiling temperature, vapor pressure, various partition coefficients, water solubility, Henry s Law constant, sorption coefficient, bioconcentration factor, and diffusion properties. Reactivities by processes such as biodegradation, hydrolysis, photolysis, and oxidation/reduction are also critical determinants of environmental fate and such information may be needed for modeling. Unfortunately, measured values often are not available and, even if they are, the reported values may be inconsistent or of doubtful validity. In this situation it may be appropriate or even essential to use estimation methods. 

The data presented here may be helpful in modeling the environmental fate of surfactants. A recent modeling study for LAS (Mackay et al., 1996) has indicated that, due to negligible gas-water exchange, LAS interphase transfer via the gas phase will not occur. Instead, the aqueous phase is the central compartment for environmental fate of surfactants. Besides biodegradation, which reduces the amount of surfactants in the environment, sorption determines the partitioning of the surfactants between the aqueous and the solid phase. 

Lee, Y., 2005. Regional scale assessment of environmental fate and transport of dioxins by a multimedia model (KoEFT-PBTs). PhD dissertation, Seoul National University, Seoul, Korea. 

Schenker, U., Soltermann, R, et al (2008) Modeling the environmental fate of polybrominated diphenyl ethers (PBDEs) the importance of photolysis for the formation of lighter PBDEs. Environmental Science and Technology, 42(24) 9244-9249. 

Although the FIFRA microcosm has a number of advantages, there are also compromises. The few experiments that have been conducted and the variance in methodologies have not provided an accurate representation of the repeatability or replicability of the experiments. In addition, the method is somewhat local specific since the temperature, diurnal cycle, and to some extent the experimental organisms are controlled by the local environmental conditions. On the other hand, the sensitivity to local conditions can also act as a more accurate model of local fate and effects of the test material. 

Mathematical Models for Environmental Fate and Exposure of Chemicals... 

In multimedia box models, the environmental fate of a chemical is described by a set of coupled mass-balance equations for all boxes of the model. These equations include terms for degradation, inter-media exchange such as settling and resuspension of particles, and transport with air and water flows [19,20]. Equations for different boxes are coupled by inter-media exchange terms (linking different environmental media) and terms for trans-... 

Gouin T, Hamer T (2003) Modelling the environmental fate of the polybrominated diphenyl... 

Quigley MM (1998) A predictive model for environmental fate and transport of the toxicity of leachates from highway construction and repair materials, MS Thesis. Department of CivU, Construction, and Environmental Engineering, Oregon State University, CorvaUis, OR... 

Feijtel, T., Veerkamp, W., Koch, V. and Niessen, H. (1995) Use of environmental fate models in the risk assessment of substances. Toxicol. Model., 1, 5-19. 

---