Chemical fate and transport models

The goal of this paper Is to present the current status of model validation and field testing of chemical fate and transport models other papers in this symposium discuss the state-of-the-art of modeling specific processes, environments, and multimedia problems. The process of model validation, and its various components, is described considerations in field testing, where model results are compared to field observations, are discussedp an assessment of the current extent of field testing for various processes and media is presented and future field testing and data needs are enumerated. 

Comparisons between observed data and model predictions must be made on a consistent basis, i.e., apples with apples and oranges with oranges. Since models provide a continuous timeseries, any type of statistic can be produced such as daily maximums, minimums, averages, medians, etc. However, observed data are usually collected on infrequent intervals so only certain statistics can be reliably estimated. Validation of aquatic chemical fate and transport models is often performed by comparing both simulated and observed concentration values and total chemical loadings obtained from multiplying the flow and the concentration values. Whereas the model supplies flow and concentration values in each time step, the calculated observed loads are usually based on values interpolated between actual flow and sample measurements. The frequency of sample collection will affect the validity of the resulting calculated load. Thus, the model user needs to be aware of how observed chemical loads are calculated in order to assess the veracity of the values. 

Kolset, K. and A. Heiberg. 1988. Evaluation of the "Fugacity" (FEQUM) and the "EXAMS" chemical fate and transport models a case study on the pollution of the Norrsundet Bay (Sweden). Water Sci. Technol. 20 (2) 1-12. 

It is now universally accepted that chemicals of commerce and those that may be formed inadvertently by processes such as combustion should be subjected to evaluation for their possible adverse effects on humans, the environment, and its various ecosystems. Earth surface processes are continually active with atmospheric, oceanic, and terrestrial media forces that foster chemical mobilization with long-range chemical transport within continental land masses, across the oceans, and on a global scale between the hemispheres. Monitoring data from remote locations provide evidence of this transport and these assertions are confirmed by the theoretical results of a variety of multimedia chemical fate and transport models. At the local level, the other geographic extreme, chemical sources are more intense and the pathways shorter and the impacts are therefore more severe. Anthropogenic substances have been mobilized and now exist in every nook, cranny, and recess of the physical environment and within many biological species. 

For an aquatic model of chemical fate and transport, the input loadings associated with both point and nonpoint sources must be considered. Point loads from industrial or municipal discharges can show significant daily, weekly, or seasonal fluctuations. Nonpoint loads determined either from data or nonpoint loading models are so highly variable that significant errors are likely. In all these cases, errors in input to a model (in conjunction with output errors, discussed below) must be considered in order to provide a valid assessment of model capabilities through the validation process. 

To construct models of this sort, we combine reaction analysis with transport modeling, the description of the movement of chemical species within flowing groundwater, as discussed in the previous chapter (Chapter 20). The combination is known as reactive transport modeling, or, in contaminant hydrology, fate and transport modeling. 

SRC. 1994b. Fate and transport models Model outputs for the chemical -hexane. Syracuse, NY Syracuse Research Corporation. 

The results from this case study can be used as input to the general comprehensive RRR fate and transport model (i. e., which includes volatilization, photolysis, biodegradation, and sorption/desorption modules) in order to predict organic leachate-generated chemical loads and concentrations at highway boundary or landfill sites. 

Physical and Chemical Properties. The physical and chemical properties of carbon tetrachloride have been well-studied, and reliable values for key parameters are available for use in environmental fate and transport models. On this basis, it does not appear that further studies of the physical- chemical properties of carbon tetrachloride are essential. 

Jury, W.A., Ghodrati, M. (1989) Overview of organic chemical environmental fate and transport modeling approaches. In Reactions and Movement of Organic Chemicals in Soils. SSSA Special Publication No. 22, Sawhney, B.L., Brown, K., Editors, pp. 271-304, Soil Science Soceity of America and Society of Agronomy, Madison, Wisconsin. 

Exposure assessment qualitatively and quantitatively characterizes the potential for exposure to occur in particular circumstances and includes an estimate of dose when possible. The assessment includes an estimation or measurement of chemical concentration in the contact media (e.g., soil, water, air, a particular food crop, a consumer product), an estimation of the length of time over which contact will occur, characterization of potential routes of exposure (inhalation, ingestion, and skin contact), and the likelihood for a chemical to be absorbed through those routes. In certain circumstances, direct measurements or fate and transport modeling may be used to estimate chemical concentrations in ambient media. For certain assessments, a quantitative estimate of the total dose of a chemical over a particular time frame and in the given circumstances is made. 

Exposure assessment includes both qualitative and quantitative evaluations of the potential for exposure to site-related chemicals to occur. Assessments commonly address both current and likely future uses of the property (e.g., residential, commercial, industrial, and agricultural). Typically, a conceptual model is developed that summarizes how site-related chemicals may contact receptors (e.g., humans, wildlife, and ecological). The model includes identification of chemical sources, impacted media, potential movement through the environment, identification of the appropriate exposure scenarios, and identification of the points at which contact between receptors and site-related chemicals are likely to occur. Chemical concentrations in environmental media may be estimated based on site data and using statistical analyses and/or fate and transport modeling. An estimate of the dose (intake) attributable to contact with environmental media through significant and completed pathways is made for chemicals of concern at... 

Hathaway DL, Andrews CB. 1990. Fate and transport modeling of organic compounds from a gasoline spill. In Petroleum hydrocarbon and organic chemicals in ground water Prevention, detection and restoration. Houston, TX S. S. Papadopulos and Associates, Inc., 563-576. 

In the air phase, under pressures normally found in the environment, fugacity equals the pressure exerted by the chemical s vapor. (At higher pressures, vapors do not exactly obey the ideal gas law, and a correction must be applied this is small enough to ignore for practical purposes of fate and transport modeling in the environment.) By combining the ideal gas law (Eq. [1-29]) and Eq. [1-34], it is evident that the fugacity capacity for air is 1/RT, for all chemicals ... 

Exposure Assessment. As noted above, the Risk Assistant software is intended to build on EPA s existing information base on environmental fate and transport modelling, extending it to risk-relevant exposure calculations. Accordingly, it does not incorporate mathematical models of the environmental transport and fate of chemicals, but takes as its starting point user-specified data on environmental concentrations of chemicals to which people might be exposed. The Additional Analyses discussed below, however, do include tools to assist the risk assessor in selecting appropriate transport models. 

On a second level, opportunities exist because the current level of geochemical studies in the environmental field is low. We know that a distribution constant Kj is usually insufficient to describe the chemical processes (see 10.3), but most fate and transport models have employed this concept. Geochemical modeling is generally under-used. 

Hemond, H. R, and Fechner-Levy, E. J. (2000). Chemical Fate and Transport in the Environment, Academic Press, San Diego, CA. Mackay, D. (2001). Multimedia Environmental Models— The Fugacity Approach, Lewis Publishers, Boca Raton, FL. 

The mass-balance model offers a conceptually simple and elegant approach to organizing available information about chemical fate and transport for purposes of... 

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