Chemical fate modeling

The American Society for Testing and Materials (ASTM)119 has developed a standard protocol for evaluating environmental chemical-fate models, along with the definition of basic modeling terms, shown in Table 20.17. Predicting fate requires natural phenomena to be described mathematically. 

Definitions of Terms Used in Chemical Fate Modeling... 

Soil compartment chemical fate modeling has been traditionally performed for three distinct subcompartments the land surface (or watershed) the unsaturated soil (or soil) zone and the saturated (or groundwater) zone of a region. In general, the mathematical simulation is structured around two major cycles the hydrologic cycle and the pollutant cycle, each cycle being associated with a number of physicochemical processes. Watershed models account for a third cycle sedimentation. 

Thus, the process of model testing and validation (considered synonymous) should ideally include the steps of calibration (if necessary), verification, and post-audit analyses. I indicate "ideally" because in many applications existing data will not support performance of all steps. In chemical fate modeling, chemical data for verification is often lacking and post-audit analyses are rare (unfortunately) for any type of modeling exercise. 

Parameters for which measured values are not clearly defined or readily available are often determined through calibration with observed data. In watershed chemical fate modeling, calibration has traditionally been associated with hydrologic parameters (e.g., erodibility coefficients, scour and deposition rates) because the required flow and sediment data are often available. Although initial parameter values can always be estimated, calibration is usually recommended to account for local and spatial variations. 

Keywords Chemicals, Fate, Modelling, Risk assessment... 

Webster E, Mackay D, Di Guardo A, Kane D, Woodline D (2004) Regional differences in chemical fate model outcome. Chemosphere 55 1361-1376... 

SimpleBox was created as a research tool in environmental risk assessment. Simple-Box (Brandes et al. 1996) is implemented in the regulatory European Union System for the Evaluation of Substances (EUSES) models (Vermeire et al. 1997) that are used for risk assessment of new and existing chemicals. Dedicated SimpleBox 1.0 applications have been used for integrating environmental quality criteria for air, water, and soil in The Netherlands. Spreadsheet versions of SimpleBox 2.0 are used for multi-media chemical fate modeling by scientists at universities and research institutes in various countries. SimpleBox models exposure concentrations in the environmental media. In addition to exposure concentrations, SimpleBox provides output at the level of toxic pressure on ecosystems by calculating potentially affected fractions (PAF) on the basis of species sensitivity distribution (SSD) calculus (see Chapter 4). 

The remainder of the chapter contains several, practical, mathematical, vignette chemodynamic models. These are simple but useful chemical fate models for obtaining numerical results when the dominant or controlling transport process is known. Those selected for presentation have stood the test of time, having proved useful to the authors on numerous occasions. 

It is obvious to the user at this juncture that the subject of environmental chemical fate models enjoys many individual mass transfer processes. Besides this, the flux equations used for the various individual processes are often based on different concentrations such as Ca, Cw, Cs, and so on. Since concentration is a state variable in all EC models, the transport coefficients and concentrations must be compatible. Several concentrations are used because the easily measured ones are the logical mass-action rate drivers for these first-order kinetic mechanisms. Unfortunately, the result is a diverse set of flux equations containing various mechanism-oriented rate parameters and three or more media concentrations. Complications arise because the individual process parameters are based on a specific concentration or concentration difference. As argued in Chapter 3, the fiigacity approach is much simpler. Conversions to an alternative but equivalent media chemical concentration are performed using the appropriate thermodynamic equilibrium statement or equivalent phase partition coefficients. The process was demonstrated above in obtaining the overall deposition velocity Equation 4.9. In this regard, the key purpose of Table 4.2 is to provide the user with the appropriate transport rate constant compatible with the concentration chosen to express the flux. Eor each interface, there are two choices of concentration... 

The equations above are formulated to calculate time-averaged wet particle and wet gaseous deposition flux over several rain events. This approach is particularly useful for calculating steady-state solutions to mass balance chemical fate models, since the time-variant rate of deposition with rainfall is represented as a continuous process, that is, the model assumes a constant light drizzle of rain. However, the effects of intermittent rain events on average chemical concentrations in air over several rain events are not well described for all chemicals by assuming constant rainfall [22,23]. 

The long-term global average precipitation rate ((/r) is 9.7 x 10 m/h, equivalent to 0.85 m/year, and this value has been recommended as a generic value for chemical fate modeling [6,21], However, rain rate is highly variable geographically and seasonally. 

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