Chemical fate modeling terms

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... 

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. 

This symposium concerns models for predicting the fate of chemicals in the environment. Strictly speaking, the topic of this paper does not fall into the usual definition of fate models. However, every fate model has at least one source term. Although the source term for one fate model may be the output of another fate model (as when air transport models provide the deposition rates that are the inputs to an aquatic fate model), the chain always has to be traced to the original sources, whether they are natural or associated with human activities. 

In this paper, we characterize the various sources for chemicals in the environment and discuss methods for describing the releases from them in terms sufficiently quantitative for use by fate models. 

Ionization. Many organic chemicals contain functional groups that dissociate to yield charged species. The toxicity and chemical reactivity of the uncharged (neutral) molecule and its charged ions can be very different. Differences in reactivity of ionic species can be accommodated in fate models when rate constants are expressed in terms of the individual species. 

In order to achieve that an environmental fate model is successfully applied in a screening level risk assessment and ultimately incorporated into the decisionmaking tools, the model should have computational efficiency and modest data input. Moreover, the model should incorporate all relevant compartments and all sources of contamination and should consider the most important mechanisms of fate and transport. Although spatial models describe the environment more accurately, such models are difficult to apply because they require a large amount of input data (e.g., detailed terrain parameters, meteorological data, turbulence characteristics and other related parameters). Therefore, MCMs are more practical, especially for long-term environmental impact evaluation, because of their modest data requirements and relatively simple yet comprehensive model structure. In addition, MCMs are also widely used for the comparative risk assessment of new and existing chemicals [28-33]. 

Chemical transport and transformation have been a part of environmental science and engineering for decades. Air pollutant plume dispersion modeling and surface water quality stream modeling are mature elements of EC, commonly termed chemical fate and... 

This eqitation is used to describe chemical fate in the atmosphere, in waterbodies, and in the subsitrface environment, whenever spatial variability is large and the use of box models that assitme homogeneous conditions is inappropriate. It is the relative magnitude of the parameters V, E, and k that determines the character of a chemical s fate in a phase. One way to illustrate this is the calcirla-tion of the term E kA (sometimes called the Damkbhler nttmber). If that term is much larger than 1, diffusive processes are fast relative to advective processes within the life-time of the chemical indicated by 1/k. A value much smaller than 1, on the other hand, indicates that advective transport is faster than diffusive transport within that time scale. 

If mathematical fate models are used it would be desirable to interpret the results in terms of the chemical structure. As a first step in this direction, this paper will present some conclusions concerning the similarity of chemicals with respect to their environmental fate. The surface water model EXWAT... 

The species continuity equation (CE) is an expression of the Lavoisier general law of conservation of mass. Equation 2.1 presents the CE in vector form and provides the proper context for the various types of chemical mass transport processes needed for chemical modeling and fate analysis. In Section 2.2.2, the mass accumulation portion of the CE is highlighted as the principal term for assessing chemical fate in the media compartments. This term includes reaction, advection, diffusion, and turbulent transport and dispersion processes. Because the magnitude and direction of this term reflect the sum total of all processes, this term uniquely defines chemical fate. In Equation 2.2, the steady-state CE minus the reaction term is commonly referred to as the advective-diffusive (AD) equation. It provides the appropriate starting point for addressing the various transport processes associated with the mobile phases in near-surface soils. 

Depending on the relevant chemical/biological process(es) assumed to be affecting contaminant fate, the last term on the right-hand-side of Eq. (18) can be modeled using the applicable equation(s) presented above. 

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