Fate models

In this step, the assessor qiuuitifies tlie magnitude, frequency and duration of exposure for each patliway identified in Step 2. Tliis step is most often conducted in two stages estimation of exposure concentrations and calculation of intakes. The later estimation is considered in Step 4. In tliis part of step 3. the exposure assessor determines the concentration of chemicals tliat will be contacted over the exposure period. E.xposure concentrations are estimated using monitoring data and/or chemical transport and environmental fate models. Modeling may be used to estimate future chemical concentrations in media tliat are currently contaminated or tliat may become contaminated, and current concentrations in media and/or at locations for which tliere are no monitoring data. The bulk of the material in tliis chapter is concerned witli tliis step. 

Because of the influence of the ligand, physicochemical properties and environmental fate modelling derived from them are often uncertain for the organotins. 

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... 

ASTM, Standard practice for evaluating environmental fate models of chemicals, Annual Book of ASTM Standard, American Society for Testing and Materials (ASTM), E978-84, Philadelphia, PA, 1984. 

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 ... 

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. 

We first describe human activities that can cause releases of chemicals these are usually of greatest concern to fate models, because they suggest where interventions can be made and environmental concentrations can be reduced. We then classify releases by their form, medium of entry, and spatial and temporal patterns. After briefly noting the most usual quantitative... 

Environmental fate models require information on the distribution of releases over time and space. Basically, sources can be described in terms of their dimensionality and releases in terms of their temporal distribution. 

Our concept of a volume source (see Figure 2) is intentionally vague, because few good examples exist. However, photochemical smog is produced over a volume of air is this just part of a fate model or should it be considered a source ... 

There are also several possibilities for the temporal distribution of releases. Although some releases, such as those stemming from accidents, are best described as instantaneous release of a total amount of material (kg per event), most releases are described as rates kg/sec (point source), kg/sec-m (line source), kg/sec-m (area source). (Note here that a little dimensional analysis will often indicate whether a factor or constant in a fate model has been inadvertently omitted.) The patterns of rates over time can be quite diverse (see Figure 3). Many releases are more or less continuous and more or less uniform, such as stack emissions from a base-load power plant. Others are intermittent but fairly regular, or at least predictable, as when a coke oven is opened or a chemical vat... 

Aquatic fate models must account for several kinds of phenomena, including transport and transfer processes that move a compound among ecosystem segments and compartments, and... 

Ionization, sorption, volatilization, and entrainment with fluid and particle motions are important to the fate of synthetic chemicals. Transport and transfer processes encompass a wide variety of time scales. Ionizations are rapid and, thus, usually are treated as equilibria in fate models. In many cases, sorption also can be treated as an equilibrium, although somtimes a kinetic approach is warranted (.2). Transport processes must be treated as time-dependent phenomena, except in simple screening models (.3..4) ... 

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. 

From these data, aquatic fate models construct outputs delineating exposure, fate, and persistence of the compound. In general, exposure can be determined as a time-course of chemical concentrations, as ultimate (steady-state) concentration distributions, or as statistical summaries of computed time-series. Fate of chemicals may mean either the distribution of the chemical among subsystems (e.g., fraction captured by benthic sediments), or a fractionation among transformation processes. The latter data can be used in sensitivity analyses to determine relative needs for accuracy and precision in chemical measurements. Persistence of the compound can be estimated from the time constants of the response of the system to chemical loadings. 

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. 

Bonazountas, M. J. Fiksel (1982). ENVIRO Environmental Mathematical Pollutant Fate Modeling Handbook/Catalogue, EPA Contract No. 68-01-5146, Arthur D. Little, Inc., Cambridge, MA 02140. 

The Role of Multimedia Fate Models in Chemical Risk Assessment... 

Another case of multimedia fate modeling may be exemplified by human inhalation exposure estimates for PCB spills. The spill size is estimated considering both spread and soil infiltration. Volatilization calculations were carried out to get transfer rates into the air compartment. Finally, plume calculations using local meteorological statistics produced ambient concentration patterns which can be subsequently folded together with population distributions to obtain exposures. 

Numerous examples of fate models are reviewed in other papers in this symposium. For example, single media models are covered for air by Anderson (2 ), for water by Burns ( 3), and for soil and groundwater by Bonazountas (4). 

Because the significance of exposure has only been considered over the past few years, there is not as wide a selection of exposure models available as that for fate models. The latter have been applied for several decades to the calculation of ambient exposure levels compared with some standard values. Papers illustrative of human exposure assessments in this symposium include one on airborne pollutant exposure assessments by Anderson (2), a generic approach to estimating exposure in risk studies by Fiksel (5), and a derivation of pollutant limit values in soil or water based on acceptable doses to humans by Rosenblatt, Small and Kainz (6). 

In this symposium a comprehensive overview of the risk estimation step and its relationship to the output of multimedia fate models is given in the paper by Fiksel (5). Examples of the application of and linkage among the various techniques are also presented in that paper. 

We note that multimedia fate modeling constitutes a central link in the chain of calculations forming a risk analysis. Although regulatory mandates, as described above, have... 

---