degree of chemical and biological activity. The other layer is a deeper, inactive layer in which chemicals are relatively isolated from the water column. Deposition and resuspension of mineral and organic matter to sediments occur continuously in any water body and are an important mechanism for transferring particle-bound contaminants to the sediment layer.

Multimedia Environmental Models

For substances released from waste-incineration facilities, the ambient concentration and deposition fluxes are determined by the partitioning and transport rates of the substances between the different compartments of the environment. Evaluating how chemicals are transported between such compartments requires a model that characterizes multiple environmental media, (i.e., air, soil, vegetation, surface water, sediments, and so forth) in combination. Efforts to assess human exposure to contaminants in multiple media date back to the 1950s when the need to assess human exposure to radioactive fallout and releases led to an assessment framework that included transport both through and among air, soil, surface water, vegetation, and food chains (USNRC 1975, 1977; Hoffman et al. 1979; Moore et al. 1979; Baes et al. 1984a,b; Whicker and Kirchner 1987). Efforts to apply such a framework to nonradioactive organic and inorganic toxic chemicals have been more recent and now are becoming as sophisticated as those extant in the radionuclide field. The first widely used multimedia compartment models for organic chemicals were the “fugacity” models described by Mackay (1991).1 Fugacity models have been used extensively for modeling the transport and transformation of nonionic organic chemicals in complex environmental systems. Modified fugacity and fugacity-type models have also been used for ionic-organic and inorganic species, including metals. The advantage of the typical multimedia fugacity-type model is the simplicity with which it treats each of the compartments as being well mixed, and allowing for flows and mass transfer between all compartments, and degradation within compartments. Such treatment is clearly an oversimplification but the models, by the judicious selection of compartments to correspond to the penetration depth of the pollutants, can lead to insightful conclusions on the major pathways, reservoirs, and persistence in the environment.

More-recent multimedia models used for assessing releases from incinerators use various approaches. Air dispersion is handled by standard Gaussian plume models, with modification to incorporate wet and dry deposition of materials from the plume. The deposition models are multi-layer transport models, incorporating a well-mixed upper layer in the main plume, an intermediate shear

1  

The term “fugacity” is used in thermodynamics to refer to a measure of the tendency of a substance to escape by some chemical process from the phase in which it exists.



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