BIO 220 Mechanisms and Pathways that Transport Toxins to the Environment
BIO 220 Mechanisms and Pathways that Transport Toxins to the Environment
Consider the most recent oil spill in the Gulf of Mexico. Describe the various mechanisms and pathways that transport toxins to the environment. Note: Avoid only talking about the oil spill affecting people in Mexico.
The main pathway for pollutants to get into the environment from a waste-incineration facility is, as for many other sources, through emission to the atmosphere. A large number of substances have been detected—most of them at very low concentrations—in the gaseous and particulate emissions from waste incineration. Among the emitted pollutants are metals and other noncombustible matter; acid gases; and products of incomplete combustion that include a large number of organic compounds as well as oxides of nitrogen, sulfur, and carbon. These pollutants are partitioned among the gas and particulate phases of the stack emissions from an incineration facility. As the pollutants disperse into the air, facility workers and people close to a facility might be exposed directly through inhalation or indirectly through consumption of food or water contaminated by deposition of the pollutants to soil and vegetation. Other people can be exposed through a different mix of environmental pathways after the pollutants travel some distance in the atmosphere; go through various chemical and physical transformations; or pass through soil, water, or food. As part of estimating the amount of incineration-released contaminants that people are exposed to and the patterns of such exposure, investigators seek to track the concentration and movement of, and changes that occur in, the contaminants as they move through the environment from the incineration facility to a point of contact with people. Such information is also helpful in determining the contribution of incineration to the mix of environmental contaminants from all sources.
This chapter provides a review of the environmental dynamics of substances emitted from waste-incineration facilities and the pathways that could result in human exposure to such contaminants. The chapter is not intended to provide a comprehensive examination of the many aspects considered because such an examination is beyond the committee’s task. To illustrate some of the important considerations with respect to environmental dynamics and exposure, particular attention is given to the main substances of concern that are discussed in Chapter 5 from a health-effects perspective. The chapter also examines approaches for estimating environmental concentrations that are used to estimate human exposures. As an illustration of how incineration facilities contribute to environmental concentrations at different geographical scales and for different agents, information is provided on particulate matter, various metals (cadmium, arsenic, mercury, and lead), dioxin-like compounds, carbon monoxide, and hydrogen chloride.
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TRANSPORT PATHWAYS IN THE ENVIRONMENT
Substances released from combustion sources are ultimately dispersed among, and can at times accumulate in, various environmental compartments (e.g., soils, vegetation, indoor dusts, animals, and humans). Some contaminants that are released from incineration facilities are likely to contribute primarily to environmental compartments on a local scale (within 10 km). However, others that are more persistent in the environment, can be distributed over much greater distances—even up to a regional scale over hundreds of kilometers. Most of the substances released from incineration facilities to air do not remain in air but are deposited to soil, vegetation, or surface water and can come into contact with humans through a series of complex environmental pathways that include transport through several environmental media (see Figure 4-1).
BIO 220 Mechanisms and Pathways that Transport Toxins to the EnvironmentMultimedia 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.
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BIO 220 Mechanisms and Pathways that Transport Toxins to the Environment
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 layer where the wind-speed increases regularly with height, and boundary-layer near the ground or vegetation surface. Transport of material deposited on the ground is handled largely by compartment models, with pathways of human exposure elaborated to varying degrees, with inter-compartmental transfer rates based on physical modeling, empirical correlations, or fugacity-type approaches. Examples of such models, with descriptions, are given in Lorber et al. (1994); Slob et al. (1993); EPA (1990, 1997b, 1998a).
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ASSESSING HUMAN EXPOSURE TO ENVIRONMENTAL CONTAMINANTS
The issue of assessing human exposure to contaminants has been addressed in previous reports of the National Research Council (e.g., NRC 1991b, 1994). Exposure to a substance of concern is defined as contact at a boundary between a human and the environment at a specific concentration for a specific period (NRC 1991b). Human exposure assessment involves measuring or estimating the concentrations of specific substances in each exposure medium, and the time individuals or populations spend in contact with each such medium. Human activity patterns directly affect the magnitude of exposure to substances present in different indoor and outdoor locations. Assessing exposure to contaminants emitted as a result of waste incineration involves characterization of the rates and patterns of incineration emissions, tracking of the emitted material through the environment, and characterizing the amount of human contact with the material. In addition to incineration, other sources (for example, motor vehicles, coal-fired power plants, industrial manufacturing facilities, and some naturally-occurring sources) contribute to the total concentration of contaminants to which humans are exposed. Sexton et al. (1994) and Pirkle et al. (1995) discuss data bases that are available to help establish total exposure concentrations. Incineration facilities add some incremental amount to the total ambient concentrations in the environment for many pollutants, such as nitrogen oxides, sulfur dioxide, particulate matter, volatile organic compounds. For selected pollutants, such as dioxin, incinerators might collectively contribute major fractions of observed ambient concentrations as discussed later in this chapter. A particular incinerator, however, might be the dominant source at a particular location for concentrations of nitrogen oxides, sulfur dioxide, or particulate matter, and may, but not necessarily, be the dominant source for the dioxins.
Exposures to a substance of concern might be dominated by contacts through a single environmental pathway or they might reflect contacts through multiple pathways. Table 4-2 shows some of the pathways of exposure. All possible routes by which contaminants enter the body of an exposed person must be considered—inhalation, ingestion of food or drink, and absorption through skin because such patterns directly affect the magnitude of exposures to substances present in different indoor and outdoor environments.
