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EPA-Expo-Box (A Toolbox for Exposure Assessors)

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Indirect Estimation
(Scenario Evaluation)

Fate and Transport

Fate and transport covers movement of substances in the environment and chemical/biological reactions that affect the nature of the substance. The figure below shows some transport processes that might occur following release of a contaminant.

Transport: Movement Within and Between Environmental Media
Transport: Movement within and between Environmental Media

Transport can occur within a medium. For example, in the figure, a chemical released to the air is shown moving away from the stack via advective, dispersive, and diffusive processes. These same processes might also occur in the surface water. Transport can also happen at the interface between two environmental media. For example, chemicals present in the air as vapors or sorbed to particles can be transferred into the soil via a range of different processes occurring at and across the air-soil interface. Transport of chemicals can also occur between abiotic and biotic media. In other words, people and wildlife, as part of the environment, can be exposed to chemicals released initially to abiotic environmental media (Thibodeaux, 1996).

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The other part of "fate and transport" is chemical transformation of a contaminant in the environment. This is typically assumed to occur within a medium. Some of the types of transformations that can occur in the environment are shown in the figure below.

Transformation: Chemical Changes within a Medium
Transformation: Chemical Changes withiin a Medium

Once released into the environment, the form and distribution of stressors among various media or environmental compartments can be affected by the physical and chemical properties of the stressor (e.g., water solubility, vapor pressure, partitioning).

Characteristics of the environment can also impact fate and transport. Some categories of characteristics that can influence stressor fate and transport include:

  • Soil and sediment properties, such as particle size and porosity
  • Climate and meteorology, which cover properties like wind speed, evaporation rate, and rainfall amount
  • Surface water and groundwater properties, including flow, temperature, and pH
  • Other properties of the ecosystem, including microbial populations, topography, and indigenous species

The specific influences that environmental characteristics can have on stressors’ fate and transport patterns are widely varied. In exposure assessments, fate and transport of stressors is typically evaluated via some level of modeling—application of mathematical representations of the processes that distribute and transform stressors in the environment.

Sources are available that provide information on physicochemical properties that affect fate and transport. Other resources provide tools for evaluation of fate and transport based on media type.

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