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

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Soil and Dust

Fate and Transport

Fate and transport processes “link” the release of contaminants at a source with the resultant environmental concentrations to which receptors can be exposed. When a contaminant is released from a source, it is subject to transport and transformation in the environment. Compounds can also transfer from an environmental medium to biota, a process referred to as bioconcentration or bioaccumulation.

Migration Process Examples Relevant to Soil and Dust
Transport
  • Migration of a contaminant through soil via soil pore water (transport within a medium)
  • Deposition and resuspension of a contaminant between soil and ambient air or indoor dust and indoor air (transport between media)
  • Migration of soil contaminants by erosion and runoff flow to nearby soil (transport within a medium) or adjacent aquatic systems (transport between media)
  • Leaching of surface soil contaminants to subsoil (transport within a medium) and groundwater (transport between media)
Transformation
  • Organic breakdown or biodegradation of a compound in soil by a biotic organism, such as bacteria (chemical change)
  • Inorganic metals that dissolve in soil pore water (physical change)
Transfer – Environment to Biota
  • Contaminated soil or dust particles can be deposited onto plant surfaces and become translocated to different plant tissues.
  • Plants growing in contaminated soil can take up the contaminants from soil pore water through their roots.
  • Contaminants in vegetation or soil may be consumed by grazing or foraging animals that bioaccumulate the contaminants in their tissues.

For additional information on the uptake of contaminants from soil to plants and animals used as a source of food, see the Food Module in the Media Tool Set of EPA-Expo-Box.

Soil—Some Important Physicochemical Factors
  • Soil/water partition coefficients (Kd or Koc)—Kd describes a compound’s partitioning between soil solids and soil solution at equilibrium; Koc provides an indication of the extent to which an organic compound partitions between solid and solution phases in soil. Higher Kd or KOC values correlate to chemicals that are less mobile in soil (i.e., they are more likely to sorb to soil particles).
  • Vapor pressure—an indicator of how likely a compound will remain in the gaseous phase; the higher a chemical’s vapor pressure, the more likely that it will be found in the gas phase and move out of soil.
  • Octanol/air partition coefficient (Koa)—describes partitioning between air and aerosol particles, air and foliage, and air and soil; higher values of Koa indicate a tendency to sorb onto solid surfaces—vegetation, soils, aerosol particles, etc.
  • Water solubility—an indicator of mobility in soil; compounds with low water solubility would be expected to associate primarily with organic (carbon-rich) particles in soil and have less mobility compared with compounds that are more soluble and are likely to migrate with soil pore water.

Information on soil properties may be useful for fate and transport modeling of these soil contaminants. Contaminants can be physically or chemically attached to soil particles or can be trapped in the pore spaces between soil particles. Soil characteristics such as texture (percent sand, silt, and clay), structure (arrangement of the soil particles), pore space, and percent organic matter play a large role in the transport processes of contaminants through soil. For example, chemicals can move quickly through sandy soils because compared with silt and clay they have larger particle sizes, larger pore spaces, and lower organic matter; therefore, contaminants do not attach easily to sandy soils and they drain quickly. Chemicals can attach more easily to soils with a higher content of clay and organic matter. In addition, sandy soils generally do not contain a large amount of soil organisms compared with other soil types. Soils with more aggregation and smaller pore spaces will slow the movement of water and generally have a higher diversity and population of soil organisms that can metabolize the contaminant. Soil adherence to skin is also a function of soil type and particle size. Other properties of soil that could affect chemical fate and transport include soil pH, ionic strength, and presence of other pollutants.

Certain physicochemical properties of the pollutant are also important (see box). Physicochemical properties data will help determine whether a chemical is likely to remain in the soil, partition to other media, or transform physically, chemically, or biologically after release. Climatic conditions such as precipitation can also determine how contaminants are physically transported through media (e.g., by contributing to runoff, drainage, leaching).

Special Considerations for House Dust

Pollutants can accumulate in house dust over time and become “trapped” in carpets, curtains, and upholstery. House dust is not as exposed to moisture, sunlight, and the frequent temperature/climate changes that typically aid in the breakdown of chemicals in the outdoor environment. In addition, pollutants may be less mobile in indoor environments where dispersion is limited (Paustenbach et al., 1997).

Tools for assessing exposure to contaminated soil and indoor (settled) dust (via ingestion or dermal contact) are provided in this module. Indoor settled dust includes particles in building interiors that have settled onto objects, surfaces, floors, and carpets. These particles may include soil particles that have migrated or been tracked in from the outdoors. Settled indoor dust and airborne indoor dust that has been inhaled and subsequently swallowed represent potential ingestion exposures (U.S. EPA, 2011).

Outdoor settled dust includes particles that have deposited (by wet or dry deposition) onto outdoor objects and surfaces. It is not possible to distinguish between soil and outdoor settled dust; outdoor settled dust would generally be present on the soil surface. Therefore, when talking about soil and the soil ingestion pathway, we are including both soil and outdoor settled dust (U.S. EPA, 2011).

Tools for evaluating inhalation exposures to indoor or outdoor dust that is airborne are provided in the Air Module in the Media Tool Set of EPA-Expo-Box. It is important to recognize the cycle of deposition and resuspension for some contaminants, in which the inhalation, dermal, and ingestion exposure pathways might all be relevant.

There are a number of sources that provide information that is useful in predicting fate and transport of contaminants in soil and dust.

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Data

There are a number of sources that provide data that are useful in predicting environmental fate and transport of contaminants in soil and dust. Environmental fate and transport information is sometimes obtained from field studies. These studies can help us understand what happens to parent compounds and their breakdown products after they are released to the environment. Data generated can provide quantitative environmental fate characterization to assist in the estimation of exposure to a chemical.

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Models

Fate and transport models can be used to estimate concentrations of contaminants at the point-of-contact for a receptor population. Models might also be used to describe the multimedia transport and fate of pollutants from soil to other media. A variety of mathematical methods or models—each with specific data needs—are available or are under development to characterize soil contamination and describe the multimedia transport and fate of pollutants in the environment. There are also several resources that contain information on input parameters necessary for fate and transport models.

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