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

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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 (e.g., air, soil) to foods (e.g., plant, animal)—a process referred to as bioconcentration or bioaccumulation.

Migration Process Examples Relevant to Food
  • Migration of a contaminant through soil used to grow crops; dispersion of contaminant through ambient air within agricultural areas (transport within a medium)
  • Chemicals in soil that leach to groundwater used to irrigate crops; deposition of a contaminant from air to soil used to grow crops (transport between media)
  • Organic breakdown or biodegradation of a compound in agricultural soil by soil microbes (chemical change)
  • Inorganic metals that dissolve in pore water of agricultural soil (physical change)
Transfer – Environment to Food
  • Airborne contaminants that attach to plant leaves and become translocated to different tissues of a fruit or vegetable crop
  • Soil contaminants that contact plant roots and become transported into edible plant tissues
  • Grazing or foraging animals used as a source of food consume contaminated soil, water, or plants and bioaccumulate the contaminants in their tissues

For additional information related to the environmental fate and transport of chemical contaminants from water and sediment to fish and shellfish, refer to the Aquatic Biota Module of the Media Tool Set.

What about Breast Milk?

Contaminants may find their way into human milk of lactating mothers because mothers are themselves exposed. Lipid-soluble chemical compounds (e.g., dioxin) accumulate in body fat and may be transferred to breast-fed infants in the lipid portion of human milk. Water soluble chemicals (e.g., salts of metals) also may partition into the aqueous phase and be excreted via human milk. Some researchers (e.g., Travis et al., 1988) have suggested approaches for estimating breast milk contaminant levels using biotransfer factors.

Bioconcentration refers to direct transfers of the chemical from the surrounding environmental medium into the plant or animal (e.g., a chemical that is transferred from soil into a plant through its roots); for animals, it does not account for uptake by ingestion. Bioaccumulation is the uptake of a substance from an environmental medium through all routes, including food chain transfers. For example, accumulation of substances can occur through ingestion of contaminated plants or animals. Biotransformation refers to the alteration of a contaminant in the body. In addition, some chemicals can biomagnify as they move up the food chain.

Bioconcentration factors (BCFs) and biotransfer factors (BTFs) are sometimes used to estimate contaminant concentrations in plants or animals. BCFs and BTFs are derived from information on the concentrations in the food and the amount of chemical that is taken up by the plant or animal or is present in the surrounding environmental media.

BCFs represent the relationship between the concentration of contaminant in the vegetation and the corresponding soil, water, or air concentration. For example, a soil-plant BCF measures a chemical’s ability to accumulate in plant tissue and is the ratio of contaminant concentration in plants to the concentration in soil. Similarly, an air-plant BCF (which could be mass-based or volume-based) is defined as the ratio of contaminant concentration in aboveground plant parts to the contaminant concentration in air (see below).


The BTF is an empirical ratio relating the chemical concentration in biota, such as produce, livestock, or animal products (such as eggs), to the amount of chemical to which the plant or animal is exposed in soil or feed (or other media). For example, an animal BTF for milk or beef represents the ratio of the concentration in the food to the rate of chemical intake by the animal (e.g., mass of chemical per day) as shown below.


BCFs and BTFs are sometimes estimated based on the physicochemical properties of the contaminants. For example, Travis and Arms (1988) published a methodology that uses the octanol-water partition coefficient (Kow) as a predictor of BTFs in beef cattle.

The following tools provide information on the fate and transport of contaminants in the food chain and information on BCFs and BTFs. Additional information on estimating concentrations of contaminants through the use of models may be found in the Concentrations tab of this module.

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