EPA-Expo-Box (A Toolbox for Exposure Assessors)
- Fate & Transport
- Calculation Tools
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 Aquatic Media|
|Transfer – Environment to Biota||
For additional information related to the environmental fate and transport of chemical contaminants in water and sediment, refer to the Water and Sediment Module of the Media Tool Set.
Bioconcentration refers to direct transfers of the chemical from the surrounding environmental medium into the animal—it does not account for uptake by ingestion. For a fish, bioconcentration of a substance in the water includes direct uptake from water through its gills. Bioaccumulation is the uptake of a substance through ingestion of contaminated plants or animals (i.e., indirect food chain uptake). In many cases, the term bioaccumulation is used as a general term to refer to the uptake of a substance from an environmental medium through both direct and indirect routes. Some chemical pollutants can bioaccumulate in fatty tissues or bind to muscle tissue of fish and shellfish. Even very low concentrations of these pollutants in the water or sediment can result in fish or shellfish tissue concentrations high enough to pose health risks to consumers.
Bioaccumulated contaminants might be transferred up the food chain—a process referred to as trophic-level transfer or biomagnification (see figure on right). Consider a simplified fish food web; omnivorous and carnivorous fish would accumulate more of a contaminant through their diet than planktivorous fish due to the transfer of chemicals up the food chain (i.e., through consumption of other contaminated animals). This has implications for humans who tend to eat fish that are higher in the food web.
In natural environments, the ratio of the chemical concentration in an animal to the chemical concentration in its environment (through all routes, including food chain transfers) generally is referred to as a bioaccumulation factor, or BAF. A BAF relates the concentration of a contaminant in fish tissue to the amount of chemical to which the fish is exposed through ingestion of food as well as through direct contact. A bioconcentration factor (BCF) can be measured, but must be evaluated under controlled situations to avoid indirect uptake through the food chain since it is the ratio of chemical concentration in the animal to chemical concentration in the water only. A biota-sediment accumulation factor, or BSAF, is analogous to a BAF; it is an empirical partitioning ratio relating concentration in sediment to the concentration in an aquatic organism, including benthic organisms and higher trophic level fish. U.S. EPA Biota-Sediment Accumulation Factor Data is a resource that contains approximately 20,000 biota-sediment accumulation factors (BSAFs) from 20 locations, mostly Superfund sites, for nonionic organic chemicals and pesticides; fresh, tidal, and marine ecosystems are included in the data.
In general, chemicals with BAFs or BCFs greater than or equal to 1,000 (equivalent to Kow of 4.2) are considered high concerns for bioaccumulation; chemicals with values below 250 are deemed as low concerns and the rest are classified as medium concerns. BAFs/BCFs greater than 5,000 (equivalent to Kow of 5.0) indicate chemicals that are of high risk concern (U.S. EPA, 2010, 2000a)).
The environmental fate of a chemical contaminant in water will be dictated by its chemical and physical properties and its propensity for biotic and abiotic transformation. A summary of key physicochemical factors that are likely to affect partitioning and fate of chemical contaminants in aquatic media is provided below.
|Physicochemical Property||Definition||How Does This Property Affect Partitioning and Fate in Aquatic Media?|
|Half-life||Time required for one-half of the original mass of the chemical to be degraded, transformed, or destroyed in a particular medium.||
Values can provide an indication of persistence in sediment, surface water, or biota. For the same chemical across different media, half-lives can vary by orders of magnitude. For the purpose of defining chemicals that are persistent, EPA sets half-life criterion of > 2 months for water and sediment. Chemicals with a half-life of > 6 months are considered a “high risk concern” (U.S. EPA, 2010)EPA’s GCSOLAR is a program that computes half-lives of pollutants in the aquatic environment.
|Vapor pressure||Indication of how likely it is that a compound will evaporate or convert from the liquid phase to 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 water).|
|Henry’s law constant (KH)||Ratio of vapor pressure to water solubility; provides an index of partitioning for a compound between atmospheric and aqueous phases||Higher values of KH are associated with compounds that preferentially partition to air rather than to water.|
|Water solubility||Measure of the maximum amount of a chemical that will dissolve in pure water||Compounds with high solubility are likely to be mobile in water and are less likely to sorb to sediment or suspended particles in water or to bioaccumulate, and they are usually biodegradable.|
|Lipophilicity||Ability of a chemical to dissolve in fats, oils, lipids||The higher a chemical’s lipophilicity, the greater its potential to bioaccumulate in aquatic organisms (plant or animal).|
|Octanol/water partition coefficient (Kow)||Partitioning of organic chemicals between octanol (a nonaqueous, nonpolar solvent and reasonable surrogate for lipids, fat) and water||Higher Kow indicates that more of the chemical partitions into octanol (and an affinity for lipids); a lower Kow value typically correlates with a higher water solubility and suggests the compound partitions preferentially to water. The Kow is correlated with the potential for a chemical to bioaccumulate in organisms. BAFs/BCFs of 1000 and 5000 are equivalent to log Kow values of 4.2 and 5, respectively (U.S. EPA, 2010).|
|Ratio of the sorbed concentration (to suspended particles in water, sediment) to the concentration of the chemical dissolved in the aqueous solution||Indicates a compound’s potential to bind to sediment. Partitioning behavior will depend on the components of the solid matrix, the physical complexities of the solid matrix, and other factors. As a result, Kd is highly variable across different environments.|
|Organic carbon/water partition coefficient (Koc)||Ratio of chemical sorbed to organic carbon (component of suspended particles and sediment) to the chemical concentration dissolved in the surrounding water||For suspended or benthic sediments that have very low organic carbon, the amount of sorbed organic chemical will also be low (and dissolved concentration in water will be relatively high) and vice versa.|
Characteristics of the aquatic environment—including surface water flow rate, temperature, and pH, as well as meteorological factors such as sunlight and precipitation—can also impact fate and transport. For example, water solubility is a pH- and temperature-dependent parameter. In general, as temperature increases, the solubility of given solid increases. In contrast, the solubility of a gas generally decreases with increasing temperature. Acidic (low pH) conditions in surface water tend to increase the solubility of metal salts that might otherwise precipitate out of solution. So, acidic conditions can lead to higher dissolved concentrations of some metals, sometimes resulting in more toxic conditions for organisms living in the water.
Organic vs. Inorganic and PBTs
Organic compounds that enter water can be transformed in two ways—by abiotic or biotic processes. During abiotic processes, compounds chemically react and degrade in the environment through reactions initiated or assisted by exposure to light, water, or oxygen. During biotic processes (biodegradation), breakdown of the chemical occurs by a biotic organism, such as bacteria. The reaction or degradation products can themselves be contaminants of concern. (See the Other Organics Module of the Chemical Classes Tool Set for more information and resources for assessing exposure to organic compounds.)
Inorganic compounds also undergo transformation reactions, including some that are similar to the chemical and biological reactions that involve organic compounds—for example, reactions with light, water, or oxygen. However, inorganic compounds cannot be broken down beyond the metal or other species that is the basis of the compound—that is, they cannot be “completely” degraded. (See the Inorganics and Fibers Module of the Chemical Classes Tool Set for more information and resources for assessing exposure to inorganic compounds.)
Changes in speciation and complexation reactions are important for inorganics. These can result from reduction/oxidation reactions (redox reactions) that change the valence of the inorganic species and can affect the solubility, mobility, or other characteristics of the substance. Precipitation and dissolution of inorganic compounds in water are also important processes that directly affect the subsequent fate and transport of metal salts and other compounds.
Persistent, bioaccumulative, and toxic (PBT) contaminants are chemicals (organic or inorganic) that are persistent in the environment, bioaccumulate in food chains, and are toxic, posing risks to aquatic systems and human consumers of aquatic biota. For the purpose of defining chemicals that are persistent, EPA sets a biodegradation half-life criterion of >2 months for water and sediment. Chemicals with a half-life of >6 months in water and sediment are considered a "high risk concern." Contaminants with a Kow value that is ≥4.2 are considered to have high bioaccumulation potential. Toxic chemicals are those that are associated with a range of adverse human health effects, including effects on the nervous system, reproductive and developmental problems, cancer, and genetic impacts. They also have the potential to pose a risk via food chain toxicity. EPA priority PBTs include polychlorinated biphenyls (PCBs), dioxins, various pesticides (e.g., aldrin, dieldrin, chlordane, DDT, hexachlorobenzene, mirex, toxaphene), mercury, and alkyl-lead. Profiles of priority PBTs are available here: http://www.epa.gov/pbt/pubs/cheminfo.htm.
PCBs, dioxins, and many pesticides are semivolatile organic compounds meaning they have slow volatilization rates from the solids (e.g., sediments) or liquids (e.g., water) that contain them. They have low solubility in water and exist mostly sorbed to particles (i.e., sediment, suspended materials in water).
EPA has identified PCBs as an important chemical risk concern from fish consumption because they are so resistant to breakdown in the environment, and exposure can result in severe health effects in humans. PCBs build up in fish to levels hundreds of thousands of times higher than the levels in water (http://www.epa.gov/pbt/pubs/pcbs.htm). The Other Organics Module of the Chemical Classes Tool Set provides information and resources on assessing exposure to PCBs.
In general, very low levels of dioxins are found in water. In aquatic systems, dioxins undergo sedimentation and burial in aquatic sediments where degradation then occurs at a very slow rate. Dioxins are very persistent and highly lipophilic, so they have great potential to bioaccumulate in aquatic organisms. They can also biomagnify up through the food chain. The Other Organics Module of the Chemical Classes Tool Set provides information and resources on assessing exposure to dioxins.
Like PCBs and dioxins, some pesticides are very resistant to breakdown and biomagnify up through the food chain. For example, it can take more than 15 years for DDT to break down in the environment. DDT breakdown products—DDD and DDE—DDE and DDD—are also PBT pollutants. Many of the PBT pesticides such as DDT, dieldrin, chlordane, hexachlorobenzene, and mirex are banned from use in the United States (since the 1970s or 1980s), but due their persistence are still lingering in the environment. See the Pesticides Module of the Chemical Classes Tool Set for more information and resources on assessing exposure to pesticides.
Burning coal leads to emissions of elemental mercury and divalent mercury. Divalent mercury can deposit to surface water, where it can be transformed to methyl mercury (MeHg) by anaerobic microbes. This chemical transformation is of particular concern because MeHg readily bioaccumulates in fish (unlike divalent mercury), and MeHg is a potent neurotoxin in humans. MeHg builds up more in some types of fish and shellfish than others, depending on what the fish eat, which is why the levels in species vary. MeHg accumulates in the muscle tissue of the fillet and cannot be removed by trimming fat from the fish or cooking it, adding to the potential for exposure. EPA provides information related to the fate and transport of mercury in aquatic environments in Volume III of its 1997 Mercury Study Report to Congress. The Inorganics and Fibers Module of the Chemical Classes Tool Set provides information and resources on assessing exposure to mercury.
Inorganic lead may bioconcentrate in some aquatic biota, particularly benthic organisms such as bottom feeding fish and shellfish like mussels. Biomagnification of inorganic lead is not believed to be significant in aquatic organisms; however, alkyl-lead compounds might significantly accumulate in both fish and shellfish. Alkyl-lead accumulates in "soft tissues" particularly the liver, kidneys, muscles, and brain. The Inorganics and Fibers Module of the Chemical Classes Tool Set provides information and resources on assessing exposure to lead.
Environmental models can help to inform the fate and transport and the environmental concentration components of exposure assessment. Monitoring data can be used with environmental fate and transport models to better characterize exposure concentrations for aquatic media. When measured concentrations are not available, models can be used to estimate media concentrations and potential exposure concentrations in lieu of environmental data.
A bioaccumulation model can be used to predict fish tissue concentrations as a simple linear product of food or media concentrations and a bioaccumulation factor. A bioenergetics model is a type of bioaccumulation model that accounts for the exchange of chemical mass between multiple levels of a food web (i.e., biomagnification). Models that may be useful in estimating uptake and bioaccumulation of chemicals in aquatic biota are described below.