Element 2: Environmental Fate and Transport
This section describes factors to consider when evaluating fate and transport of environmental contaminants, the second element of the exposure pathway evaluation. It provides guidance for issues that you might need to consider on some sites. Not every site requires a comprehensive fate and transport analysis to categorize exposure pathways.
“Fate and transport” refers to how the nature of contaminants might change (chemically, physically, or biologically) and where they go as they move through the environment. Fate and transport evaluations help you determine how likely it is that 1) contaminants have moved or will move beyond the source area, and 2) contamination could migrate and exposures could occur beyond the sampled areas. The fate and transport analysis is generally a qualitative exercise that does not require quantitative evaluations (modeling studies).
Depending on your site, you might consider different types of information when evaluating fate and transport. The following categories of information may be useful:
- Possible transport processes that can carry a contaminant away from its source.
- Physical, chemical, and biological factors that influence the persistence and movement of a contaminant within and across environmental media, which can be important in determining whether opportunities for human exposure exist.
- Site-specific environmental conditions, such as climate and topography, that determine how contaminants move through the environment at a given location.
The extent to which you examine fate and transport issues depends on many factors, such as the availability of site-specific environmental data sets, the complexity of site issues, and community health concerns. If you determine that the nature and extent of contamination in all relevant media have been adequately characterized after reviewing pertinent studies, you might need little or no fate and transport evaluation. But if you are not able to adequately characterize the fate and transport of contamination, you cannot rule out that contaminants traveled to relevant site-specific media.
You can often obtain pertinent fate and transport information in site investigation reports. All Superfund remedial investigation reports, for example, include contaminant- and media-specific fate and transport information. When evaluating and interpreting fate and transport information, you might need to consult technical experts (e.g., hydrogeologists, air modelers), especially when more quantitative analyses are needed to characterize affected media.
When Is a Fate and Transport Evaluation Required?
An evaluation might be required to answer questions such as these:
- What is the likelihood of contamination migrating from a surficial aquifer to a deeper aquifer that serves as a drinking water source?
- What is the direction and path of a particular groundwater plume?
- What is the potential for soil or sediment contaminants to accumulate in plants, animals, or fish?
- What is the likelihood of a groundwater contaminant volatilizing and migrating via vapor intrusion into indoor air?
- What is the likelihood that degradation of volatile organic compounds is producing additional contaminants that may be of health concern?
Using Judgment When Evaluating Fate and Transport
Health assessors often use their professional judgment when evaluating environmental fate and transport. For example, suppose that at your site, there was a massive release of polychlorinated biphenyls (PCBs) into a river, and sampling studies found elevated levels of PCBs in fish tissues. Based on your understanding of how PCBs bioaccumulate, you can safely assume that part of the PCBs detected in the fish probably originated from the spill. You do not have to run a hydrology and bioaccumulation model to prove that fate and transport exists, nor do you have to step through every contaminant and physical property of PCBs to evaluate their fate and transport.
Fate and transport are interdependent processes. Fate is what eventually happens to contaminants released to the environment. Some fraction of the contaminants might simply move from one location to the next; other fractions might be physically, biologically, or chemically transformed; and others still might accumulate in one or more media. Transport is the movement of gases, liquids, and particulate solids within a given medium and across interfaces between water, soil, sediment, air, plants, and animals.
When evaluating sites, you need an overall appreciation of the primary release processes, intermedia transfer mechanisms, and transport pathways that might influence the ultimate fate of site-related contamination. Depending on site issues, understanding these basic fate and transport mechanisms (see table below) can help you understand the implications for possible past, current, and future exposures. In addition, find out how chemical and site-specific factors may affect contaminant transport in this fate and transport resource.
Ways That Fate and Transport Mechanisms Can Influence Potential Exposure Points
Understanding a contaminant’s physical, chemical, and biological properties (see table) can provide insights into its behavior in the environment and help you focus on transport mechanisms of possible significance. However, it’s important to remember that the scientific community’s understanding of chemical and physical properties is the result of laboratory studies in highly controlled conditions.
This contaminant property information does not allow us to always predict completely how contaminants will behave under real-world environmental conditions. Laboratory studies also do not usually reflect the multiple variables and influences found in the environment such as chemical mixtures, biological and chemical processes in the environment, and geochemical conditions of soils and geologic materials. Thus, health assessors should not rely too heavily upon theoretical and laboratory studies to predict the fate and transport of site-specific contaminants. Site-specific environmental measurements that reveal how much and where contamination exists are always preferred.
|The maximum concentration of a chemical that dissolves in a given amount of pure water.||Environmental conditions, such as temperature and pH, can influence a chemical’s solubility, which, in turn, also affects a contaminant’s volatilization from water. Solubility provides an important indication of a contaminant’s ability to migrate in the environment: highly soluble compounds will tend to move with groundwater, while insoluble compounds do not.|
|Density of liquid||A liquid’s mass per volume.||For liquids that are insoluble (or nonmixing) in water, liquid density plays a critical role. In groundwater, liquids with a higher density than water (called dense non-aqueous phase liquids or DNAPL) may penetrate and preferentially settle to the base of an aquifer, while less dense liquids (called light non-aqueous phase liquids or LNAPL) will float.|
|Vapor pressure||A measure of the volatility of a chemical in its pure state.||Vapor pressure largely determines how quickly contaminants will evaporate from surface soils or water bodies into the air. Contaminants with higher vapor pressures will evaporate more readily.|
|Henry’s Law Constant||A measure of the tendency for a chemical to pass from an aqueous solution to the vapor phase. It is a function of molecular weight, solubility, and vapor pressure.||A high Henry’s Law Constant corresponds to a greater tendency for a chemical to volatilize to air.|
|Organic carbon partition coefficient (Koc) (often called the adsorption coefficient)||The sorption affinity of a chemical for organic carbon and consequently the tendency for compounds to be adsorbed to soil and sediment.||A high Koc indicates that organic chemicals bond tightly to organic matter in the soil so less of the chemical is available to move into groundwater or surface water.
|Octanol/water partition coefficient (Kow)||A chemical’s potential to accumulate in animal fat by representing how a chemical is distributed at equilibrium between octanol and water.||A contaminant with a higher Kow is more likely to bioaccumulate.|
|Bioconcentration factor (BCF)||A measure of the extent of chemical partitioning at equilibrium between a biological medium, such as fish or plant tissue, and an external medium, such as water.||This factor can be qualitatively used to evaluate the potential for exposure via the food chain. A high BCF represents an increased likelihood for accumulation in living tissue.
|Transformation and degradation rates||Consider physical, chemical, and biological changes in a contaminant over time.||Chemical transformation is influenced by hydrolysis, oxidation, photolysis, and biodegradation. A key transformation process for organic pollutants is aqueous photolysis, often in the form of photochemical reactions. Chemical transformation is expressed in different rates, including reaction rate constants and half-lives.
Biodegradation is a significant environmental process in soil. Precise estimations of chemical-specific transformation and degradation rates are difficult to calculate and to apply because they are subject to site-specific physical and biological variables.
Media-specific half-life provides a relative measure of how persistent a contaminant might be in a particular environmental medium.
ATSDR’s Toxicological Profiles (Tox Profiles) are a unique compilation of information on hazardous substances. They include details on various topics such as contaminant-specific properties, fate and transport, and where contaminants are found in the environment.
Many climatic as well as geologic and hydrogeologic factors can affect—speed up, slow down, or even stop—how contaminants move through the environment and whether human exposures occur. For example, precipitation, topography, hydrology, hydrogeology, and soil type indicate how quickly water-soluble contaminants will enter groundwater, while temperature and other factors affect whether and how quickly contaminants will volatilize into the air. Pertinent information is usually documented in site investigation reports and other information sources.
Climate can be important when determining the likelihood of contaminant movement in a setting. The following are some of the key climatic factors affecting environmental fate and transport:
- Annual precipitation and evaporation rates can help determine how surface-water runoff, groundwater recharge rates, and soil moisture content could be influencing contaminant migration at a site. Precipitation also promotes the removal of particulates and soluble vapors from the atmosphere. Note that the land topography and local surface water flow patterns will affect the migration and deposition of contaminants.
- Temperature conditions affect the volatilization rate of contaminants: chemicals are more likely to evaporate in warmer environments. In addition, ground temperature can affect the movement of contaminants as frozen ground cover can increase runoff and inhibit groundwater recharge. Also, frozen soils can act as a cap and increase the lateral spread of soil gas.
- Wind speed and direction influence the dispersion and volatilization of airborne contaminants, as well as fugitive dust generation rates. Knowing the prevailing wind patterns for a site can help provide a qualitative understanding of downwind locations, increasing your ability to accurately evaluate potential air exposures. However, you should not rely solely on the prevailing wind direction when identifying potentially exposed populations. For example, prevailing wind directions may suggest areas of long-term pollutant effect from a particular emissions source, but winds may also periodically blow from other compass directions during certain times of the year. Therefore, emissions may have short-term air quality effects in all compass directions around a site, with the extent of these effects determined by how often a location was downwind from the facility. For some sites, you may evaluate wind roses, which are graphical displays of the statistical distribution of wind speeds and wind direction measurements. See the resources below for examples of wind roses and details on how to read them.
- Seasonal conditions can be a major factor affecting contaminant migration if precipitation and temperatures vary greatly from one season to another. For example, the extent and distance of contaminant migration will be dramatically different during a period of heavy rain versus a heavy snow. Another factor affected by seasonal conditions is the soil vapor intrusion pathway (refer to resources in the box), where homes are more tightly sealed in cold and hot weather, thereby reducing the rate at which fresh outdoor air comes into homes and replaces indoor air.
Geologic and Hydrogeologic Conditions
Geologic and hydrogeologic conditions will influence how fast and in what direction contaminants in soil and groundwater might move, and ultimately if and how contaminants might reach people. Consider these conditions when deciding whether available sampling data are sufficient to characterize exposure points. GRASP can help develop maps to illustrate these conditions at your site (see example GRASP maps). The following are some key considerations:
- Groundwater hydrology and geologic composition affect the direction and extent of contaminant transport in groundwater. To understand a site’s groundwater flow patterns, review site reports or U.S. Geological Surveyexternal icon or state geological surveyexternal icon data to identify groundwater flow direction, hydraulic conductivity (water-transmitting characteristic), gradient, water table contours, and possible discharge points (e.g., seeps, springs, surface water). Note: when available, site-specific information is most desirable because non-specific maps may not accurately represent site-specific hydrologic and geologic conditions.
- The physical characteristics of aquifers beneath or near a site, especially the porosity and permeability of their geologic materials, will greatly influence the vertical and lateral movement of contaminants in groundwater. Note the presence and continuity of confining layers and rapid recharge areas, such as sinkholes and solution channels. Be aware that factors such as discontinuities in the aquitard, overpumping of the lower aquifer, and poorly installed or maintained wells piercing the aquitard can all lead to contaminant migration from an upper aquifer to a “protected” lower aquifer.
- Depth to groundwater—or the depth of the water table—is a key consideration when evaluating whether vapor-forming contaminants (e.g., volatile organic compounds) from groundwater might evaporate and migrate into indoor air. Shallow aquifers, particularly water tables at or just below building foundations, pose a greater threat for migration of vapor-forming chemicals than deeper water tables. Contaminated shallow groundwater tables may also be a direct contact concern should the water table rise during seasonal weather variations, extreme weather events (e.g., flooding of a basement through sump pits), or digging into the soils.
- Wells installed within aquifers can affect groundwater flow and direction. Pumping rates of high-capacity municipal, industrial, or agricultural wells can influence localized groundwater flow patterns and could affect contaminant transport in the capture zone (the aquifer in the area surrounding the well).
- Soil characteristics, such as configuration, composition, porosity, permeability, and cation exchange capacity ultimately influence the rates of percolation (or rainwater infiltration), groundwater recharge, contaminant release, soil gas migration, and transport. For example, you might view a site composed largely of clay soils differently from a site with sandy soils since many contaminants tend to adsorb readily to clay. Regardless of soil type, however, the greatest sorption will typically be to organic material.
Remember: Human-made objects, such as sewers, culverts, and drainage channels, can create or alter exposure pathways by changing the movement of contaminants through the environment.
- Ground cover and vegetative characteristics of the site influence rates of soil erosion, percolation, and evaporation. Releases to a paved surface may be carried long distances by surface water runoff, while releases to soils might be confined to a smaller area.
- Topography, the relative steepness and elevation of the site, will affect the direction and rate of surface water runoff, the rate of soil erosion, and the potential for flooding.