EPA-Expo-Box (A Toolbox for Exposure Assessors)
- Fate & Transport
As described in the EPA’s Guidelines for Exposure Assessment (U.S. EPA, 1992), exposure is dependent upon the intensity, frequency, and duration of contact. Exposure magnitude is usually expressed as the concentration of contaminant per unit mass or volume (e.g., μg/g, μg/L, mg/m3, ppm) within the environmental media to which exposure occurs.
Characterizing contaminant concentrations for an exposure scenario is typically accomplished using one or more of the following approaches:
- Sampling the bulk media with which the receptor is expected to come into contact and analyzing the media to measure contaminant concentration
- Using existing, available measured concentration data collected for related analysis or compiled in databases
- Modeling the concentration distribution based on source characteristics, media transport, and chemical transformation processes (i.e., modeling fate and transport)
Methods for Determining Contaminant Concentrations
Stressor concentrations are typically measured or estimated in air, water, soil, food or food webs, microenvironments, surfaces, biota, or a combination of any of these, depending on the purpose and scope of the assessment.
Environmental concentrations or exposures can be measured directly through media sampling or monitoring and analysis or indirectly estimated using models. A common approach for quantifying exposure for risk assessment is to combine the use of environmental monitoring data with model outputs. This approach integrates measured concentrations, information on source/stressor formation, and the measured and estimated effects of fate and transport processes.
Environmental monitoring can provide concentration data in relevant media for calculating exposure and dose at a specific location and for a given time period. Monitoring also allows us to identify and fill data gaps. By monitoring pollutants in environmental media, individuals are better able to know what is in the environment and where receptors might be potentially exposed (U.S. EPA, 1999a).
As noted above, when measured concentrations are not available, models are often used to estimate media concentrations and potential exposure concentrations. The outcome of fate and transport modeling is often a database of estimated chemical concentrations in environmental media, within microenvironments, or on surfaces. Depending on the assessment, the modeling approach can be characterized in multiple ways including:
|Mechanistic (i.e., based on theories of physical processes)||OR||Empirical (i.e., based on observed experimental data)|
|Deterministic (i.e., uses set of single point values)||OR||Probabilistic (i.e., uses distribution of point values from which single point values are selected randomly)|
|Steady-state (i.e., variables are assumed to stay the same over time)||OR||Dynamic (i.e., variables are assumed to change over time)|
Further, the fate and transport processes might be modeled based on one or more of these approaches:
- First principles – model is based only on established scientific laws and principles; no assumptions are employed.
- Partitioning – model is based on how transport and transformation phenomena influence the distribution of the substance in the environment.
- Mixing – model is based on identifying features in mixtures that allow stressors to be quantified by source.
- Bioaccumulation – model is based on the varying abilities of biological organisms to accumulate stressors over time at concentrations higher than those to which they are exposed.
Many resources are available describing modeling techniques, sampling techniques, and analytical methods employed for estimating or measuring media concentrations in air, water and sediment, soil and dust, food, aquatic biota, and consumer products.