3.3.4 Exposure Area Decision Units

DUs based on exposure areas are a fundamental part of many environmental investigations and are a key tool in risk assessments and risk-based decision making. For the purposes of this document, an “exposure area” is defined as an area where human or ecological receptors could come into contact with contaminants in soil on a regular basis (refer to Risk Assessment Guidance for Superfund, Vol. II [USEPA 1989b]). Examples include residential yards, schoolyards, playgrounds, gardens, areas of commercial/industrial properties, or areas designated as exposure areas through other means (e.g., state laws). Figure 3-3 depicts various types of exposure area DUs.

Figure 3-3. Examples of potential exposure area DUs.

The primary use of data from an exposure area is to estimate exposure and, subsequently, risk to human health and the environment. The data may also be used to screen sites for further study using criteria such as risk-based screening levels. (See USEPA's Soil Screening Guidance for guidance in development and application of screening levels to assess soil leaching potential [USEPA 1996a, 1996b]). This objective may be accomplished by comparison of the estimated mean concentration in the DU to action levels. If the project is more mature, data may be used to develop exposure point concentrations (EPC)s to quantify risks from exposures to contaminants by human and/or ecological receptors.1 When the decision will be based on risk assessment approaches, the DU should ideally be based upon the area where exposure is or potentially could occur. The size and placement of exposure areas depend on current use or proposed future use of the site.

When systematic planning considers data collection to support risk assessment, a primary question is "Over which area and depth do samples need to be taken to represent exposures of concern?" The exposure area could be based on current land use or the likely or possible future use of the area. In some cases, however, data and risk assessment may be required to inform the necessity for deed restrictions for a less likely land use. Site-specific information and the CSM should be used to designate exposure areas as much as possible. In cases where future land use is uncertain, location of future residences, for example, areas suspected of contamination should be sampled at a scale that is consistent with the presumed future land use as much as possible. The size and placement of exposure area DUs may need to be adjusted and the resultant uncertainties documented relative to potential future exposure and risk.

1 It should be noted that detailed exposure assessments may require evaluations of bioavailability or relative bioavailability of media contaminants, and exposure DU ISM samples may be used for this purpose. However, to be representative of exposure, bioavailability studies must be performed on ISM samples that have not been processed by grinding. If other DQOs being fulfilled by ISM samples in the exposure DU require grinding as part of sample processing, the comparability of ground vs. unground samples should be evaluated as part of the study.