3.3.4.3 Ecological exposure area decision units

Data collected to support ecological risk assessments are based on assumptions similar to those made for human health risk assessments. However, the differences between human health and ecological risk assessment mean that the DUs derived for each of these efforts may be quite different. The selection of the exposure area size during the refinement phases of a baseline ecological risk assessment should be representative of a mean EPC. A further consideration for selecting DUs for ecological risk assessment is that more than one ecological end-point species is typically evaluated and their exposure areas may overlap in some cases or be distinct in others. Obtaining the assistance of a trained ecological risk assessor/toxicologist for exposure area and DU selection and early planning in each phase of an ecological risk assessment is highly recommended.

ISM data can be used to represent estimates of maximum exposure for the purposes of screening level assessments or assessment of threatened and endangered species. This would generally be a sufficiently conservative assumption when other conservative screening level assumptions such as 100% bioavailability and 100% of the diet coming from the most contaminated media are used, but is an assumption that must be agreed on prior to sampling. When ISM is used, the project team must determine a minimum spatial scale for exposure area DUs, often based on the home range or foraging range for representative species. However, since many ecological receptors have exceedingly small or exceedingly large home ranges, it is not always possible for the scale of sampling to exactly correspond with the scale that reflects the wildlife receptors, especially when multiple receptors (multiple species) are considered. In other words, excessively small exposure area DUs may be impractical, while excessively large exposure area DUs may be meaningless. Several options could be considered: use of other sampling methods (see Section 3.1.4.2), extrapolation from sampled to unsampled areas (see Section 7.2.6), and use of statistical approaches to provide conservative estimates of exposure concentrations (see Section 4.4.4). Note that some regulatory agencies do not accept certain options.

Sampling designs for ecological risk assessments must consider not only the size and location of exposure area DUs, but also the sample processing procedures. These procedures must be consistent with the technical requirements and objectives that mandate the exposure assumptions (USEPA 1997).