4.4.4 Oversized DUs

Generally, DUs should be no larger than the exposure units used for risk assessment if risk assessment is likely to be needed for the site. However, this limit may be impractical under some circumstances. Examples might include an acute exposure scenario (e.g., single soil ingestion event for a small child) or ecological risk assessment for a species with a very small home range. In these situations, DUs are by necessity oversized, and the average concentration for the DU provided by ISM offers only a crude approximation at best of concentrations that might exist for individual exposure units within the DU. Extrapolations of estimates of dispersion (e.g., SD or CV) across DUs to calculate a 95% UCL or other upper-bound statistic should be performed with caution, as discussed above (see Section 4.4.2).

Note: A computationally equivalent approach is to use the average concentration but divide the soil criterion by the number of increments.

Another approach is to use estimates of possible upper-end concentrations within a DU to evaluate potential “worst-case” situations, but the information to derive these estimates is limited due to the nature of ISM. This is not a new issue, and an analogous problem exists for composite samples. The literature for composite sampling contains a number of approaches for estimating high-end concentrations within the sampled area. The simplest of these is to multiply the mean value from the composite (or ISM sample) by the number of increments. This method represents the situation in which all of the contaminant is present in one of the increments. Given the number of increments in a standard ISM design, this approach is extraordinarily conservative and can yield quite high values. Other approaches that are less conservative include multiplying the average concentration by the square root of the number of increments or more complicated formulas (Barnett and Bown 2002). It would be advantageous to explore approaches to “decomposite” data in the context of ISM for situations in which the upper end of the concentration range within a DU is an important component of meeting site DQOs.