3.3 Decision Units

3.3.1 Defining Decision Units

There are various approaches to defining DUs. The approach selected should be consistent with the understanding of the site reflected in the CSM and should support the objectives of the investigation. DUs may be defined in regularly spaced and equal volumes as established by exposure areas, or they may be based on irregular features of the site which define contaminant transport or receptor exposure. Alternatively, DUs may be based on an understanding of the contaminant distributions, for example, in and around source areas. Volumes of soil known or suspected to be contaminated are generally good candidates for designation as DUs because the decision over these volumes is best made separately from less-contaminated surrounding volumes. Human health or ecological exposure areas may provide the basis for the designation of DUs. This approach has the advantage that it is conceptually supported by the exposure assumptions used to derive most action levels. DUs may also be based on the needs of remediation or excavation. For example, landfill construction or other remedial approaches may dictate the dimensions of the DU. Sidewalls and floors of excavations may be designated as DUs to determine whether soil removal was sufficient.

Selection of DUs should also consider the geologic aspects of the CSM. If the boundaries between different geologic formations are important for contaminant transport or exposure, they may provide a logical demarcation of the DU. In some cases a DU may extend across more than one geologic formation or soil type, but in other situations basing DU boundaries on the geological boundaries may make more sense. Background studies may particularly require consideration of geological conditions.