3.3.2 Types of Decision Units

Two primary types of DUs are suggested: those based on the known or suspected locations and dimensions of source areas, called “source area DUs,” and those based on the size assumptions of risk assessment, called “exposure area DUs.”

A source area is defined here as a discernible volume of soil (or waste or other media) containing elevated or potentially elevated concentrations of contaminant in comparison to the surrounding soil. Source areas include the following:

  • areas with stained soil, known contamination, obvious releases
  • areas where contaminants were suspected to be stored, handled, or disposed
  • areas where sufficient sampling evidence indicates elevated concentrations relative to the surrounding soil over a significant volume of contaminated media

This definition highlights the difference between types of DUs. Source area DUs are differentiated from exposure area DUs in that the boundaries of source area DUs and the scale of sampling are based on the known or hypothesized extent of the contamination, while the boundaries of exposure area DUs are determined through the exposure assumptions of the risk scenario. There are a number of purposes for differentiating so exactingly between these two types of DUs. This approach is intended to simultaneously do the following:

  • address the concern that source areas will be “diluted out” through the use of ISM sampling by emphasizing the importance of proper DU designation for source areas
  • reiterate that action levels derived from risk assessment scenarios are based on exposure assumptions that include a specified areal extent of contamination within which a mean concentration is of interest