3.2 Nature and Intent of Decision Units and Sampling Units

Environmental decisions are often based on the risks resulting from exposure to estimated mean concentrations of contaminants in volumes of soil. In some cases, a decision for additional investigation or remedial action might be made based on a comparison of ISM sample results to published screening levels. In other investigations the estimate of the mean contaminant concentration provided by ISM samples might be used to estimate the risk to human or ecological receptors. ISM results may also be used to estimate background concentrations or to assess sources or to evaluate various stages of remedial activities. (See Section 7.2.4 for further discussion.)

ISM is a method for estimating the mean concentration of contaminants in specified volumes of soil called DUs.

  • A DU is the smallest volume of soil for which a decision will be made based on ISM sampling.
  • An SU is a volume of soil from which increments are collected to determine an estimate of the mean concentration for that volume.

While these two concepts are closely related, the subtle differences between them allow for great flexibility in how ISM data may be used to make decisions for volumes of soil. Although it is not necessary to subdivide DUs into component SUs, the option to do so allows additional types of decision mechanisms to be used with ISM data. (Decision mechanisms are discussed in Section 7 and are defined in the glossary as an algorithm or protocol that results in a decision for a volume of media).