18.104.22.168 Comparison of ISM means and "not to exceed" basis regulation
Regulatory requirements in some situations compel evaluation of concentrations within an area on a “not-to-exceed” basis. This may include screening levels, action levels, or leachability values, depending on the state. In this situation, derivation of a mean concentration by ISM alone does not satisfy the requirement. The development of statistical approaches that use ISM data in some form to estimate variability across the sampled area could overcome this challenge and allow ISM to be used in these circumstances, as discussed in Section 4.
Under some state regulations, leachability-based cleanup goals may be considered to be not-to-exceed values for any single, discrete sample collected within the targeted volume of soil. In this situation, ISM samples would not allow a direct comparison to cleanup objectives; however, it is important to consider that the cleanup goals need to consider not only the concentration but the mass of the contamination. ISM data can provide an estimate of the mass of the contaminant within the DU. This allows for better comparison to the cleanup goals. As an update to the survey, some states are moving in the direction of emphasizing mass of contamination over leachability goals by establishing a minimum volume of soil and contaminant concentrations that need to be considered for potential leaching hazard.