5.3.4 ISM Confirmation Sampling
Confirmation sampling may be performed during post-removal activities to verify that residual concentrations of target COCs are below the predetermined cleanup goals for the site. Confirmation sampling is often a requirement to achieve final clean closure certification. Confirmation samples are typically collected from the side walls and floors of an excavation to confirm that concentrations remaining after excavation are below specified concentration limits. Results from individual grab samples, an average or a 95% UCL from discrete samples, are often compared with the cleanup criteria for the site for this purpose.
The use of ISM samples to confirm excavation of a source area DU can be highly advantageous.An incremental sample result is specifically designed to estimate the mean concentration in a volume of soil designated as a DU. If excavation is performed for a site based on results from ISM sampling, it is usually because one or more DUs “failed” (i.e., had concentrations above the specified cleanup goals). Once the soil in a failed DU has been removed, the motivation for sampling the sidewalls and floor of the excavated DU is presumably to determine whether surrounding potential DUs also require remediation. If adjacent areas have already been designated as DUs, evaluated, and found to have soil concentrations within acceptable limits, confirmatory sampling in the conventional sense may not be necessary. If adjacent areas have not been adequately characterized, collecting ISM samples around the excavation can inform the need for or against further removal. In this situation, the expanded investigation requires new planning, including the designation of additional DUs and the determination of appropriate cleanup goals. One approach is to designate a volume of soil surrounding the excavated area as a new DU and sample from the walls and floor accordingly. This process is somewhat analogous to conventional confirmatory sampling. However, it is important to consider how the areas of the walls and floor relate to the volume of soil in the new DU and take increments in a manner that ensures a sample is representative of the DU. It is also important to recognize that the cleanup goal for a DU consisting of soil immediately surrounding an excavated area might be different from the original cleanup goals used for site evaluation because the objectives (e.g., addressing concern for potential for direct contact, leaching to groundwater, etc.) may be different, given the size and location of the new DU. As always, clear articulation of objectives and proper planning are essential.
In summary, the use of ISM samples to confirm excavation of a source area DU can be highly advantageous over a traditional, small number of discrete samples. The excavation floor and sidewalls should be treated as individual DUs (see Figure 3-11), with the investigation objective of determining whether the estimated mean concentration of COPCs for these areas exceeds targeted screening levels. Again, these issues should be evaluated and determined as part of the planning and DQO process. Collecting ISM samples within these areas rather than single discrete samples ensures good DU spatial coverage and a more representative estimate of mean COPC concentrations. There may be regulatory limitations to this approach, however. For example, if regulations require cleanup of releases to a not-to-exceed regulatory level (e.g., the maximum concentration determined by discrete samples), then an ISM mean concentration may not be applicable and/or accepted by the regulating authority.