3.3.3 Vertical Definition of DUs

DUs consist of volumes of soil. Therefore the depth (how far below ground) and interval (vertical dimension of the DU) of each DU must be carefully considered during planning stages. These attributes should be based on the project objectives and the CSM and should not be left to haphazard decisions in the field. Identifying the correct depth for sampling is not a simple undertaking (true no matter what type of sampling is conducted). Additionally, it is important to remember that a correctly defined DU includes the requirement that all hypothetical increments within the DU have an equal likelihood of being sampled. Therefore, a DU should not be defined to be 5 feet deep when only the first few centimeters are available to the sampling device.

Like all DUs, exposure area DUs necessarily include vertical as well as lateral components. When exposure area DUs are used, the depth and interval of DUs should be defined consistently with the exposure scenario under consideration. In many such scenarios the first few inches or centimeters at the surface is the appropriate sampling interval. However, the DU depth and interval considered acceptable for evaluation of direct exposures varies among agencies (ITRC 2007a) and risk scenarios. Evaluation of risks posed by future excavation and spreading of deeper contamination to the surface could require DU depths many feet below ground surface.

The vertical dimensions of the DU might also be determined using assumed, current, or future subsurface activities at the site. In this case, evaluation of soil leaching risks to groundwater or surface water might mean that the base of contaminated soil (if known) is designated as the vertical limit of the DU. State requirements might also serve to define the depth and interval of the DU.

When source area DUs are used and the CSM provides an understanding of where the contaminants are most likely to be located, DU depth should be based on this information. Many contaminant releases originate at the surface; therefore, once again, the first few inches or centimeters may be the appropriate sampling interval. If, however, the CSM indicates that contaminants are more likely at greater depths the DUs should be targeted accordingly. Cleanup decisions may also drive the depth of DUs; the depth interval of a remediation “lift” (e.g., a 6‑inch scrapping by a bulldozer) is often a rational approach for defining DUs. Geological strata which may affect contaminant transport may also be a method to determine the vertical extent and depths of DUs. Finally, it should be noted that DU depths may also depend on the types of contaminants involved.