3.2.1 Planning DUs and SUs
The size, shape, location, depth, and number of DUs and SUs must be clearly identified during planning. As discussed in Section 2, all contaminant concentrations in soil are heterogeneous on some scale. Therefore, the determination of the sampling scale and the related increment density is very important in all sampling situations. If a finer resolution of contaminant variability is needed to address the objectives of the investigation, then the scale of the DU is too large. On the other hand, excessively small DUs are impractical at some point. Determining the size, shape, location, depth and number of DUs is a critical component of the planning process. Likewise, the strategy behind the use of SUs as well as their size, shape, and other attributes must be carefully planned.
Because decision mechanisms and the designation of DUs and SUs are integrally related, the anticipated decision mechanism must also be considered along with the layout of the DUs and SUs during systematic planning. Each of these variables should be considered in relation to the CSM and should support and elucidate the project objectives. Basic questions about the intent of the investigation should be considered; for example, "How do the DUs and SUs fit into the overall objectives of the investigation?" and "How will the resulting data be used in the decision mechanism to address the project objectives?" are crucial. The answers to these questions should flow naturally from the understanding of the site, the CSM, and the project objectives. The designation of DUs and SUs should support and clarify the objectives of the investigation, and as those objectives are refined further during latter stages in the life cycle of the site, the DUs, SUs, and decision mechanisms should be reconsidered.