126.96.36.199 Example source area decision units
Figure 3-5 provides an example of how a source area might be investigated using ISM samples. The CSM suggests that contaminant concentrations should be highest immediately below the source (in this case an aboveground storage tank [AST]) and will gradually diminish with lateral distance from the source. Therefore, the soil immediately below the source area is designated as a single source area DU, while additional DUs are selected surrounding the source as discussed in Section 3.3.6.
Figure 3-5. Source area DU in a suspected release area.
Figure 3-5 depicts a source area DU established at the perimeter of a suspected release. The site is located within a small, industrial complex, and various chemicals are suspected as part of the release. It is also assumed that the greatest potential for contamination is associated with near-surface soils, to a depth of approximately 10 cm. The primary objective of the investigation is to determine the presence or absence of potentially significant polychlorinated biphenyls (PCBs) concentrations in the immediate vicinity of the pad.