18.104.22.168 Smaller source area decision units within exposure area DUs
In some situations, it may be advisable to designate smaller source area DUs or SUs within larger DUs, based on an understanding of contaminant distributions. Assessment of a smaller subarea might be motivated by the suspicion that concentrations are higher in that location relative to the surrounding soil. Depending on site-specific conditions and contaminants, there could be concerns about acute, subchronic, or chronic exposures to soil in these smaller DUs. A common example is the need to investigate soil around a house suspected to be contaminated with lead-based paint chips. The perimeter of the house may be designated as a separate DU and characterized separately from the larger exposure area DU consisting of the entire yard. In this example, there might also be a need to designate a separate DU because of the need to address the potential presence of large particles (i.e., large chips of potentially lead-contaminated paint) to meet the project objectives for FE. For example, particle size reduction and increased sample mass for extraction might be necessary (see Section 4 for further details).