3.3.9 Excavation Decision Units
Excavation of contaminated soil from source area or exposure area DUs allows ready access to the assumed outer margins of the DUs, much as excavation of soil exposed at the ground surface allows easy access to the bottom of the DUs. The sidewalls and floors of excavations can be treated as separate DUs and can be sampled for confirmation of adequate soil removal.
This ISM approach is applicable if the criterion for successful excavation is achieving an average concentration in soil below the action levels at the excavation boundaries. Small areas of contaminated soil within otherwise clean excavation sidewalls or floors do not necessarily pose a significant risk to human health and the environment. In these situations use of ISM (or any other sampling methodology) for "confirmation samples" may represent a traditional approach, rather than a sampling plan based on the risk assessment model or targeted removal of source areas. Once an excavation is made, it may logical to base subsequent soil removal in the sidewalls on visual information or field instrument data. Unlike field data, ISM samples collected across multiple depths in the sidewall may not lend themselves to the original definition of DUs based on exposure areas or CSMs.
Example excavation decision units
Figure 3-11 shows excavation sidewall and floor DUs which may be assessed with ISM samples.
Figure 3-11. Floor and two sidewall DUs for an excavation site.