3.3.6 Decision Units Surrounding Source Areas
Contamination often migrates away from source areas over time. In this situation, DUs must be selected to identify boundaries of contamination with sufficient resolution to meet the project objectives. However, the boundaries between source areas and clean soil are often not evident prior to sampling. Demarcation of these boundaries is often an important goal of investigation. As discussed throughout this document, discrete sample results can be subject to potentially extreme variability. Therefore, a few discrete samples are not necessarily reliable for demarcating this boundary. As depicted in Figure 3-6, a series of concentric DUs sampled using ISM may be the best alternative. In this situation hypotheses are formed concerning the limits of the contamination associated with the source area and the resolution required for the project. These hypotheses are then tested using ISM samples associated with DUs surrounding or downgradient of the source area. Concentric DUs may be appropriate when there is no readily apparent likely migration direction from the DU. Available information concerning the limits of source areas should always be used when determining DUs for ISM samples. Visual clues and field instrument data in particular should be used if available.
Recall that source areas are defined as volumes of media containing “elevated concentrations,” which are generally of concern due to their potential for leaching, vapor intrusion, or acute exposure. They are not simply volumes of media whose concentrations exceed action levels derived using chronic exposure assumptions. Therefore, after moving away from the known source area, the goal of the investigation changes from delineation of the boundaries of “elevated concentrations” within the source area to determination of DUs whose mean concentrations exceed chronic action levels. In this second situation, exposure area DUs may be more appropriate.
A source area is a discernible volume of soil (or waste or other media) containing elevated or potentially elevated concentrations of contaminant in comparison to the surrounding soil.Additional DUs are suggested in the situation depicted in Figure 3-6, because the CSM indicates that the contamination gradually diminishes in the lateral directions away from the source. To include increasingly larger volumes of soil in a single DU in this situation (for example, combining DUs 1–4 in Figure 3-6 into one DU) eventually results in dilution of the estimated mean concentration of the contaminant in the DU. This is a fact of environmental investigation; however, it is not unique to ISM sampling. Decisions based on discrete and composite sampling must also eventually be based on sample results which represent specified volumes of media, and the dimensions of those volumes of media must be determined with sufficient resolution. This rule always brings up the question “How much soil does a sample represent?” In the case of ISM, the investigator must make reasonable hypotheses about the volume of soil that is contaminated around a source area, sample that volume, and make decisions based on the results. Those DUs surrounding the known source area will be compared to action levels derived from chronic exposure in exposure areas; they are no longer source area DUs. In many situations, the volume of soil surrounding a source area that is hypothesized to be contaminated and then tested with ISM samples will be relatively small in comparison to human health exposure areas. This means that decisions based on action levels derived from chronic exposure scenarios will be more conservative than those based on large exposure areas.
In summary, definition of source area DUs should be based on an understanding of the boundaries of the known or suspected source as reflected in the CSM. Rote sizes derived from the exposure areas in risk assessments are not recommended. However, once the investigation has moved away from the discernible boundaries of the source area, DU size and shape must be selected in a manner which reflects reasonable hypotheses about contaminant migration away from the source. Further away from the source, DUs should be based on exposure areas.