3.3.5 Source Area Decision Units
Source areas are of concern because contamination can migrate from source areas to other locations and media (e.g., leaching to groundwater, volatilizing to soil gas and/or indoor air, or running off to surface water). Source areas can also result in additional releases, direct exposures, and other issues associated with gross contamination (e.g., risk of explosion, nuisance issues, or inappropriate disposal). The identification and characterization of source areas is an important and generally necessary part of a typical investigation. Source area DUs can be identified using various methods, including observation, review of site records, preliminary samples, field analytical samples, wide-area assessments, aerial photographs, interviews and site surveys.
Ideally, source areas are identified based on knowledge of the site before DU designation and subsequent ISM sampling. However, source areas can also be discovered through the interpretation of sampling results, whether discrete or ISM. When sufficient sampling evidence indicates high concentrations of contaminants are present in soil, it may be possible simply to make a decision using those sample results. However, in other situations it may be necessary to refine the sampling plan, redesignate DUs (perhaps on a revised scale) and resample.
Once identified, source areas should generally be designated as discrete DUs and sampled accordingly.Once identified, source areas should generally be designated as independent DUs (source area DUs) and sampled accordingly. If source areas are successfully demarcated from surrounding soil, ISM is a useful approach to determining the mean concentrations of contaminants within these source areas. DUs derived from exposure areas (exposure area DUs) are not generally recommended for source areas because the environmental hazards represented by the source areas are not directly related to the concept of exposure areas or the scale they represent.
Note that the definition provided here for source area does not presume that any particular type of decision mechanism or action level is required. Sample results derived from source area DUs may be compared in an appropriate decision mechanism to any type of action level. Decision mechanisms are discussed in Section 7, and source area sample results may be compared to any type of action levels (e.g., derived from risk assessment procedures, state or federal regulation, background estimates, disposal requirements, or any other selected approach).
A final point concerning source area DUs: In many situations where the source area is known to be highly contaminated, discrete samples may be the best option. In this situation, even though it is likely that contaminant concentrations are highly heterogeneous, any discrete sample is likely to result in a correct decision because the concentrations are elevated.