5.3.2 Subsurface ISM Samples
As discussed in Sections 2 and 3, DUs are by definition 3-D in nature and are intended to focus the investigation on a specified volume or mass of soil. Obtaining good spatial coverage and data quality for subsurface soils is more challenging but is still necessary. The objectives for surface vs. subsurface investigations may be similar in nature, for example, to estimate the representative concentration of targeted contaminants for targeted depth intervals (e.g., within the defined vertical limits) or to determine or confirm the lateral boundaries of the source area. For remedial purposes, the estimation of contaminant mass within the DU is also sometimes critical (e.g., mass of tetrachloroethene for design of soil vapor extraction system or mass of dioxins for design of in situ thermal desorption system). The practical application of ISM sampling must be considered during project planning, especially when considering implementing it for nature and extent investigations of subsurface contamination. Often, alternative sampling techniques (e.g., discrete sampling, field screening, or field analytical methods) may be more applicable and/or cost-efficient.
Soil samples collected as part of a subsurface investigation are intended to be representative of a specific depth interval.Soil samples collected as part of a subsurface investigation are intended to be representative of a specific depth interval. As discussed in Section 3, this trait can be described as the resolution of the data collected. Discrete soil samples from borings or excavations have traditionally been used to characterize subsurface soils. In most cases, however, discrete samples may provide less spatial coverage of the targeted depth intervals and also increase laboratory analytical costs. As discussed below, alternative sample collection approaches to improve sample data quality and reduce laboratory costs include options for ISM core sampling across targeted depth intervals.