3.3.7 Subsurface Decision Units

Because of the frequency with which subsurface contamination is encountered, subsurface DUs are an important application of ISM sampling. Some CSMs may suggest that contamination extends only to depths of a few centimeters. But in other situations the volume of interest may be situated entirely below ground surface (bgs). Therefore, DUs are inherently 3-D and necessarily extend some depth into the subsurface. Vertical depths and intervals must be carefully considered based on the CSM, previous data, screening results, and applicable state laws. Objectives for the investigation of subsurface soils might include assessment of the following:

  • leaching of contamination from soil to groundwater
  • volume of contaminated soil that may need to be removed or properly managed
  • potential for subsurface soil to be excavated during future site development and spread out at the surface, posing direct-exposure hazards
  • grossly contaminated soil

Subsurface soils should be subdivided into DUs (or SUs) in layers based on clues from the CSM and the project objectives.Subsurface soils should be subdivided into DUs in layers based on clues from the CSM and the project objectives. One to several layers may be necessary. The DUs should indicate the vertical limit of contamination. Initial subdivisions may later be revised if more exact thickness resolutions are necessary for remediation decisions than for initial investigations.

Ideally, subsurface DUs should be investigated in a manner that allows every possible increment in the DU an equal likelihood of being collected. Sampling theory also suggests that the entire cross section of the DU be sampled in each increment making up the ISM sample. In practice, however, the combined mass of the increments from a large number of borings would likely result in a sample volume that is impractical. Therefore, field subsampling plans or other compromises may be needed.

In addition to sufficient lateral coverage as with surface DUs, sufficient vertical coverage in subsurface DUs is an important consideration. Sampling approaches for subsurface soils differ from those applied to surface soils because access to the subsurface is more difficult. This does not mean that low-quality data are unavoidable for subsurface soils; sufficient coverage of the DU at depth is still necessary, and improving sampling techniques support higher-density sampling and thus higher-quality concentration estimates for subsurface soils. Section 5 goes into further detail on sampling techniques for subsurface soils.

The thickness of each DU is based on balancing factors such as the desired resolution of the investigation, potential disposal costs, and the actual time and cost of the investigation. The cost of collecting subsurface ISM samples must be balanced with the cost of analyses as well as the limitations of discrete samples discussed throughout this document. Discrete subsurface soil samples and/or field analytical methods can provide useful screening data prior to, or in conjunction with, subsurface ISM investigations. The results of the screening investigation can be used to determine the number, location, and dimensions of subsurface DUs and ISM samples.

It is important to note that assessing the potential intrusion of vapors into existing or potential buildings may be necessary. Collection of discrete soil gas samples is the recommended sample method for evaluating the vapor intrusion to indoor air route of exposure. During initial site screenings there may be some benefit for the collection of “bulk” soil samples for VOC analysis. ISM might be a useful method at some sites where VOCs may be present and should be used on a case-by-case determination. USEPA Method 5035 should be considered and used where appropriate. Several states have guidance on characterization of the vapor intrusion pathway and individual state guidance should be consulted as appropriate.

Example subsurface source area decision units

Examples of subsurface contamination include soil that has been capped by a layer of clean fill, paving material, or building slabs; leaking underground storage tanks (USTs); buried pipes; buried disposal sites; and surface spills that have spread downward. Figures 3-7, 3-8, and 3-9 depict potential approaches for investigation of shallow, deep, and isolated subsurface DUs.

Figure 3-7 depicts a former pesticide mixing operation with known spills and releases. Contaminants include pentachlorophenol, dioxins, furans, and triazine pesticides. Environmental hazards posed by these contaminants include direct exposure, leaching and contamination of groundwater. The CSM depicted in Figure 3-7 indicates that contamination extends from the ground surface downward to a relatively shallow depth that can easily be reached with a backhoe. Therefore, excavation provides easy access to the desired sample depth. Because the evaluation of leaching potential is a project objective, a focus on source area DUs is appropriate.

One-meter vertical resolution DUs selected to help isolate heavily contaminated soil from less-contaminated soil and assist in evaluation of remedial alternatives.

Figure 3-7. One-meter vertical resolution DUs selected to help isolate heavily contaminated soil from less-contaminated soil and assist in evaluation of remedial alternatives.


It is assumed that the lateral boundaries of the source area have already been determined. Most of the contamination is believed to be restricted to the upper 2–3 m of soil and will be excavated and disposed at an off-site facility. Investigation objectives include the following:

  • determine the vertical extent of contamination
  • identify and separate potential hazardous wastes

In the example shown in Figure 3-8, the CSM suggests that much deeper soils are of interest because a long-term release from the source has migrated to deeper soils. Therefore, a series of vertically stacked source area DUs extending from the ground surface to depth was chosen. Soil borings were necessary to collect the sample increments. Depending on local conditions, direct-push technology might also have been useful.

A hypothetical investigation of series of stacked source area DUs using borings.

Figure 3-8. A hypothetical investigation of series of stacked source area DUs using borings.


In the final example (Figure 3-9) contaminated soil is believed to have been overlain by clean soil, so once again soil borings are necessary. Figure 3-9 depicts an example where shallow contaminated soil has been excavated but a deeper unit of contaminated soil remains at depth. DU designation and investigation of the situations described in Figure 3-8 and in 3-9 are similar.

An example of a subsurface DU for contaminated soil overlain by clean soil that is not accessible by excavation.

Figure 3-9. An example of a subsurface DU for contaminated soil overlain by clean soil that is not accessible by excavation.


Although not shown, 30 borings per DU with incremental samples from borings across the entire depth of the DU is recommended to maximize vertical coverage.