3.5 Hot Spots

Historically, discrete soil sample results with concentrations above an action level have often been assumed to represent a significant volume of soil containing sufficiently high concentrations of contaminant to warrant concern. These assumed volumes have been considered to represent hot spots. The relative nature of this definition results in a wide range of interpretations and typically leads to subsequent remobilization and resampling intended to define the extent of the hot spot, often with insufficiently specified objectives. (Note that ITRC’s Use of Risk Assessment in Management of Contaminated Sites [ITRC 2008] lists various state criteria and guidance values for hot spots).

To further complicate the issue, the terms “hot spot” and “source area” are sometimes used interchangeably. In this document the distinction is that information about the location and likely extent of source areas are known or postulated based on a CSM at the outset of a sampling project; source area designation relies on more information than the interpretation of yet-to-be-obtained sampling results. The locations and dimensions of source areas therefore may be established or hypothesized a priori. Waste disposal units, spills, releases, and volumes of soil shown by previous sampling to have significant contaminant concentrations relative to the surrounding soil are defined as “source areas” in this document. In contrast, hot spots are considered to be soil volumes with relatively high concentrations that could be present at a site but whose locations and dimensions cannot be anticipated prior to sampling. The designation of a hot spot based on sample data alone has decision-making value only when the chemical criteria (how “hot”) and spatial dimensions (what “spot”) which will define hot spots are specified. Thus, these criteria must be agreed on by the planning team before sampling. As mentioned above, some states have established criteria for defining hot spots; therefore, it is highly recommended that project teams include their state regulators during early planning and that the project team understand the basis of the criteria used to define hot spots.

Effective detection and delineation of hot spots in heterogeneous soil matrices is a challenge. Results from both traditional low-density discrete sampling approaches and ISM sampling approaches have constraints and uncertainty associated with their interpretation. While, with enough samples, discrete sampling designs provide the user with concentration data on smaller spatial scales from very small discrete areas, there is often lack of sufficient coverage to detect the inflection points in concentration gradients that would establish the boundaries of hot spots. The primary problem with using discrete samples to search for unknown hot spots is that when a few discrete samples are collected in the presence of small- and/or medium-scale heterogeneity, isolated high concentration results can be misinterpreted as high concentrations of contaminants over significant volumes of soil. Similarly, ISM sampling using low-density or noncontiguous coverage of small hot-spot DUs over an area of concern may also be an ineffective means to detect and delineate hot spots. Areas of elevated concentration could be detected using very high-density ISM or high-density discrete sampling approaches, but for practical reasons such methods are seldom employed.

When used over relatively large areas (perhaps the size of quarter- or half-acre residential yards or larger), ISM typically captures the broad effects (i.e., proportional representation and thus higher average concentrations) of hot spots due to the improved spatial coverage within the DU, but it does not provide information on the spatial location of smaller volumes of soil containing hot spots of contaminants within the DU, nor does it indicate the magnitude of these areas of elevated concentration if they exist. To detect and delineate potential hot spots using ISM, DUs must be scaled down to be consistent with the area and depth (or volume) of soil of potential concern for hot spots. In other words, to detect a hot spot of a given size, the spatial dimensions of the DUs have to be that size or smaller. Additionally, the hot-spot DUs need to contiguously cover the area suspected of containing hot spots. While smaller DUs may provide better spatial resolution, as discussed above with discrete sampling approaches, there are practical limits on the number of DUs that can be designated, sampled, and analyzed. Therefore, using ISM to detect relatively small hot spots may also be infeasible in many situations.

To avoid the pitfalls of “chasing” areas of elevated concentration with no predefined boundary conditions and for the data to be useful for project decisions, ISM practitioners must predefine the area and depth of concern and the chemical criterion that will be used to define hot spots as part of the systematic planning process. It is also encouraged that the planning team understand and agree on the basis of the criteria to facilitate later decisions that will be made with the hot-spot DU data.

The definitions of source areas and hot spots provided in this guidance are intended to promote meaningful discussion on the purposes, limitations, ability, and need to detect and characterize volumes of soil smaller than exposure areas when concentrations are highly variable in heterogeneous particulate materials such as soil. If detection of contaminated volumes such as these is an important objective of an investigation, careful planning is vital.