2.2 Soil Heterogeneity and Variation in Contaminant Concentrations

The ratio between particles of different sizes and densities in a soil sample has a strong influence on the resulting contaminant concentration.

Taking a scoop of soil to collect and analyze as a soil sample may seem like a simple task. The critical question is whether that scoop of soil will produce meaningful data on the scale at which a decision is to be made. In other words, will results from a tiny sample provide the “right” answer for a volume of soil millions of times larger? Complications arise because soil is made of a variety of different materials which interact with contaminants in different ways. These materials generally take the form of particles of various sizes, which are composed of various mineral and organic substances. Many different kinds of soils exist, as defined by the types of minerals present and their ratios to each other and to organic carbon content. Different kinds of soils can differ widely in their physical and geochemical properties.

As a consequence of the physical and chemical properties of contaminants combined with differences in individual soil particles, contaminant atoms and molecules bind to some particles loosely, but more tightly to others. Further description of the interactions of soil with contaminants is provided in Hyperlink 3. Therefore, a sample of contaminated soil is a heterogeneous mixture of particles that are carrying different amounts of contaminant. This phenomenon is described by terms such as compositional heterogeneity (CH), microscale heterogeneity, or within-sample heterogeneity, and it creates a “nugget effect.” “Nuggets” form when contaminants preferentially attach to certain particles rather than others, such that contaminant-laden nuggets may be present in a matrix of other particles having less or no contaminant loading. Consider the effect of nuggets on concentration. Even if only one or two of these concentrated nuggets happen to be included in a very small sample when it is analyzed, a high concentration will be reported. If those same one or two nuggets were captured in a larger sample, a moderate concentration will be reported. If by chance no nuggets are present in the analyzed sample, then a low or nondetect concentration is reported. Hyperlink 4 provides an example of the “nugget effect” and how it may lead to decision errors. This is closely related to the concept of sample support, which is further discussed in this Hyperlink 5.

In contrast to this microscale heterogeneity, which occurs within a single sample, large-scale heterogeneity refers to differences in concentration from location to location across an area, in other words, differences in how contaminants are spatially distributed throughout the DU. For example, contaminants may be released from leaking drums, creating distinct but rather small contaminated areas. Or contaminants may be released by single or multiple large-volume spills, which might create large patterns that are mostly uniform in concentration within the spill area but demarcated by a fairly sharp boundary. Some contaminants, such as pesticides, might have been sprayed only along the edges or in garden pockets of a residential yard. Or pesticide leftovers might be poured out in a single spot. Atmospheric deposition is a common release mechanism with the resulting spatial pattern affected by wind strength and direction and by distance from the source.

Short-scale heterogeneity refers to concentration differences observed at the scale of colocated samples. Colocated samples are taken from the “same” location in the field generally a few inches to a few feet apart and are traditionally considered to be equivalent, meaning that their concentrations are expected to be approximately equal. However, field experience shows that colocated samples often differ in concentration, sometimes quite drastically.

Soil heterogeneity is present at different spatial scales. Each must be considered when collecting representative soil samples.

Heterogeneity at each of these different scales poses challenges for the collection of representative samples. Each calls for different sampling strategies, techniques, and QC measures to assess and improve sampling representativeness. ISM recognizes these various scales of heterogeneity and conscientiously attempts to control their effects. The following sections further discuss the differing scales of heterogeneity, and Section 2.4 provides approaches for collecting representative samples in the face of these heterogeneities.