# Hyperlink 16. To What Volume of Soil Do Discrete Samples Apply?

While the volume of soil represented by ISM samples is defined by the demarcation of the DUs, to be useful, discrete samples must also apply to some volume of soil in a decision. However, the volume of soil to which discrete samples apply is often ambiguous, determined after samples are collected and analyzed, and subject to the vagaries of the allocation approach used. Vastly different volumes of soil will be determined based on the allocation approach used. Some of the various allocation approaches are provided below.

- The sample applies to some arbitrary volume of soil determined through agreement, regulatory requirement, or professional judgment.
- The sample result is assumed to apply to all the soil until the next “dirty” or “clean” discrete sample is encountered.
- A sample applies to some volume of soil as in a tacit or actually calculated Thiessen polygon. Thiessen polygons are polygons which indicate the “area of influence” of a single point in relation to all other sampled points. They are determined by the perpendicular bisector of the lines between all neighboring points. Thiessen polygons may be calculated; however, it is it more often that the concept of “areas of influence” is tacitly used when discrete samples are employed to determine where samples are located and what their results mean in the assessment of volumes of soil.
- Discrete samples are used to populate iso-concentration maps where groups of samples collectively represent the concentration of contaminants in large volumes of soil based on their concentration, weighting, and the algorithm employed.
- The mean of a number of discrete samples collectively applies to some volume of soil.

Each of these approaches potentially results in different decisions over vastly different volumes of soil when using the same discrete data.** **