Hyperlink 9. What Area/Volume Does the Sample Represent?

With a limited number of discrete samples, the reasonableness of data extrapolation can be questioned. ISM addresses these problems. The spacing of increment locations and the shape of DUs are designed during up-front systematic planning. Consider Figure H9-1 and suppose the kidney-shaped light blue area portrays an area of surface soil for which a decision needs to be made. The area is on the order of 1/8 acre. In Scenario A, only two discrete samples are taken. In Scenario B, ISM is used. In Scenario A, the representativeness of the two discrete samples is unknown. Theoretically, one could say that each represents 1/16 acre. However, there is no objective reason to assume those samples represent the spatial area of interest. A practitioner could just as well say the two samples represent the rectangular area outlined in gold as say they represent the oval area outlined in purple. If the two results are very different and significant short-scale heterogeneity is suspected or found, a cautious practitioner may feel that each sample result can only be trusted to represent a small area around its location (i.e., the small blue circles around the sampling points). Without more data, there is no basis for deciding how far extrapolation of sample results should go.

Figure H9-1. Discrete and ISM sampling approaches for an irregularly shaped DU.

Figure H9-1. Discrete and ISM sampling approaches for an irregularly shaped DU.

In contrast, notice the intuitive appeal of Scenario B. Increments were collected from all over the area of interest and conform to the spatial boundaries of the area. When the increments are physically combined and averaged into the incremental sample, it is obvious that that single incremental sample represents the targeted area.

A real-world example is shown in Figure H9-2. For this site, DUs were selected to encompass particular hydric and density of increments are shown for each DU.

Figure H9-2. Example of multiple irregularly shaped DUs

DU1 - 2.32 acre forested tidal wetland characterized by a predominately flat topography

DU2 - 3.49 acre forested wetland with tidal tributary associated with Baily Creek moving through, characterized by moderately sloping topography

DU3 - 5.203 acre brackish tidal marsh north of Baily Creek, flat topography with saltmarsh cordgrass, saltmeadow grasses and big cordgrass in higher elevation areas

DU4 - 1.62 acre forested upland buffer with steep changes in elevation

DU5 - 0.88 acre stream bed of Baily Creek

DU6 - 1.346 acre brackish tidal marsh south of Baily Creek, flat topography with saltmarsh cordgrass, saltmeadow grasses and big cordgrass in higher elevation areas

Figure H9-2. Example of multiple irregularly shaped DUs.