1.4 How ISM Compares to Compositing
Environmental professionals recognize the act of combining increments as being similar to conventional compositing. ISM is an improved type of compositing in comparison to conventional compositing in that great attention is given to establishing the DU. ISM also requires that the total sample mass be sufficient to represent the heterogeneity of soil particles within the DU in proportion to all of the DU soil (i.e., population) and that a sufficient number of equal-volume increments are collected in an unbiased manner from throughout the entire DU so that all particles in the unit have an equal probability of being included in the sample. Thus, the incremental sample has the goal to contain all constituents in exactly the same proportion as they are present in the DU (i.e., the sample is representative of the DU). Proper laboratory processing and subsampling procedures then produce an aliquot for analysis that contains all constituents of the subsample in the same proportion in which they occur in the sample and, therefore, the DU.
ITRC’s ISM Team has found that many state regulatory agencies have been reluctant to use composite sampling and that such reluctance spills over to ISM (see survey results in Section 8.4.3). One concern expressed with composite sampling is that clean or less-contaminated soil will be mixed in with contaminated soil, therefore diluting areas of high contamination. This problem can be minimized with a clear understanding of sampling objectives that incorporate the concept of a DU.
As shown in Figure 1-2, a hypothetical DU may be sampled with several designs. This figure illustrates a discrete gridded sampling design, various composite designs, and an ISM design. While each has its advantages and limitations, a goal of incremental design is to provide a high degree of spatial coverage of the DU. It is also obvious that the incremental design is similar in appearance to the various composite designs recommended by the U.S. Environmental Protection Agency (USEPA). Compositing is discussed further in Section 2.6.2.
COMPOSITE SAMPLING DESIGNS
Source: USEPA 2002e.
Compositing based on a simple random design. Samples with the same letter are composited together
Compositing based on a systematic
Compositing with a systematic Sampling Unit block design
DISCRETE SAMPLING DESIGN
INCREMENTAL SAMPLING DESIGN