8.5.4.7 Background/geochemical limitations

Comparison of site data with background data may be necessary to establish the extent to which chemicals present are naturally occurring. As discussed in Section 4.4.3.3, the two key challenges for ISM are the likelihood of detecting differences in the populations that exist (ISM background data to ISM site data) and the inability to evaluate upper tails of the background to site underlying distributions. In addition, decision errors may be affected if the background samples are collected with different sampling designs from the site samples, including different number of increments/replicates, different sample masses, sampling protocols, depth intervals, and sampling patterns. Therefore, the results of hypothesis tests applied to ISM data sets should be interpreted with caution until these limitations can be more thoroughly studied. Even if formal statistical tests are not used, simple graphical analysis (e.g., plots grouping ISM results by study area) may be informative as a semiquantitative method for comparing background and site distributions.

Comparison of site ISM data to background discrete data using either hypothesis testing or upper tolerance limits is not recommended because the variance is represented differently in ISM and discrete sampling. Careless comparison of an ISM estimate of the mean to a discrete sample collected from soil representing background is likely to lead to decision errors in which one incorrectly concludes that the contaminant distribution on site is consistent with background conditions.