A.6.4.6 Large-scale (long-range) DH
Large-scale DH occurs due to nonrandom spatial and/or temporal distribution of bulk material particulates in the DU. The DU usage, natural, and temporal factors (e.g., floods, drought, weather patterns, matrix interactions, and multiple uses over time) contribute to large-scale DH.
For DUs with large-scale DH, the mean concentration is not the same at all locations of the DU. In statistical terms, this variation leads to a DU with multiple populations, or ISs that are not “identically distributed.” Large variability in analytical results among replicate ISM samples does not necessarily mean that the DU is heterogeneous. For example, DUs with lognormally distributed analytical results considered in other sections of this appendix represent homogeneous DUs with large variabilities (CV = 4); the DU considered in Scenario M3-D of this appendix also represents a homogeneous DU with large variability.
Large-scale DH cannot be addressed (or identified) by ISM. When large-scale DH is present, the objective should be delineation. To some extent, this can be achieved using a modified version of ISM in which the DU is subdivided into different SUs (see Section 4).