Department of Health Seal

TGM for the Implementation of the Hawai'i State Contingency Plan
Section 4.0
CHARACTERIZATION OF DECISION UNITS

4.0 CHARACTERIZATION OF DECISION UNITS

Section 3 discusses the importance of Decision Unit (DU) designation as part of the Systematic Planning process of an environmental investigation. A DU is and area and volume of soil for which a decision is to be made. In most cases this will involve an estimation of the mean concentration of contaminants of concern for each DU. This Section discusses the use of Multi Increment sampling methods to accomplish this objective.

Ideally, the entire, targeted volume of soil or other targeted media (e.g., sediment, water or air) included in a DU would be collected and sent to the laboratory for analyses. This is of course not practical under most circumstances and a representative sample (or samples) of the media must instead be collected and tested. It is important that the selected sampling approach generates precise and unbiased (“accurate”) data that meet the objectives of the site investigation. Understanding the factors involved in collecting a representative sample is therefore critical and the essence of the field of sampling theory.

1Multi Increment sampling (“MI sampling or “MIS”) methods are recommended for characterization of a DU. This sampling approach, long used in the mining and agricultural industries, is specifically designed for characterization of soil and addresses short comings of traditional discrete soil sampling methods. Of particular importance is the ability of MI sampling methods to overcome and represent small-scale, random variability of contaminant concentrations in soil that plagues traditional discrete sample site characterization approaches.

The section begins with a brief overview of “scale” in environmental investigations and the use and misuse of concepts such as “hot spots.” The results of a detailed field investigation carried out by the HEER Office in 2014 are used to demonstrate how inherent random, small-scale variability of contaminant concentrations in soil limit the usefulness of discrete sample data. The predictability of this variability is discussed in terms of sampling theory. Assumptions regarding an anticipated small-scale “uniformity” of contaminant concentrations in soil served as the basis for much of the discrete sampling site investigation guidance written in the 1980s and 1990s. Limitations on the use of discrete sample data in site investigation is summarized. A detailed analysis of these topics is provided in the reports prepared for the 2014 HEER Office field study (HDOH, 2015, b).

The section focuses on background and use of Multi Increment sampling methodologies to characterize DUs. Topics addressed include Multi Increment sample collection, laboratory processing, use of replicates to evaluate data precision, collection of subsurface Multi Increment samples, and use of MIS for volatile chemicals and characterization of stockpiles.

The document Incremental Sampling Methodology (ISM), published by the Interstate Technology Regulatory Council (ITRC), is referenced in parts of this Section (ITRC 2012). Several staff from HDOH as well as Hawai‘i consultants assisted in preparation of the guidance. The ITRC document provides a basic overview of sampling theory and “incremental sampling” methods as well as examples of Decision Unit designation under different site scenarios. The document is especially strong in laboratory processing of “incremental” samples. Discussion of the collection of incremental samples in the field is basic, due in part to the lack of significant field experience (at the time) among members of the ITRC ISM team.

The discussion of the limitations of discrete soil sampling methods in the ITRC document is incomplete, however, with the potential impression that incremental sampling and thus “Multi Increment” sampling methods are simply one available alternative to traditional discrete sampling methodologies. This is not the case and was one motivation for the more detailed, HDOH field study of discrete sample variability and reliability in 2014 (HDOH 2015). At the time the ITRC document was prepared, an analysis based on field studies of discrete sample variability and reliability was lacking (the statistical analysis included in the ITRC document was based on a computer-generated database). As discussed in detail in this Section, it should be emphasized that traditional discrete soil sample data, while potentially useful for large-scale screening purposes, fail to meet basic requirements of sampling theory for the collection of representative data and should not be used for final decision making purposes. Decision Unit and Multi Increment sampling methods are not simply “another tool in the toolbox”. This sampling strategy addresses serious deficiencies of past discrete sampling methods, and represents an entirely new set of science-based tools. DU-MIS methods are recommended to obtain scientifically defensible and representative data for contaminants in soil and sediments on projects overseen by HDOH.


1Multi Increment® is a registered trademark of EnviroStat, Inc.