Decision Unit and Multi Increment Sampling

Maui Wildfire Data: Decision Unit and Multi Increment Sampling

DOH evaluates risk to human health based on the average concentration of a substance within a specific “Decision Unit” or “DU” area and volume of soil, water, or sediment under question. For example, the question might be “Does the concentration of lead in the area of the beach frequently used by residents exceed a level that could pose a health risk for children who visit the beach on regular basis?” A single sample is then prepared by collecting and combining a large number of “increments” of the material from all parts of the DU. Including a large number of points, typically 50 or more, in the sample helps to ensure that small but potentially more contaminated areas of the beach or “hot spots” are included in the sample.

Note that DOH and other research has demonstrated that data for “discrete” or “grab” samples collected from few random locations within an area under question are not reliable for evaluating the health risk posed by any contamination that might be present. This is because the samples are typically too few and too small, and they are not properly processed and tested at the laboratory. The background and limited use of discrete sample data is discussed in the DOH Technical Guidance Manual (TGM, in particular Section 4 Appendices) and in the references cited below.

DOH obtains consistently reliable data by following a three-step process:

Designate site-specific, risk-based, or remediation-based areas and volumes of soil for testing, referred to as DUs.
Prepare a single, representative sample for each DU by combining a minimum of 1 to 2 kilograms of soil or sediment (for example) from many points [default 50] within each DU, referred to in DOH guidance as a Multi Increment Sample (MIS).
Properly process the sample at the laboratory (e.g., air drying and sieving to target particle size) to ensure that a representative subsample that meets minimum mass requirements (e.g., 10 grams) is collected and tested.

Sections 3, 4, and 5 of the DOH TGM discuss the basis and implementation of such DU-MIS investigation approaches.¹ The HEER Office webpage provide links to in-depth, recorded DU-MIS training webinars.

DOH’s Solid and Hazardous Waste Branch (SHWB) informally required the use of DU-MIS data at sites overseen by that office since the publication of initial HEER Office guidance in 2009 and formally adopted a requirement for the use of DU-MIS data in final decision-making in a technical memorandum dated September 23, 2019. DOH’s Clean Water Branch (CWB), working in conjunction with the SHWB, requires the collection and use of DU-MIS data for all dredged material that is proposed for reuse in upland areas.

The CWB also requires the collection of MI-type samples for monitoring turbidity in surface water during dredging projects.^2,3 The collection of DU-MIS data is also required for disposal of soil by several landfill operators in Hawaii and is being formally incorporated into landfill permits as those permits come up for renewal. In conclusion, it has been the position of the HEER Office and DOH in general for some time that, while discrete sample data can be useful for initial, gross characterization of contaminated soil and sediment and initial assessment of risk, DU-MIS data are required for final decision-making.

References

¹ Hawaii Department of Health. (2016). Technical Guidance Manual. Office of Hazard Evaluation and Emergency Response. https://health.hawaii.gov/heer/tgm

² Hawaii Department of Health. (2015, August 14). AMAP guidelines (version 1). Clean Water Branch. https://health.hawaii.gov/heer/files/2021/07/HDOH2015d.pdf

³ Hawaii Department of Health. (2015, August 14). AMAP sampling notes (helpful hints and supporting information). Clean Water Branch. https://health.hawaii.gov/heer/files/2021/07/HDOH2015d.pdf