Department of Health Seal

TGM for the Implementation of the Hawai'i State Contingency Plan
Subsection 3.5


This Section provides example decision units for commercial/ industrial, residential, school, large area, subsurface, stockpile, and sediment sites. Examples of both exposure area and spill area decision units are included. A mixture of both types of decision units is often appropriate. The examples provided are based in part on site investigations in Hawai´i, although the placement of DUs noted in the figures has been modified to reflect lessons learned or emphasize specific points discussed in the text.


Figure 3-17 depicts a simple spill area DU placed around a former transformer pad. The purpose of the decision unit is to investigate the presence or absence of PCB-contaminated soil in the immediate vicinity of the pad. An area extending approximately 3 feet out from the pad was selected as the DU. The pad appeared to drain to the side of the DU shown. A second DU of similar shape and size was placed on the other side of the pad. Triplicate Multi Increment samples were collected within the DU (i.e., three separate Multi Increment samples, refer to Section 4). The flags denote the location of increments collected for the first sample (approximately 30 increments per Multi Increment sample). Samples were to be tested for PCBs.

Figure 3-17. Decision Unit Designated to Investigate PCB Contamination Beside Former Transformer Pad DU extended outward three feet from stained side of pad. Flags denote location of increments collected for Multi Increment sample.

Figure 3-18. Designation of Spill Area (red) and Exposure Area (blue) DUs at a Former Electrical Power Plant to Determine the Magnitude and Extent of PCB-Contaminated Soil. Former Transformer Storage and Repair Operations Located in Upper Left Area of the Property

Figure 3-18 depicts DUs designated for a former electric power plant. A review of historical data and previous discrete sample data suggested potential significant contamination of soil with PCBs in the area of the property where transformers were formerly stored and repaired. Relatively small DUs are designated across this area. This provides a high resolution for the distribution of PCBs across the area in order to maximize removal or remedial options. Small, perimeter DUs are designated around this area in hopes of confirming an outer boundary of clean soil. The remainder of the property where significant PCB contamination is not anticipated is divided into somewhat larger, Exposure Area DUs appropriate for the current, commercial use of the property.

Decision units designated for a former agricultural, pesticide storage and mixing area are depicted in Figure 3-19. Relatively small (100-2,000 ft2), Spill Area DUs are designated in the former mixing tank area to evaluate potential leaching hazards posed by the triazine herbicides ametryn and atrazine (depicted in red, Figure 3-19). The DUs are designated based on obvious or suspected areas of high contamination. For example, obvious or suspected spill areas were identified on the ground under elevated mixing and storage tanks, under the floor or the storage building and in a low lying drainage area adjacent to the tanks and building (Figure 3-20). The use of small DUs helps to better assess potential leaching hazards from this area as well as optimize future remediation actions by minimizing the volume of potentially clean soil included in the DUs.

Figure 3-19. Example Spill Area (red), Exposure Area (blue), and Perimeter Area (blue, outside ring) Decision Units for a Former Pesticide Mixing and Storage Area

Figure 3-20. Spill Area Decision Units Designated Beneath Pesticide Mixing and Storage Tank

The remainder of the area is known from previous investigations to be contaminated with arsenic and pentachlorophenol-related dioxins with no known localized spill areas. This area was divided into eight 5,000 ft2 Exposure Area DUs representing hypothetical, residential lots (see Figures 3-19 and 3-20). Two rings of lot-size, perimeter DUs were then designated around the site to establish a clean boundary and ensure that contamination associated with the pesticide mixing area has been adequately defined (refer to Figure 3-19). Decision units in the outer ring (not depicted) are tested as needed if samples from an inner ring DU failed action levels.

Figure 3-21. Decision Units to Investigate a Proposed, Four-Acre Hotel Site DUs A through C represent exposure areas based on the proposed hotel design. DU-D represents a suspected spill area identified during initial site investigation actions

Figure 3-22. Example Designation of DUs Around House Perimeter and Yard

Multi Increment soil samples are collected in each DU (with triplicate samples collected in two of the DUs) and are used to evaluate direct exposure hazards and leaching hazards. The full suite of COPCs is tested for in each sample. The results of the investigation and a summary of the subsequent Environmental Hazard Evaluation are carried forward as an example in Section 13.

Figure 3-21 depicts Decision Units for a proposed commercial development on a four-acre site known to be contaminated with arsenic. The property was divided into four DUs. DUs A through C represent exposure areas. DU-D represents a suspected spill area identified during initial site investigation actions. This was an attempt to isolate the most heavily-contaminated soil on the property to as small an area as possible in order to minimize future remediation and long-term management costs.


Soil contamination concerns for residential properties are normally limited to the presence of lead-based paint residue and organochlorine termiticides (e.g., Technical Chlordane) in soil surrounding the perimeters of wooden homes constructed prior to the mid 1970s. Termiticides could also be present beneath a building slab or in soil exposed in a crawl space, or less commonly in open areas in the yard.

Figure 3-22 depicts typical decision unit designations to investigate these potential concerns. A narrow DU (or DUs) is designated around the immediate perimeter of the home, typically within three to five feet of the foundation. The perimeter could be divided into separate DUs for testing if there is a reason to think that these areas could be different (e.g., more recent utility work or landscaping along one side of the home). The remainder of the yard or different sections of the yard can then be characterized separately as single or multiple Exposure Area DUs.


The investigation of large, high-density residential areas for potential soil contamination concerns is approached in a similar manner as done for individual homes. Suspect spill areas are targeted as separate DUs for characterization (e.g., lead-based paint and termiticides around building perimeters).

The identification and management of lead-contaminated soil or soil treated with organochlorine termiticides can be a significant challenge for the redevelopment of large housing complexes. Detailed characterization around and beneath each building is often not practical (e.g., redevelopment of military or public housing complexes where dozens or hundreds of buildings will be demolished or renovated).

As an alternative, detailed characterization can be conducted for a select number of buildings (e.g., 10%) constructed during the same time period and by the same builder, with the assumption that the use of lead-based paint or termiticides around the buildings would be similar. The results could then be applied to the remainder of the buildings in order to prepare initial soil management plans. Stockpiled soil following home demolition should be tested prior to reuse or disposal if past treatment with termiticides is possible.

Figure 3-23. Example Designation of DUs for Investigation Pesticide-Contaminated Soil at a Public Housing Complex

Figure 3-24. DUs Designated to Test for the Potential Presence of Lead-Contaminated Soil at a School

As discussed in Subsection 3.4.4, the presence of termiticide-treated soil under building pads can be evaluated by the collection of soil samples from a small number of borings through the building slab. The combined sample is processed and tested in the same manner as done for a MI sample. Although the resulting data will not be reliable for estimation of mean termiticide concentrations under the slab, the presence or absence of the chemicals can be used to prepare initial soil management plans for the complex as a whole.

Open areas planned for use by residents can also be tested separately as Exposure Area DUs. This can include play areas or lawns used for recreational activities. Exposure Area DUs can also be designated for yards located in suspect areas of the property where the specific location of potential spill areas is not known.

Figure 3-23 depicts DUs designated for a public housing complex suspected of being constructed in an area where pesticide mixing and storage may have taken place in the past. Relatively small (100s to a few 1,000 ft2) DUs were designated for characterization in areas of highest concern. Designated DUs included small backyards, large open areas and playground areas. The DUs reflect exposure areas appropriate for the housing complex and also provide good resolution of mean pesticide concentrations in soil across the site.


Designation of DUs for characterization of potential soil contamination at schools typically represents a combination of approaches used for commercial/industrial facilities and high-density residential complexes. Potential spill areas should be characterized separately with DUs sized to reflect the suspected extent of contamination, as well as to optimize anticipated removal or remedial actions. These could include maintenance yards, suspected termiticide-treated soil around buildings perimeters, garden areas where persistent pesticides may have been used in the past, and other areas of bare soil where children or staff may have periodic exposure.

Figure 3-24 depicts DUs designated for a school to test for the presence of lead-contaminated soil associated with dumping prior to construction of the campus. The DUs largely reflect easily recognizable exposure areas. This might include playgrounds or other gathering areas as well as gardens or open areas between buildings.

In this example a focus was made on barren areas of soil exposed in otherwise thick lawns including soil along walkways, under outdoor tables and in areas of high foot traffic. Field screening of samples from barren areas within the main campus was carried out using a portable XRF (see Figure 3-24). Soil from subareas of the upper campus was ultimately combined and tested as a single MI sample after field screening indicated similar low levels of lead within the preliminary DU as a whole.


Figure 3-25. Grouped Lots for Decision Units at a Proposed Residential Site Exposure area DUs for a former golf course based on clusters of planned houses. Red cross-hatched areas indicate suspected arsenic-contaminated soil as determined by field-based XRF.

Figure 3-26. Apparent leak under valve of large fuel tank designated as a small DU in order to document the presence of a release.

The configuration of DUs with respect to the planned redevelopment might also be desirable, although this could complicate usage of the data should redevelopment plans change in the future. An example is depicted in Figure 3-25. In this case, a former 100+-acre golf course, the property was known to contain elevated concentrations of arsenic that would require partial removal. Development plans were used to divide the property into DUs that consisted of four to five housing units each. Each DU was then tested separately for arsenic. A backhoe was used at each increment location to collect samples at depth. This allowed a three-dimensional image of soil that exceeded cleanup levels to be developed and incorporated into the site grading and soil removal plan.


Characterization of DUs as small as a few square feet and/or a very small volume of soil or sediment might be required under some circumstances. Examples include testing of upper few inches of soil under a leaking tank valve to document the presence of a release (Figure 26) or testing of sediment in a storm sewer vault to assess runoff from a known or suspect, contaminated area (Figure 3-27). Another example includes the desire to test a small, easily accessible portion of a suspected much larger spill area, for example the rapid collection of a soil sample from an illegal dump site (Figure 3-28). This might be done in order to document the presence of a release, identify specific contaminants of concern and establish the relative magnitude of potential contamination within the larger area as a whole.

Figure 3-27. Stormwater drainage vault designated as a DU for testing of sediment runoff from a contaminated property. Representative sample or entire volume of sediment is collected and submitted to the laboratory for processing and analysis.

Figure 3-28. Designation of small, targeted DU within a suspected much larger area of contamination at an illegal dump site in order to document presence of contamination and identify potential contaminants of concern.

The size of DU will necessarily be site specific but should be made large enough to minimize “false negatives” and collection of a sample from a single and inadvertently clean location (e.g., three feet-by-three feet or approximately one-square meter). This is similar to the concept of “judgmental,” “biased” or “subjective” sampling under past, discrete sample collection methodologies. The data collected in judgmental sampling is only representative of the specific DU area tested, however, and while it can be used to initiate the need for a larger-scale investigation it cannot be used to quantitatively assess overall risk posed by the release.

In such cases it is important to specifically designate a DU and associated DQOs for characterization and to collect the sample in a nonbiased, “probabilistic” or “statistical” manner that ensures the resulting data will be representative of the targeted soil or sediment (refer to Section 4). This includes the need to collect a minimum, 1-2 kg of material in order to address Fundamental Error and to collect the sample from multiple, random locations within the DU in order to capture and represent random, small-scale heterogeneity (see Subsection 4.2.5). The sample must be processed for analysis at the laboratory in accordance with Multi Increment protocols again in order to address Fundamental Error (e.g., dried, sieved and subsampled; see Subsection 4.2.6). If the soil is to be tested for volatile chemicals then multiple increments from the DU can be placed into methanol in the field and submitted to the laboratory for analysis (minimum 300g recommended; see Subsection

This approach achieves the same objectives as traditional, judgmental or “grab” discrete samples but provides much more defensible data for decision making. The potential for “false negatives,” where a small mass of unrepresentatively clean soil is inadvertently collected and/or tested by the laboratory, is also minimized (refer to Subsection 4.3).

In some cases collection of the entire DU volume of soil or sediment as a single sample might be feasible. Examples include small volumes of sediment in stormwater sumps being tested to assess contaminant runoff from a property (Figure 3-27). This ideal scenario, referred to as to as "direct inference" (AAFCO, 2015), negates the need for the collection of a Multi Increment sample (and replicates) and eliminating potential field error (e.g., <2kg of material present). Importantly, the sample must be processed and subsampled for analysis at the laboratory in accordance with Multi Increment protocols in order for the data to be considered representative (refer to Subsection 4.2.6).

The concept of very small DUs also applies to targeted “DU Layer” of soil or sediment in a core where decisions are to be made on data for single boreholes (see Subsection 3.4.4). If the targeted interval of the core is less than a few feet in length then it is usually practical and even desirable to submit the entire core interval to the laboratory for processing and testing. In other cases subsampling of the core will be required to reduce the sample to a manageable mass (see Subsection and Section 5).


Figure 3-29 depicts designation of a thin horizon that represents the top of a former dump area as a subsurface DU for characterization. The area was covered by clean fill material following closure of the dump. Subsequently, buried debris was removed as part of a redevelopment project. Soil around the perimeter of the former dump was tested for the presence of heavy metals and dioxins to determine if additional excavation was required.

Figure 3-29. Subsurface Burn Layer Targeted for MI Sample Collection Following Excavation of Former Dump Area

Figure 3-30. Cross Section of Subsurface DU Layers Designated at Former Pesticide Mixing Area Facility (white indicates anticipated clean soil)

The pesticide mixing area depicted in Figures 3-19 and 3-20 is used to depict a more detailed designation of DU layers for vertical characterization of the extent and magnitude of contamination. The resulting data might, for example, be used to estimate the volume and mass of soil that require excavation and disposal or the total mass of a contaminant present in soil for design of in situ treatment. Both the area and volume of DUs should be summarized in investigation work plans.

Refer again to Figure 3-19. Eight DUs of approximately 5,000 ft2 each were designated for the outer area known to be contaminated by arsenic and dioxins. These contaminants primarily pose direct-exposure concerns. The DU area reflects the default size recommended for a hypothetical, single-family home residential lot. Three DUs are designated for the inner area contaminated with triazine herbicides (ametryn and atrazine), ranging in area from approximately 1,000 ft2 to 2,000 ft2. These areas primarily pose leaching and groundwater impact hazards.

Subsurface DU layers designated for the site are depicted in Figure 3-30. The thicknesses and depths of DU layers are assigned based on the anticipated depth of contamination and the desired resolution of the site investigation in terms of soil volumes for development of a remedial action plan. A higher resolution (i.e., smaller DU layer areas and volumes) increases the cost of the investigation but helps to minimize the inclusion of clean soil in treatment or removal plans. Maximum DU volumes of a few hundred cubic yards are recommended for contaminants that primarily pose direct exposure hazards (refer to Clean Fill Guidance; HDOH, 2017d). Maximum DU volumes of a few tens of yards are recommended for contaminants that pose significant leaching hazards.

In this example, arsenic and dioxin contamination in the outer part of the mixing area is assumed to be largely surficial due to the lack of distinct spill areas. Three vertical DU layers were designated (see Figure 3-30): 1) 0 to 0.5 feet, 2) 0.5 to 2.0 feet, and 3) 2.0 feet to 3.0 feet. The volumes of soil associated with the respective layers in each of the 5,000 ft2 DUs are approximately 90 cubic yards, 280 cubic yards and 185 cubic yards, respectively. It is anticipated that the upper two layers will require removal. The lowermost layer is anticipated to be clean. This resolution was determined to be acceptable for development of a followup remedial action plan for this area.

Soil below a depth of two feet is divided into two DU layers in order to better assess the depth (and volume) of contamination by triazine herbicides. This is accomplished in this example by the use of smaller DUs in comparison to the arsenic- and dioxin-contaminated area. Four vertical DU layers were designated (see Figure 3-30): 1) 0 to 0.5 feet, 2) 0.5 to 2.0 feet, 3) 2.0 feet to 5.0 feet, and 4) 5.0 feet to 10.0 feet. The volumes of soil associated with the respective layers range from 20 to 50 cubic yards in shallow DU layers anticipated to be most heavily contaminated up to 185 to 370 cubic yards in deeper DU layers anticipated to be relatively clean. Contamination is anticipated to be concentrated in the upper three layers.

As discussed in Section 5, characterization of the DU layers can be accomplished by trenching and/or the installation of multiple borings. Trenching and testing of DU layers exposed in sidewalls was used to estimate depth of contamination in the outer areas. Trenching in this area was also desired to determine the presence or absence of burial pits common to these types of sites that might have been missed in previous investigations. Borings were installed in the triazine spill area, with core increments from each targeted layer collected, subsampled, and combined to produce a bulk MI sample for each layer. Triplicate samples could be collected from alternative boring locations for all or select DU layers.


Figure 3-31. Stockpile Segregated into DU Volumes for Testing Based on Planned Reuse

Figure 3-32. DU Designation for an Investigation of Former Sugar Mill Drainage Canal DU-1 is 75 feet long and averages 10 feet wide (750 ft2). DU-2 and DU-3 are 250 feet long and again average 10 feet wide (2,500 ft2). DU sediment volume is estimated at 15 yd3 and 50 yd3, respectively.

A DU volume of 100 cubic yards (yd³) is recommended for a stockpile with an unknown history (Figure 3-31). This represents the approximate volume of soil needed to cover a hypothetical 5,000 ft2 residential lot to a depth of six inches (see HDOH, 2017d; see also Subsection 3.4.2). Testing of stockpiles with an unknown history typically focuses on toxic and persistent chemicals such as arsenic, lead, organochlorine pesticides and PCBs. The presence of heavy oil is also typically tested (e.g., TPH-o). If concentrations of contaminants in each 100 yd³ volume do not exceed Tier 1 action levels then it is assumed that the soil does not pose direct exposure concerns for unrestricted reuse (e.g., residential). See Appendix 3-A, Guideline for Evaluation of Imported and Exported Fill Material, for additional information pertaining to sampling of stockpiles.


The size of sediment DUs will vary widely depending on the nature of the release and the objectives of the investigation. Example shallow water sediment DUs are presented in Figures 3-32 through 3-36. The examples are taken from actual sites, although the details have in some cases been modified to illustrate specific points.

Figure 3-32 depicts sediment DUs designated for a drainage canal that once carried waste water from a sugar mill. Testing of surface soil at discharge points suggested that sediment in the canal might be contaminated with mercury (used as a fungicide). A relatively small DU is designated for the area of the canal immediately downstream of the discharge area (DU-1 in Figure 3-32). Two additional and somewhat larger DUs are designated for areas of the canal further downstream (DU-2 and DU-3). Two DU layers are designated, 0-6 inches and 6-12 inches. It is anticipated that contamination might be greater at depth, due to the long time interval since the cessation of operations at the facility.

The next example illustrates a single sediment DU designated at the outfall of a wastewater pipe. A single DU is designated given the anticipated similarity of impacts within the small area (Figure 3-33). The upper six inches of sediment is targeted for characterization. An 30+ increment MI sample is collected by maneuvering around the perimeter of the ponded area.

Figure 3-33. DU Designated for Characterization of Sediment at the Mouth of a Wastewater Pond Outfall "Xs" indicate increment collection locations

Figure 3-34. Sediment DUs Designated for a Spill of PCB-based Transformer Oil Beside a Small Stream The DUs cover approximately 500ft2areas to a depth of six inches (approximately10cyds per DU).

Figure 3-34 depicts hypothetical DUs for a PCB spill suspected to have entered a small stream. The area outlined in red depicts the upland area impacted by the spill. Spill Area DUs as described above are used to characterize this area, including Perimeter DUs to confirm that the edges of significant impact have been identified.

Relatively small DUs, depicted in yellow, are then designated in the stream itself for characterization of sediment. The location and size of the DUs might be based on stream flow characteristics (e.g., focus on individual depositional areas, including pools and bars) and the maximum volume of sediment to be included within a DU with respect to ecological and remedial considerations. In this example, DUs approximately 500 ft2 in area were considered appropriate. In this example sediment cover in the stream was very thin, three to six inches in most areas, and the entire volume of sediment within each DU was targeted for characterization (approximately 5-10 cubic yards per DU). A Multi Increment sample was collected in each DU with triplicate samples collected in 10% of the DUs.

Figure 3-35 depicts a much larger sediment investigation carried out in the upper part of a spring-fed fifty-acre estuary suspected to have been impacted by historic arsenic-contaminated wastewater and runoff from past agricultural operations in the area. The pond is tidally influenced.

Figure 3-35. Decision Units Designated for Characterization of Arsenic-Contaminated Sediment in an Estuary

Figure 3-36. Sediment DU Layers Designated for the Estuary Example Depicted in Figure 3-35

In the example, DU-1 was placed to characterize sediment in the immediate areas of a former sugar mill and a former Canec production facility (used to make arsenic-infused, termite-resistant press-board panels from waste sugarcane fibers.). The remaining DUs reflect sediment areas more distant from the former Canec plant site. The area of the pond encompassed by DUs 2 through 5 are relatively low energy and characterized by fine silts. The lower area of the pond, DU-6, is higher energy due to focused tidal action and characterized by a mix of silts to medium-grained sand. A narrow, high-energy area between DU-4 and DU-5 was not sampled due to a lack of sediment.

Vertical DU Layers may be assigned based on factors that include observations from initial test cores (e.g., distinct layering, grain size, aerobic versus anaerobic zones, etc.), characterization of benthic zones for use in ecological risk assessments, estimated depositional depth since closure of an industrial facility formerly located in the area, and/or desired resolution for potential remedial actions. In this example, the sediment in each DU was ultimately divided for testing into three DU Layers for characterization (Figure 3-36): 1) 0-4 inches, 2) 4-8 inches and 3) 8-12 inches. Methods for the collection of MI sediment samples from the DUs are reviewed in Section 4.


Sediment targeted for dredging operations can be tested in place as described above or in stockpiles after dredging, as discussed in Subsection 3.5.7. Decision unit area and volume designation is based on targeted contaminants of concern and related environmental hazards as well as planned reuse or disposal of the dredge material. Offshore disposal of dredge material is overseen by the US Army Corps of Engineers in coordination with local environmental agencies.

Note that the use of dredge material from salty or brackish water bodies as fill material in upland areas is not recommended due to potential salinity problems. Soil salinity is evaluated in terms of Electrical Conductivity (EC) and Sodium Absorption Ratio (SAR; see HDOH, 2016). Salt intolerant plants begin to be affected by soil salinity at an EC greater than 2 mS/cm (1 milliSiemen/centimeter = 1 millimho/centimeter; see also Blaylock, 1994). Soil with an EC of greater than 32 mS/cm is toxic to even salt tolerant plants. Soil EC values of over 100 mS/cm were reported for samples of saline, dredge material collected by the HEER Office, indicating high toxicity to plants (HDOH, 2014).

Excess sodium associated with saline sediment can also cause soil to harden and form clods when dry, impeding the uptake of water during rainfall or irrigation and again reducing plant growth. A SAR value greater than 5.0 indicates sodic soils that could inhibit plant growth (see Dickson and Goyet,1994) Soil SAR values of over 200 were reported for the same dredge material noted above that was tested by the HEER Office (HDOH, 2014).

Other potential environmental concerns associated with the reuse of dredge material include runoff of saline water during rain events, as well as leaching of salt and impacts to underlying groundwater. (Note that while a high sodium content can inhibit leaching and runoff, this will also ensure that the soil remains saline and unusable for a long period of time.) Dredge material from heavily developed harbors and in the vicinity of urban runoff can also contain trace levels of other pollutants, including termiticides, petroleum, PCBs, and lead.

While the use of dredge material for beach replenishment and as fill in shoreline or near-shoreline areas may be appropriate on a case-by-case basis, the general use of dredge material from saline water bodies for fill material is not recommended without prior review and approval by HDOH. The HDOH Office of Solid and Hazardous Waste should be contacted for additional guidance and regulatory information on the reuse of dredge material.