Department of Health Seal

TGM for the Implementation of the Hawai'i State Contingency Plan
Section 4.4
COMMON DU-MIS INVESTIGATION MISTAKES AND PROBLEMS

4.4 COMMON DU-MIS INVESTIGATION MISTAKES AND PROBLEMS

4.4.1 INAPPROPRIATELY SIZED DUs

The designation of Decision Units for site characterization is discussed in Section 3.4. It is important to ensure that DUs are appropriately sized to meet site investigation objectives. Decision Units should ultimately be sized to address potential environmental hazards posed by contaminants in soil at the site. This always includes direct exposure and depending on the contaminant can also include leaching, gross contamination and other concerns (see Section 13).

Direct exposure concerns under current site conditions are most directly evaluated through the designation of Exposure Area DUs (see Section 3.4.2). As discussed below, however, separate characterization of known or suspected spill areas within an exposure area is still recommended. Leaching, gross contamination and other concerns are most directly evaluated based on Spill Area DUs. The latter requires a more detailed understanding of the locations of potential heavy contamination (i.e., "spill areas") based on the site history, field observations, and interviews with people knowledgeable of the site and related information. Spill Area DUs are commonly a few hundred to a few thousand square feet in size and typically smaller than Exposure Area DUs that might be designated at the same site. The maximum size of a Spill Area DU for characterization purposes is generally set to the maximum DU size likely to be acceptable for exposure areas (e.g., default HDOH residential exposure area of 5,000 ft2; see Section 3.4.2).

Failure to adequately identify and characterize suspect spill areas at the beginning of an investigation can have several consequences. Foremost is the need to identify suspect spill areas as a basic objective of an environmental investigation under the State Contingency Plan (refer to Section 2). If historical information or field observations suggest that contamination might be concentrated in a specific area of a site then this area must be characterized separately from anticipated clean areas (i.e. areas suspected to have only low levels of contaminants, below HDOH Tier 1 EALs). The inclusion of small areas of heavy contamination (e.g., a few hundred to a few thousand square feet) with large areas of otherwise clean soil for characterization can also cause the entire DU to fail and unnecessarily drive up cleanup costs.

Assume for example that an older building on a 5,000ft2 lot is to be demolished and a new home constructed. The entire lot might be considered to represent a single, "Exposure Area" DU for evaluation of direct exposure risk (Section 3.4.2). Soil around the perimeter of the existing house is, however, suspected to have been treated with Technical Chlordane (chlordane), widely used in the past as a termiticide. Exceptionally high concentrations of chlordane in this area could erroneously imply that the entire property is contaminated above soil action levels.

This highlights the need to characterize the house perimeter as a separate, Spill Area DU, with the remaining area of the yard tested as an Exposure Area DU (see Figure 3-20 in Section 3). The perimeter of the house will likely be flagged for potential direct exposure concerns. If the new house is to be constructed on the existing foundation then exposure to treated soil in this area can subsequently be minimized by placing gravel, landscaping or pavement around the perimeter.

Contamination associated with spill areas can also extend below the depth of soil included in the original Exposure Area DU. This deeper soil could potentially be excavated during future redevelopment and spread out across the surface, resulting in a higher exposure area concentration of chlordane than estimated from the original investigation.

Significant disagreement between replicate samples can indicate the presence of a localized spill area(s) within an initially large DU. If this occurs and the resulting data are inadequate for decision making (see Subsection 4.2.7), then the original DU should be subdivided into smaller DUs for re-characterization. This situation can be avoided for contaminants known to be subject to potential exceptionally high small-scale variability (e.g., lead shot, PCBs, etc.) by designating reasonably small DUs up front and increasing the number and/or mass of increments collected within a DU (e.g., no more than a few hundred to a few thousand square feet; see Section 3.4.3; see Subsection 4.2.2).

The use of inappropriately small DUs can also interfere with an efficient site investigation. Decision unit sizes are guided by the need to address risk and optimize remedial efforts. While a strong resolution of contaminated versus clean areas is desirable, the use of excessively small DUs (e.g., less than a few hundred square feet) to characterize an area is generally not beneficial and unnecessarily adds to the cost of the investigation.

4.4.2 DATA GAPS BETWEEN SURFACE DU's OR SUBSURFACE DU LAYERS

Traditional discrete sampling methods require extrapolation of contaminant concentrations between individual sample points, where data are not available. As discussed in the HDOH field study of discrete sample variability, extrapolation between discrete data points can be highly unreliable (HDOH 2015, b). Under a DU-MIS investigation approach, the data generated represent the mean contaminant concentration for a designated area rather than a single point. The use of adjoining DUs and subsurface DU layers minimizes gaps in data obtained for a site. This helps avoid the need for additional characterization should contamination be found as well as help optimize remedial actions. Data gaps for precise delineation of the lateral or vertical extent of a spill area might be acceptable under some circumstances but should be reviewed and discussed on a site-by-site basis.

Perimeter DUs surrounding suspect spill areas of heavy contamination should ideally be placed immediately adjacent to the Spill Area DU, with no gaps of untested soil present (see Section 3.4.5). Multiple rings of DUs might be advantageous in case inner DUs unexpectedly fail action levels. If gaps are unavoidable, for example due to buildings or other access limitations between spill areas and anticipated clean areas, then contamination in the untested area of soil should be assumed to be similar to that identified for the primary spill area unless additional information suggests otherwise.

The same need to minimize data gaps holds true for subsurface soil. Traditional discrete sampling of subsurface cores involved testing of soil at widely-spaced intervals at depth below the ground surface (e.g. every 5 feet). Contamination was typically assumed to extend halfway between points where concentrations above and below action levels were reported. Under a DU-MIS investigation approach the entire depth of soil targeted for sample collection is divided into separate but adjoining, DU layers for representative sampling and characterization (see Section 3.4.4). Extrapolation across data gaps is not necessary or desirable.

4.4.3 INADEQUATE NUMBER OF INCREMENTS AND MASS

Sampling theory requires that a sample of adequate mass be collected from an adequate number of points within a targeted DU to capture and represent distributional heterogeneity within the DU and to estimate a reliable mean (refer to Subsection 4.1). Recall that the number of increments collected and the representative sample methodology used is independent of the size of the DU (refer to Subection 4.2.2). The number of increments may vary somewhat based on the form of the contaminant (e.g. more for lead nuggets or PCB droplets) or other suspicions about the degree of contaminant heterogeneity, but increasing increments in such cases would apply to both small and larger DUs as well. The number of increments collected and the representative sample methodology used is independent of the size of the DU (refer to Subsection 4.2.2). The number of increments may vary somewhat based on the form of the contaminant (e.g. more for lead nuggets or PCB droplets) or other suspicions about the degree of contaminant heterogeneity, but increasing increments in such cases would apply to both small and larger DUs as well.

A minimum of 30 to 75+ increments per DU is recommended, with a default of 50 for sites where the nature of contamination is uncertain (see Subsection 4.2.2). If the target contaminant does not show an unusual degree of heterogeneity in the DU soil, then approximately 30-50 increments are typically adequate to determine a representative mean concentration (determined by the collection and analysis of field replicate samples). For contaminants or situations where there is a relatively high degree of contaminant heterogeneity in the DU, larger numbers of increment (and/or larger masses for increments) are typically needed to obtain representative mean values. The adequacy of the number and mass of increments included is tested through the collection of replicate samples (see Subsection 4.2.7)

An adequate mass and number of increments to obtain a representative sample is required for both surface soil as well as subsurface soil, discussed below. If a less-than-recommended number of increments can be collected from a targeted DU, especially in the case of subsurface soil, then field replicate data is crucial to help evaluate the usefulness of the data for decision-making. In general, using fewer increments than recommended increases the likelihood that the data may not prove to be adequately representative. Any limitations of the data identified should be discussed in the investigation report, as well as the potential need for more reliable characterization in the future.

Some sampling guidance documents and training classes have suggested that increments initially collected from a DU be combined into smaller "sampling unit" subsets for separate testing in order to provide a better understanding of contaminant distribution variability within the DU (e.g., ITRC 2012). For example a DU might be divided into four subareas with 8 increments collected from each "SU" and combined and tested separately. This approach suffers from several shortcomings. Most importantly, DUs should be appropriately sized to the desired scale of decision making at the start of the investigation. If better resolution might be needed for an initially large DU then the DU should simply be subdivided into smaller DUs with a multi increment sample of adequate mass and number of increments collected from each DU.

Testing of poor quality samples from DUs when a proper number of increments could have been collected is wasteful of investigation resources and should be avoided. The resulting data cannot be assumed to be representative of the area where the combined increments were collected (see HDOH 2015, b). From a field perspective, the added time and cost to collect an adequate number of increments (e.g., 30 to 75+) from each smaller area is also negligible, especially given the importance of the resulting data in decision making.

Collecting an adequate mass of soil (e.g., 1-2 kg) is usually feasible for a project, as is the collection of an adequate number of increments from exposed, surface soil. The collection of a large number of increments from subsurface soil DU layers might not be practical, however, due to cost or access issues (see Subsection 4.2.8.2). If this is the case then limitations on the reliability of data should be clearly discussed in the investigation report. Replicate data from at least 10% of the DUs are especially important in such cases (see Subsection 4.2.7). Data for other DUs should be adjusted as necessary in accordance with Subsection 4.2.7. If this adjustment indicates that contamination above levels of potential concern could in fact be present, then the soil should be included in remediation work plans and/or managed under a site EHMP until such time that it is more accessible.

4.4.4 IMPROPER INCREMENT SPACING

As a shortcut in the field it can be tempting to collect large numbers of tightly spaced increments from a few widely spaced lines within a DU (see Figure 4-10). While this approach might address sampling theory requirements in terms of the mass and number of increments used to prepare a bulk MIS sample, it may not be representative of mean contaminant concentrations within the DU. The described approach does not meet the sampling theory requirement of randomly located increments and is therefore unacceptable. There are three options - purely random, systematic random and stratified random, with systematic random increment collection demonstrated to produce the most reliable results.

Unevenly spaced increments can cause localized areas of heavy contamination within the DU to be both over or under represented by the resulting bulk sample data. This can also cause replicate samples to fail and require re-characterization of the DU, wasting resources and unnecessarily extending the time and cost required to complete the project.

Sample data are most reproducible when increment locations are distributed at evenly spaced locations, referred to as "systematic random" (see Figure 4-9). Increments should be equally spaced in both the x and y axis directions. While simple in concept this can be complicated to implement in the field without prior practice and experience.

4.4.5 IMPROPER INCREMENT SHAPE

Gardening trowels are easy to use and decontaminate in the field for the collection of soil samples. Such tools are prone to collect wedge-shaped increments, however. This can bias the subsequent MI sample to the upper portion of the targeted DU layer, where the greater mass of soil was collected, and call into question the representativeness of the data in terms of the site investigation objectives. Note that this bias would not necessarily be reflected in replicates samples collected from the same DU, since the same error is carried forward in each individual sample.

Trowels should be avoided when tools that allow the collection of more core-shaped increments can be utilized (e.g., sampling tubes). A core-shaped increment is ideal, since it equally represents the targeted DU layer in both the vertical and lateral direction (see Subsection 4.2.5.2). The use of trowels and/or other tools might be unavoidable for hard-packed or gravely soils, however (see Section 5.3). If this is the case then an effort should be made to collect cylindrical-shaped increments that are equally representative of the full thickness of the DU. This approach might also be required for dry, loose soils that would otherwise fall out of sampling tubes or not be evenly extracted with drills or other coring equipment. Non-coring sampling alternatives may result in the collection of larger individual increment masses and larger bulk MI samples. This needs to be considered when planning the investigation and coordinating with the laboratory.

4.4.6 CO-LOCATED DISCRETE SAMPLES AND INCREMENT SPLITS

Field studies carried out by HDOH indicate that contaminant concentrations within a single sample or increment and co-located samples or increments can vary by orders of magnitude in an unpredictable and random manner (see HDOH 2015, b). The concentration of the contaminant in a simple subdivision of the discrete sample or increment (sometimes referred to as a split) or otherwise co-located sample/increment could well have no bearing on the concentration of the contaminant in the increment collected from the same location. Attempting to combine small groups of co-located increments into bulk MI samples for testing similarly poses the same risk of non-representativeness as described above.

Note also that replicate samples should not be collected from the same (or co-located with) initial increment locations (see Subsection 4.2.7). While technically a separate sample, the precision of the DU-MI sample data is accurately assessed by the collection of replicate samples from widely separated and completely independent locations.

4.4.7 INADEQUATE LABORATORY PROCESSING

Inadequate processing of a MI sample negates the field representativeness of the sample and the validity of the resulting data. The resulting data reported by the laboratory can be considered to be no more useful than a single discrete sample collected from within the DU area.

It is important to ensure that the laboratory that receives the MI samples has a written standard procedure in place to properly process and collect a subsample for testing (refer to Subsection 4.2.6). For non-volatile contaminants this includes drying, sieving and subsampling in accordance with sampling theory methodologies. Request a copy of the laboratories Standard Operating Procedure (SOP) for incremental sample processing and testing. Ideally the lab should be visited and the procedures used to manage Multi Increment samples demonstrated.

4.4.8 INADEQUATE SUBSAMPLE MASS FOR ANALYSIS

The mass of soil collected in the field and extracted for analysis by a laboratory is dictated by sampling theory (see Subsection 4.1). A minimum subsample mass for analysis of 10 grams is recommended for soil samples sieved to the <2 mm particle size (Subsection 4.2.6). When possible, a larger subsample mass (e.g., 30+ g) is preferable to help further reduce the potential lab subsampling error and improve the precision of laboratory subsample replicates (see Subsection 4.2.7.2). Grinding (milling) of samples to a smaller particle size can allow for collection of a smaller lab subsample where appropriate for the contaminant or specified in a standard lab method (see Subsection 4.2.6.3). Such cases should be discussed with the laboratory and the HEER Office during sample investigation planning.

Standard laboratory methods for testing of metals in soil only require one gram or less to meet analytical needs. Unless the bulk sample has been ground, however, this is inadequate to ensure that the resulting data will be representative of the sample collected. The need to extract a larger mass of soil for metals analysis should be clarified with the laboratory prior to the initialization of field work.

Extraction of a larger subsample mass and/or grinding of the sample might be required if laboratory replicate samples indicate poor subsampling precision (see Subsection 4.2.7.2). This should be discussed with the laboratory prior to submittal of the samples and procedures for retesting of samples included in the investigation work plan and instructions to the laboratory.

4.4.9 LACK OF FIELD REPLICATE SAMPLE DATA

The need to collect replicate data might seem redundant with experience gained for a specific contaminant or a geographical area (Subsection 4.2.7). For example, 30-increment MI samples have been routinely demonstrated to generate reproducible data for most former sugarcane-growing soil sites contaminated by arsenic-based pesticides in Hawai‘i (e.g., see HDOH 2015). The representativeness of a DU sample can only be evaluated and documented if replicate samples are collected, however. Routine collection of field replicates is required to demonstrate that correct sampling procedures were utilized (e.g. number of increments, systematic random sample spacing, correct increment shape and adequate sample mass, field handling/processing procedures, etc.).

The precision of MI samples can decrease as the mean concentration of a contaminant increases. Unanticipated areas of localized contamination within DUs can also lead to decreased precision of normally acceptable MI samples. Field studies carried out by HDOH indicate that the concentration of a contaminant can vary by an order of magnitude or more in replicate samples collected from the same DU, even when an MI sample consists of greater than 50 increments (HDOH 2015, b). Under some circumstances even the higher recommended default of 75 increments per sample could be inadequate to demonstrate a representative mean contaminant concentration in a DU, such as when contaminants are distributed in a very heterogenic "nugget" form (e.g. lead pellets, or lead paint chips).

Testing of large numbers of discrete samples from a DU, for example with a portable XRF (see Section 8), can provide a semi-quantitative indication of the degree of small-scale variability within the DU and provide an indication of the relative number of increments necessary to collect a representative MI sample (e.g., greater number of increments needed for increasing heterogeneity; see Subsection 4.3). Statistical methods used to estimate the number of discrete samples needed to estimate the mean concentration of a contaminant within a DU (USEPA 2013b) are not, however, directly translatable to the number of increments required under an MI investigation and cannot be used as a substitute for the collection of replicate samples. This is due to multiple factors, including consistency in the manner in which the individual discrete samples were collected (e.g., shape, mass, etc.) and perhaps more importantly the mass of soil represented by each sample data point in comparison to the mass of soil typically represented by a single increment.

4.4.10 REVERSION TO DISCRETE SAMPLING

Perhaps the most egregious error in site investigations is a reversion to discrete sampling due to real or perceived difficulties for the collection of proper MI samples in the field. This is especially common for characterization of subsurface soil. Sampling theory and the use of Multi Increment samples to characterize soil is not just one alternative to past discrete sampling methods, it is a much needed update.

The concept of “DUs” was an inherent part of past, discrete soil sample investigations (see Section 3.4). Discrete soil sample collection points were typically designated based on a desire to characterize contamination in one area versus another. As discussed below, the area intended to be represented by a single, discrete sample point (or cluster of sample points) is designated as a separate DU for characterization. A large-mass, Multi-increment sample is then collected from multiple (e.g. 30-75+) locations within this area rather than reliance on a small, discrete soil sample collected from a single location. The number of DUs designated for a particular investigation not coincidentally corresponds with the number of discrete soil samples or clusters of samples that might have been collected under past approaches.

The unreliability and inefficiency of discrete sample data remains the same regardless of the nature and location of the targeted soil. Consideration of sampling theory is still required to ensure that the resulting data are technically defensible and useful for decision making purposes. The fact that a targeted layer of soil is covered by additional soil that must first be penetrated for the collection of an MI sample cannot be used as a reason to revert to discrete sample collection approaches.

Targeted DU areas and layers, rather than single horizons, must always be designated as part of a site investigation regardless of the manner used to characterize the soil (Section 3.4.4). Methods to collect MI samples from subsurface DU layers are described in Subsection 4.2.9 and Section 5.4. As is the case for surface soil samples, subsurface samples must be of adequate mass and distribution within the DU to address fundamental error. Samples must also be processed at the laboratory in accordance with multi increment subsampling methods. If an ideal number of increments cannot be included in a DU layer sample due to access or cost limitations then limitations regarding the reliability of the resulting data must be assessed and discussed based on a review of the replicate sample data. Identification of data limitations is also important where single borings are used for decision making purposes (see Section 3.4.4).

Another error sometimes encountered in site investigations is a reversion to the collection of a single discrete sample when the targeted DU is very small, for example <100 ft2 or even <10 ft2 or less. Sampling theory is independent of DU area and volume (Subsection 4.1). A minimum 1-2 kg sample must still be collected from the DU in order to address fundamental error. If collection of the recommended default number of increments from the DU is somehow not practical then this should be noted and replicates collected and reviewed to determine precision of the sampling data. Any limitations identified through analysis of the replicate data should be discussed when reporting the results. The sample must be processed and subsampled for testing at the laboratory in accordance with multi increment sample methods.

If the DU is so small that the entire volume of soil is to be collected and submitted to the laboratory, then processing and subsampling in accordance with Multi Increment sampling methods are still required (e.g., testing of sediment in a small sump). In this sense the soil submitted is not a true "sample" in terms of sampling theory, since the entire DU volume of interest is collected for analysis. The use of Multi Increment sampling methods to collect a representative sample from the DU in the field was not necessary. Any error in the resulting data would be fully attributable to laboratory subsampling and analysis errors, since the entire mass is not being analyzed and a laboratory subsample must be collected.

Similar concerns and requirements as noted above also apply to the characterization of sediment that happens to be covered by a layer of water. Simplistic contouring between discrete sample points cannot be assumed to be reliable beyond the gross recognition of large contaminant patterns (see HDOH 2015b). Decision Unit layers, rather than single horizons should be designated and targeted for characterization (see Section 3.4). Increments collected within a DU must be of adequate shape, number and mass to address fundamental error and generate a representative sample. It is possible that fewer numbers of increments might be adequate to collect a representative sample of sediment from designated DU areas, due to the nature in which the contaminant was released and the sediment deposited. This issue has not been evaluated in detail in the field to our knowledge, however. Limitations on the reliability of resulting data when an adequate number of increments cannot be collected must be discussed in the investigation report.

4.4.11 DU-MIS INVESTIGATIONS UNDER TSCA

The investigation, cleanup, verification and disposal of soil contaminated with polychlorinated biphenyls (PCBs) is regulated under 40 CFR § 761.61 (PCB remediation waste) of the Toxic Substances Control Act (TSCA; USEPA 1998h). The Hawai‘i State Contingency Plan also authorizes HDOH to require the investigation and remediation of PCB-contaminated properties (refer to Section 2). This joint authority has caused problems as USEPA lags behind HDOH in the transition to multi increment sampling methods from outdated discrete sampling methods prescribed in 40 CFR 761.61(a) self-implementing on-site cleanup and disposal of PCB remediation waste and associated guidance documents (e.g., USEPA 1985, 1986).

Use of alternative procedures is provided for in 40 CFR 761.61(c)(1) risk-based disposal approval, subject to the approval of the USEPA Regional Administrator:

Any person wishing to sample, cleanup, or dispose of PCB remediation waste in a manner other than prescribed in paragraphs (a) or (b) of this section ... must apply in writing to the EPA Regional Administrator in the Region.

A Memorandum of Understanding (MOU) that outlines a technical and regulatory pathway for the incorporation of DU-MIS investigation methods under TSCA is currently being pursued between HDOH and USEPA Region IX. This MOU would then be referenced for continued investigation and remediation of PCB-contaminated sites under HDOH oversight following methods described in this guidance manual, with notification and allowance for review and comment made to USEPA Region IX.

Figure 4-30. Limited “Compositing” and “Dilution” Allowed Under TSCA to Reduce Laboratory Costs. Soil combined across separate “sample areas” or “contaminated zones,” referred to in HDOH guidance as “Decision Units (DUs)” represents a composite sample. This can lead to a potential dilution of a higher PCB concentration in otherwise separate “hot spots,” referred to as “Spill Area DUs” by HDOH. Under TSCA the laboratory result must be divided by the number of discrete samples, or more specifically otherwise separate areas represented by the composite sample for comparison to the screening level. This ensures that no single area, i.e., DU, exceeds the target screening level.


Figure 4-31. Theoretical Compositing of Multi Increment samples. Multi Increment samples from separate DUs combined into a single sample for processing and testing at the laboratory. The laboratory data are divided by the number of samples (DUs) included in the composite sample for comparison to screening levels. Note that a single MI sample collected within a single DU is not a composite. Compositing of MI samples is not allowed under HDOH site investigation guidance. Refer to Section 3 of HDOH Technical Guidance Manual for information on designation of Decision Units at contaminated properties.

Until such an arrangement has been made, responsible parties are encouraged to contact the TSCA office of USEPA Region IX when concentrations of PCBs in soil greater than 50 mg/kg are reported for MI samples. Under TSCA, soil with a concentration of >50 mg/kg PCBs must be disposed of at a hazardous waste landfill in the mainland US. Workplans for DU-MIS investigations at such PCB sites must be approved on a case-by-case basis by both HDOH and USEPA Region IX.

Of particular concern under TSCA is the need to minimize "dilution" of heavily contaminated soil with soil from surrounding, clean areas in sample data. Doing so might cause a conflict with Section 761.1(b)(5) of TSCA regulations, which states "No person may avoid any provision specifying a PCB concentration by diluting the PCBs, unless otherwise provided." This concern can be avoided by designation of well-thought-out and researched Spill Area DUs at known or suspected PCB release sites in accordance with this guidance document and in coordination with HDOH. If PCB concentrations >50 mg/kg are identified in any DU then USEPA Region IX may also request to review and approve DUs designated for characterization of the site.

Dilution, as described under TSCA, can occur when samples intended to represent distinctly different areas (i.e., DUs) of a site are intentionally combined for a single analysis. The use of "composite" samples is also limited under TSCA regulations and guidance (e.g., USEPA 1985, 1986). As interpreted by HDOH, a Multi Increment sample is not a composite sample in the sense used in TSCA. A sample becomes a "composite" when soil from what should otherwise be separate DUs is combined. Under TSCA, each individual discrete sample is assumed to potentially represent an individual, PCB "contaminated zone" or "sampling area," referred to in this guidance as "Spill Area DU" (see Section 3.4.3)(USEPA 1985):

The PCB level is assumed to be uniform within (a contamination zone/spill area) and zero outside it.

The spacing of individual discrete samples was based in part on the anticipated size of a spill area in order to ensure that at least one sample was collected from each potential area (USEPA 1987):

The decision maker must determine… the acceptable probability of not finding an existing contaminated zone in the suspected area. For instance, it might be determined that a 20 percent chance of missing a 100ft-by-100ft (10,000ft2) contaminated zone is acceptable but only a 5 percent chance of missing a 200ft-by-200ft (40,000ft2) zone is acceptable.

Under this scenario, TSCA regulations and associated guidance allow soil from multiple DU areas to be combined or "composited" into a single sample for analysis in order to reduce the total cost of laboratory analysis (Figure 4-30; USEPA, 1985, 1987, 1998h). This in effect allowed intentional "dilution" of suspect spill areas with surrounding areas of cleaner soil that should otherwise be separately characterized. The resulting data therefore had to be divided by the number of samples included in the composite, however, in order to ensure that no single "sampling area" exceeded the target cleanup level. A maximum of ten discrete samples was permitted to be included in a single composite, based on a target cleanup level of 10 mg/kg and a laboratory detection level of 1 mg/kg. Note that risk assessment guidance was still under preparation at the time that TSCA guidance and regulations were being prepared and the concept of "exposure areas" and risk were still not widely understood.

Under a more up-to-date, DU-MIS investigation, "compositing" in the sense initially intended under TSCA guidance would involve the intentional combination of Multi Increment samples collected from separate DUs into a single sample for testing. (Figure 4-31) The resulting data would again need to be divided by the number of DUs and MI samples included in the composite, however, in order to ensure that no single DU area might exceed the target cleanup level.

Although this would save on analytical cost, compositing of MI samples is not allowed under HDOH guidance. An independent MI sample, representing what in the past might have been a single discrete sample, must instead be collected from each DU and individually tested for comparison against target action or cleanup levels. Intentional inclusion of suspect spill areas with anticipated clean areas for characterization as a single DU could be interpreted to violate the "anti-dilution" clause in TSCA regulations. For these reasons it is important to closely coordinate DU designation at PCB-release sites with HDOH and, as necessary, with USEPA Region IX.

As noted earlier, the intentional mixing of known or anticipated contaminated areas (i.e., "Spill Areas") with clean areas as part of a site investigation is poor practice. Doing so risks unnecessarily increasing the area and volume of soil requiring removal or long-term management. Relatively small DUs, usually a few hundred to a few thousand square feet, should be designated for characterization within suspect spill areas (refer to Section 3.4.3). Perimeter DUs of a similar area and volume should be designated in anticipated clean areas around suspect spill areas. The maximum size of DUs in outer, anticipated clean areas should be limited to the size of current or anticipated exposure areas (default residential exposure area 5,000 ft2; see Section 3.4.2). These approaches will help ensure that the investigation and cleanup PCB-contaminated soil is carried out in an efficient and effective manner.