INCREMENTAL SAMPLING METHODOLOGY

Transcription

1 INCREMENTAL SAMPLING METHODOLOGY 1. INTRODUCTION Incremental sampling methodology (ISM) is a structured composite sampling and processing protocol having specific elements designed to reduce data variability and increase sample representativeness for a specified volume of soil under investigation. Variability in measured contaminant concentrations between discrete soil samples is due primarily to the particulate nature of soil and heterogeneity in the distribution of contaminants. The elements of ISM that control data variability are incorporated into (a) the field collection of soil samples and (b) laboratory processing and subsampling procedures. ISM is designed to obtain a single aliquot for analysis that has all constituents in the same proportion as an explicitly defined volume of soil. Properly executed, the methodology provides reasonably unbiased, reproducible estimates of the mean concentration of analytes in the specified volume of soil. In 2009, the ITRC convened the ISM Team to prepare this guidance document, which focuses on soil sampling. ISM addresses all the sources of sampling error in a systematic fashion (Gy 1998, Smith 2006). Other approaches to soil sampling have not emphasized reducing sampling error as much as ISM. Because this methodology requires change from traditional approaches, the ISM Team found it necessary to go into detail about the theory as well as the application of incremental sampling. The team found this to be a valuable exercise and it should be valuable to the reader as well. Because a good deal of new terminology is introduced by ISM, the reader is directed to the glossary in Appendix E. Section 1.8 also illustrates some key terms used in this document. The ISM Team recommends that, as with any well-conceived sampling approach or plan, all members of the investigation project team (e.g., consultants, regulators, geologist, analytical chemists, risk assessors and toxicologists) be involved in the entire ISM development process. 1.1 Summary of ISM as an Environmental Sampling Approach The elements of ISM that control data variability are incorporated into (a) the field collection of soil samples and (b) laboratory processing and subsampling procedures. Like all sampling approaches, ISM should be applied within a systematic planning framework. Figure 1-1 shows a general ISM flow process. One of the first steps in such a framework is to have the investigation project team establish a working conceptual site model (CSM). Once the CSM has been agreed to, the project team defines the data quality objectives (DQOs) and determines the appropriate decision unit (DU) size(s) and location(s). DUs are based on projectspecific needs and site-specific DQOs; both considerations specify and constrain the appropriate end use of the data. The size of a DU is site-specific and represents the smallest volume of soil about which a decision is to be made (USEPA 1999, Ramsey and Hewitt 2005, HDOH 2008a, ADEC 2009). In some cases a DU comprise smaller units known as sampling units (SUs), as discussed in Section 3. The requirement to explicitly and appropriately define the DU that each incremental sample represents is a key component of ISM and is discussed in detail in Section 3. 2

2 * The statistical performance of the 95% UCL calculation depends on the properties of the data set and the sampling design. Note that ProUCL or FLUCL does not currently include the statistical algorithms for handling ISM data (see Section 4.0 and Appendix A). ** See Section 7. Figure 1-1. ISM flowchart. ISM planning includes the development of an ISM protocol for the number of increments and replicates to be collected for each ISM sample. An incremental sample is created by collecting many (usually ) equal-volume increments in an unbiased manner from throughout the 3

3 entire DU. The combined increments (frequently totaling a kilogram or more) are typically processed at the laboratory and subsampled to provide an analytical aliquot of only a few grams that is used for analysis. The final analytical aliquot is the target sample. ISM is designed to provide an unbiased, statistically valid estimate of the mean value of an analyte within the DU. Through adequate spatial coverage of the DU as well as disciplined handling, processing, and subsampling of the single sample formed from the increments collected, ISM works to overcome major sources of error in both sampling and subsampling of soils that have often been apparent with current sampling practices. By design, ISM provides complete spatial coverage within the DU; however, ISM does not provide information on the spatial distribution of contaminants within the DU. Should this spatial variability be important to the decisions being made, a smaller DU should be used. ISM may not be appropriate in certain situations (see Section 8 for further information on the limitations on ISM). 1.2 Traditional Investigation Approach Limitations ISM addresses major sources of sampling error and increasing sample representativeness. Soil sampling is typically done to characterize a site. Historically, the majority of soil samples collected has been discrete samples. Collection of discrete samples is sometimes preferred or mandated by regulatory agencies (see Section 8). Over the years, consultants, environmental scientists, and regulators have become aware of a number of recurring challenges, problems, and deficiencies associated with collecting soil samples as discrete, composite, or any other sampling method, including the following: Lack of clear environmental objectives at the initiation of the investigation Often the primary objective is to find contamination with little clarity as to how the data will be used to determine whether identified contamination poses unacceptable risks to human health and the environment and often leading to lengthy delays in completion of project and expenditure of funds available for site investigation before adequate characterization is completed. Poor spatial coverage of areas targeted for investigation and inadequate sample density Generally, a minimum of discrete samples is needed for an adequate characterization of a targeted area and volume of soil; however, only a small number (e.g., <10) of discrete samples are commonly collected to characterize large areas of suspected contamination. The degree of coverage is typically controlled by the amount of funds available for laboratory sample analysis, thus limiting the number of samples needed to provide a representative and statistically valid characterization of a targeted area. Laboratory aliquots prepared for analysis not necessarily representative of the field sample Aliquots prepared by random selection of a single, small mass of soil from the field sample container are not representative of the larger volume of soil delivered to the laboratory. Traditional soil sampling and analysis methods impart a level of uncertainty in the use of data generated to identify potential environmental hazards associated with contaminated soil and to support decisions for or against remediation. In large measure, ISM is evolving to address these limitations. 4

4 1.3 How ISM Addresses Traditional Investigation Approach Limitations The fundamental question with all soil sampling, discrete or incremental, is representativeness. In reviewing sampling results, environmental professionals often find themselves asking, What does the sample concentration we get back from the lab represent? With incremental sampling that question is purposefully rephrased as, What does the (incremental) sample have to represent? and that question is used to shape the project planning and establishment of DQOs well before any sample is actually collected. The major problem with discrete soil sampling is the extreme magnitude between the mass of the subsample analyzed by the laboratory and the mass of the target population (area to be investigated or sample volume collected), which can be on the order of 1 in 10 million or more. This increases the chance that the sample misses contamination, which will consequently not be represented in the analytical results at all. ISM builds a sample from increments to provide a good representation of the DU and so is more likely to capture even heterogeneous contamination. ISM requires that the project team address the spatial dimensions associated with the analyte concentration that is of interest. That is, the project team must define the DU to be represented by each incremental sample. This requirement is inherent in any soil sampling effort but must be addressed head-on and with great deliberation in ISM. ISM forces the project team to confront the inherent heterogeneity in soil by defining the scale at which heterogeneity will be addressed. ISM does this early in the project life cycle by getting stakeholder agreement on the dimensions of the DUs from which samples will be collected. The scale issue is present for all sampling approaches but has typically not been made an integral part of the sampling strategy as it has with ISM. Furthermore, once the scale of the DU has been decided, the concept of hot-spot delineation within the DU should be moot. If it is not, then the DU may not be appropriately sized and should be reevaluated (see Section 3.5 for further discussion on hot spots). ISM addresses common errors associated with sampling soils. As such, ISM embeds the concept of quality assurance (QA)/quality control (QC) in a meaningful way into planning, design, field sampling and sample processing, as well as laboratory work, by explicitly addressing all of the activities necessary to build an ISM sample that will be representative of the DU of interest. Traditional QA/QC approaches have focused primarily on laboratory procedures, particularly those that take place after a subsample of soil has been extracted, and do not address the major sources of error that occur well before an extract solution is introduced into an analytical instrument. 1.4 How ISM Compares to Compositing Early in the project life cycle, ISM forces the project team to confront the inherent heterogeneity in soil by defining the scale at which heterogeneity will be addressed. Environmental professionals recognize the act of combining increments as being similar to conventional compositing. ISM is an improved type of compositing in comparison to conventional compositing in that great attention is given to establishing the DU. ISM also 5

5 requires that the total sample mass be sufficient to represent the heterogeneity of soil particles within the DU in proportion to all of the DU soil (i.e., population) and that a sufficient number of equal-volume increments are collected in an unbiased manner from throughout the entire DU so that all particles in the unit have an equal probability of being included in the sample. Thus, the incremental sample has the goal to contain all constituents in exactly the same proportion as they are present in the DU (i.e., the sample is representative of the DU). Proper laboratory processing and subsampling procedures then produce an aliquot for analysis that contains all constituents of the subsample in the same proportion in which they occur in the sample and, therefore, the DU. ITRC s ISM Team has found that many state regulatory agencies have been reluctant to use composite sampling and that such reluctance spills over to ISM (see survey results in Section 8). One concern expressed with composite sampling is that clean or less-contaminated soil will be mixed in with contaminated soil, therefore diluting areas of high contamination. This problem can be minimized with a clear understanding of sampling objectives that incorporate the concept of a DU. As shown in Figure 1-2, a hypothetical DU may be sampled with several designs. This figure illustrates a discrete gridded sampling design, various composite designs, and an ISM design. While each has its advantages and limitations, a goal of incremental design is to provide a high degree of spatial coverage of the DU. It is also obvious that the incremental design is similar in appearance to the various composite designs recommended by the U.S. Environmental Protection Agency (USEPA). Compositing is discussed further in Section

6 COMPOSITE SAMPLING DESIGNS (Source: USEPA 2002e) DISCRETE SAMPLING DESIGN INCREMENTAL SAMPLING DESIGN Figure 1-2. Sampling designs. 1.5 Purpose The purpose of this technical guidance document is to advance the appropriate use of ISM for sampling soils at waste sites and potentially contaminated land or properties. In doing so, this document addresses those challenges that constrain or prohibit use of ISM. Some of these challenges may be directly associated with ISM, but as just described, others may be associated with questions poorly addressed in traditional soil sampling approaches using discrete samples. The challenge for developing this document on ISM is quite broad. See Section 8 for a detailed discussion on regulatory challenges and survey issues regarding ISM and how they can be successfully addressed. While the focus of this document is on sampling shallow soils, other interests and areas, including sampling of deeper soils, are also discussed. In addition, some of the limitations 7

7 associated with traditional soil sampling practices are not so much attributable to the reliance on discrete samples as they are due to the lack of clear and quantifiable sampling objectives to achieve project goals. Meeting sampling goals is discussed in Sections 2 and 3 of the document as part of the planning process, as well as part of the sampling design. ISM usage is increasing in the environmental field. Currently, two states, Alaska and Hawaii, use ISM based on guidance documents that each state has recently developed. In addition, USEPA SW-846 Method 8330B applies incremental sampling procedures for explosive residue field sample collection and laboratory analysis. Thus, it is timely for this document to be issued. Again, it is the intent of the ISM Team that this document advance the appropriate use of ISM, as well as to expand the list of chemical contaminants that can be addressed confidently by ISM. 1.6 Frequently Asked Questions Table 1-1 conveys many challenging points taken on by the ISM Team in preparing this document, including key points and frequently asked questions that are addressed within the referenced sections. Although ISM has advantages over traditional soil sampling practices, it may not be appropriate given the current state of technology for all sampling applications (e.g. low-level VOC analysis, possibly metal speciation, etc.) It is anticipated that technology advances will allow these limitations to be addressed. Table 1-1. Crosswalk for frequently asked questions on ISM Key Point/Question Reference How can a regulator (or anyone) better assess ISM? Section 1. Introduction What is ISM and what are the advantages/ disadvantages of using it? Currently, two states, Alaska and Hawaii, use ISM based on guidance documents that each state has recently developed. In addition, USEPA SW-846 Method 8330B applies incremental sampling procedures for explosive residue field sample collection and laboratory analysis. Although ISM has advantages over traditional soil sampling practices, it may not be appropriate for all sampling applications. All sections Contact the ISM Team with questions Participate in ISM Internet-based training Sections 1.3, 2.6.3, 3.5, and 8.5 Is ISM compositing? Sections 1.4, 2.6.2, and Is ISM data more representative than discrete Sections 1.3, 2.3.2, and 2.6 data? When should ISM not be employed? Figure 1-2, Sections 3.1, 8.3, and 8.5 Section 2. Nature of Soil Sampling and Increment Sampling Principles Is ISM really based on Gy s Theory? Section 2.5 Why do we care about the mean value in a DU? Section 2.1 Section 3. Systematic Planning and Decision Unit Designation What is a DU, and how is it established? Sections 3.2 and 3.3 At what types of sites can ISM be used? Section 3.3 8

8 Key Point/Question Reference Can ISM be used at any point of an Sections 3.1, 3.2, and 3.3 investigation? How many increment and replicate samples Sections 3.1, 2.5.6, 4, 5.3, , and should be collected? Appendix A How can ISM be used in risk assessments? Sections 3.1 and 3.3 Can ISM be used for ecological investigations? Sections 3.1, 3.2, 3.3, 4.4.4, 7.1, and Can ISM be used to delineate contamination? Section 3 Can I use ISM when needed to determine Sections 3.1, 3.2, 3.3, 8.3, and 8.5 whether contamination is a leaching concern? How do you compare ISM background samples Sections 3.1, 3.2, 3.3, , 7.2.4, and to background generated from discrete samples? Does ISM mask areas of high concentration or Sections 3.5, 8.2, and 8.5 hot spots? What soil sampling depth should be used with Sections 3.1, 3.3, 5.3.1, and ISM? Section 4. Statistical Sampling Designs for ISM How do you calculate a 95% upper confidence Section 4.2 limit with ISM data? Can ISM data/results be compared to discrete Sections and data/results? Section 5. Field Implementation, Sample Collection, and Processing How do you sample for volatile organic Section compounds (VOCs) with ISM? How do you ship VOC ISM samples? Section Section 6. Laboratory Sample Processing and Analysis What contaminants are most suitable for ISM? Section 6.1 Do ISM samples require more laboratory sample Section 6.2 and preparation? What effects does sample processing (grinding, Section 6.2 etc.) have on contaminant concentration? How are DQOs addressed in the laboratory? Section How do you address low-level reporting Sections requirements of VOCs with ISM? Section 7. Making Decisions Using ISM Data How do you use ISM data? Section 7 Section 8. Regulatory Concerns with ISM What are the regulatory challenges and what are possible solutions? Section 8 Are there cost savings when using ISM instead Section of discrete only sampling? Is subsurface ISM sampling cost-effective? Section Can ISM data/results be compared to regulatory Sections criteria (e.g., not to exceed )? 9

9 1.7 Document Organization This document is organized into 11 sections which reflect the ISM Team s best effort at presenting a wealth of information concerning ISM in a logical and cohesive manner. Beyond the mechanics of collecting ISM samples, much attention has been given to the planning process, particularly in Sections 2 6. Section 2 presents the nature of soil sampling and fundamental ISM sampling principles. Section 3 focuses on systematic planning and how to determine a DU. Section 4 covers the statistical basis of ISM sampling design, the results of statistical simulations and the effects of changing the number of increments, replicates and the effects of sample patterns. NOTE: Hyperlinks This guidance was developed as Web-based document. The blocks of information presented online as Hyperlinks are contained in Appendix G. Section 5 provides information on sampling tools, field sampling collection, and field handling procedures. Section 6 presents the current practices and options available for laboratory processing and subsequent analysis. Section 7 covers what to do with ISM data. Section 8 summarizes the regulatory concerns with ISM and the ISM survey results. Section 9 provides selected case studies as examples. Section 10 includes input from stakeholders. Section 11 provides the list of additional materials referenced throughout this document. Appendix A presents additional details regarding the simulation studies used to evaluate the performance of alternative ISM sampling strategies. Appendix B presents August 2009 Survey Results. Appendix C presents Case Studies. Appendix D includes ISM Team Contacts. Appendix E includes the Glossary. Appendix F provides a list of Acronyms. 10

10 1.8 Key Terms This document includes new terminology introduced by ISM, and Figure 1-3 provides some key terms. See Appendix E for a glossary of additional terms. Figure 1-3. Key ISM process terms. 11