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by Douglas M. Chatham
Senior Chemist


The field duplicates program should be eliminated from most environmental projects. Additional sampling locations would provide more information than duplicates. Field duplicates provide no information about accuracy and questionable information about precision. A much better evaluation of site precision could be obtained by dividing the set of environmental samples into statistical sub-sets. Elimination of field duplicates would reduce analytical costs by at least 5%.


Any and all QC which contributes to the quality of the data or are required for other reasons should be included regardless of arguments presented in this paper. For each QC sample or analysis proposed, Project Managers (PMs) and Quality Assurance Project Officers (QAPOs) should ask what that determination contributes to the quality of the data and whether it helps meet the project DQOs. If a QC sample contributes nothing toward the DQOs, an argument should be made against incurring the cost for that sample.
Collect and analyze field duplicates for Level IV (CLP) projects only. Eliminate or greatly reduce the requirements for field duplicates for Levels I, II, and III projects, unless it is necessary to establish statistical measures of uncertainty in the definition of extent of contamination.


The two types of Field Duplicates are split samples and co-located samples. A split sample is a sample which has been thoroughly blended and split between two containers. Often, the split samples are sent to different laboratories. Split samples are intended to measure the precision of the whole sampling and analysis procedure. Most often, if they contain anything to measure, split samples are a measure of how thoroughly the sample was blended before being split. There is no way to determine an effect on the rest of the samples at the site. Co-located samples are samples taken in the same location but not blended. The intent of co-located samples is to measure sampling precision or the variability of the matrix.

";When designing experiments or procedures, it is important to keep in mind that the overall objective is accuracy. It naturally follows that those in charge of a project should ask whether additional measurements really contribute to the accuracy of a method, or simply to its precision.
In today's business world cost is very important, and each extra measurement adds to the cost of a project. We all know that precision is important, but we need to take a closer look at the costs and benefits to the customer when expenses are increased for the sake of improving precision without necessarily increasing accuracy."(1)
Often, the stated purpose of field duplicates is to measure the precision of the complete process from sampling through analysis. This is nice-sounding phraseology in a work plan, but what can you do with the results? Due to the potentially large variability inherent in the media being sampled particularly for soils and sediments, one sample location out of twenty probably will not represent the sampling or matrix variability. The result is that these measurements are often reported as measures of ";precision";, but they have no effect on the flagging or the use of the data. As stated above, the source of the greatest variation in environmental analytical results is the variability of the media. Comparable results (<40% RPD) are seldom achieved from co-located duplicate soil samples, even with the best efforts of the best sampling technicians available. A statistical evaluation of all sample results at a site should be used to measure the precision and representativeness of the sampling program. These statistical measures may provide confidence intervals for establishing extent of contamination in a medium. For duplicates to adequately assess sampling precision, they would have to be statistically selected as a subset of the samples taken at the site and evaluated as a subset to determine if the subset results in the same overall values as the original set of samples.

Types of Investigations

The CERCLA preliminary assessment (PA) and the RCRA facility assessment (RFA) are used to identify any releases or migration from a facility based on the existing information and apparent visual evidence. The PA or RFA are typically the first step in the CERCLA or RCRA process. Samples are not usually taken during these assessments.
The CERCLA Site Investigation (SI) and the RCRA Confirmation Sampling Process (CS) are used to confirm or disprove suspected releases to an environmental medium based on information obtained during the PA or RFA. The SI or CS process is generally limited in scope; the goal is to gather sufficient data to confirm the presence or absence of contamination at Potential Release Locations (PRLs), and to provide a basis for the scope of the CERCLA Remedial Investigation (RI) or the RCRA Facility Investigation (RFI) process or a No Further Action decision. The data quality levels for SIs or CSs are generally required to be Level IV (CLP) or Definitive data (2) since the data is required to be litigation quality. These investigations are usually the first ones conducted at most sites and historical data will be limited or non-existent. An expansion of the sampling scope would provide more information than duplicates.
Additional sample points would provide more useful data than field duplicates for all projects following the CERCLA SI or RCRA CS studies. This includes the CERCLA Remedial Investigations (RI), Feasibility Studies (FS), and Remedial Design/Remedial Action Studies (RD/RA), the RCRA Facility Investigations (RFI), Corrective Measures Studies (CMS), and Corrective Measures Implementations (CMI), and all cleanup verifications and monitoring programs.


1. EPA, Risk Assessment Guidance for Superfund, Volume I, Human Health Evaluation Manual (PartA), p. 5-5, EPA/540/1-89/002, December 1989.
2. EPA, Test Methods for Evaluating Solid Waste, Physical/Chemical Methods, SW-846, 3rd Edition, Final Update 1, November 1990.

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