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Classifying Drinking Water Contaminants for Regulatory Consideration: Committee on Drinking Water Contaminants, Water Science and Technology Board, Board on Environmental Studies and Toxicology, National Research Council

Most people would agree that protecting our drinking water supply by regulating the maximum allowable content of hazardous contaminants is desirable. What is of considerable debate is which contaminants should be regulated, how many should be regulated, and what process will be used to select them. The U.S. Environmental Protection Agency (EPA) currently has enforceable National Primary Drinking Water Regulations for more than 80 inorganic and organic chemical, radionuclide, and microbial contaminants and groups of related contaminants under the Safe Drinking Water Act (SDWA). The 1996 SDWA Amendments were intended to further this protective approach by requiring EPA to periodically develop a list of contaminants that are currently unregulated and may pose a health risk. The agency must then select from each list-called the Drinking Water Contaminant Candidate List, or CCL-at least five contaminants for regulatory decisions every five years. In addition to supporting the mandated development of drinking water regulations, each CCL is intended to be the source of priority contaminants for the EPA's drinking water program as a whole and, to include research, monitoring, and guidance development. However, the specifics of developing the CCL and the manner in which the five or more contaminants are ultimately selected for regulatory decisions are not specified in the legislation. The EPA requested assistance from the National Research Council (NRC) in addressing these difficult issues. This project has been conducted in two phases. The first phase was completed in July 1999 and resulted in two reports. The first of these, Setting Priorities for Drinking Water Contaminants, examines past approaches used by federal agencies, state and local governments, public water utilities, and other organizations for establishing priorities among drinking water contaminants and other environmental pollutants. It also recommends a phased decision process, associated time line, and related criteria to assist EPA efforts to set priorities and decide which contaminants already on a CCL should be subjected to regulation development, increased monitoring, or additional research. The NRC also convened a workshop on `emerging' drinking water contaminants and published the proceedings in a second report entitled Identifying Future Drinking Water Contaminants. Preceding a dozen papers presented by government, academic, and industry scientists at the workshop is a short committee report that outlines a conceptual approach to the creation of future CCLs. In that report, the committee strongly urged EPA to consider the benefits of a more carefully considered and detailed description of the requirements of a CCL development process, especially regarding the identification of critical drinking water contaminants for regulatory activities from among tens of thousands of potential candidates. The second phase of the study focused on refining specific methods and processes to identify and narrow a very broad universe of potential contaminants into a smaller, more focused list for planning and action by interested parties. The specific tasks of the second phase are as follows: Identify and evaluate a process to narrow, focus, and prioritize contaminants from a preliminary list for inclusion on a smaller, more manageable list of contaminants, including chemical and microbiological contaminants. The process and methods will include simple (semi) quantitative tools to cull the broad preliminary list of contaminants. The tools that are developed to narrow and focus future drinking water contaminant lists will be tested using validation case examples of currently regulated contaminants. Explore the feasibility of developing virulence-activity relationships (VARs, now termed virulence-factor activity relationships or VFARs) for microbial contaminants. If a scientifically sound basis for developing VFARs is determined to be feasible, the committee will provide initial guidance and recommendations for interested parties on the steps necessary to construct and use VFARs. Time and resources permitting, the committee will provide specific recommendations of methods for narrowing the broadest universe of contaminants to a smaller, preliminary contaminant list. The Committee on Drinking Water Contaminants of the NRC addresses these three issues in the following report. We have recommended what may be considered a bold and innovative approach to selecting contaminants for inclusion on future CCLs. In our second report, we recommended that the CCL be developed in a two-step process. Initially, the `universe' of potential drinking water contaminants is identified by considering many possible categories and sources of contaminants. A preliminary CCL, or PCCL, is culled by a screening process and expert judgment from this universe. Then the CCL is selected from the PCCL using a more refined process in conjunction with expert judgment. In this report, we provide initial guidance and several recommendations for how to accomplish the first step in this process. However, the bulk of the committee's effort for this final report was focused on providing a detailed paradigm for selection of the CCL from the PCCL. To this end, we have recommend that EPA develop and use a set of selected contaminant attributes to evaluate the likelihood that a contaminant or group of related contaminants would occur in drinking water at sufficient concentrations or prevalence to pose a public health risk. To make this determination, we recommend that the agency use a prototype classification algorithm in conjunction with expert judgment. Although this approach requires considerable initial investment by EPA, we feel that it represents a superior approach to relying exclusively on expert judgment or ranking schemes such as those reviewed in our first report. The committee has gone so far as to develop a demonstration algorithm to test the efficacy of this approach, and the results are compelling. Last but not least, the committee concludes that the construction and eventual use of VFARs within EPA's drinking water program is indeed feasible and merits careful consideration. We also provide some initial guidance and recommendations for their application herein. The committee is grateful for the support of this project by Michael Osinski and his colleagues at EPA's Office of Ground Water and Drinking Water. In addition, we would not have been successful in our endeavors without the contributions of several experts who gave presentations to us during our first two meetings and aided in focusing our discussions. They include Fred Hauchman, EPA Office of Research and Development; Robert Clark, EPA Office of Research and Development; Kenneth Beattie, Oak Ridge National Laboratory; and Betty Olson, University of California at Irvine. We have been highly fortunate as a committee to have the significant contributions and guidance of Mark Gibson, study director of this project and staff officer in the NRC's Water Science and Technology Board, and Carol Maczka, former program director for toxicology and risk assessment of the NRC's Board on Environmental Studies and Toxicology.

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