Setting Priorities for Drinking Water Contaminants (1999)

The provision of safe drinking water has been a major triumph of twentieth-century U.S. public health practice and has been an important factor in the improvement of the health status of U.S. communities since the turn of the last century. Nevertheless, chemical and microbiological contaminants still occur in drinking water supplies. Further, waterborne disease has not been entirely eliminated in the United States, as was evident in the major cryptosporidiosis outbreak that affected some 400,000 Milwaukee residents in 1993. The continuing presence of contaminants in water supplies and occurrences of waterborne disease serve as reminders that the system for regulating drinking water in the United States needs to be reassessed periodically.

The most recent update of U.S. policies for ensuring the safety of public water supplies occurred in the summer of 1996, when Congress enacted the Safe Drinking Water Act (SDWA) Amendments of 1996. These amendments substantially revised the process for regulating public water supplies. One of the major changes was the creation of a new policy for establishing standards for contaminants that currently are unregulated. The amendments require that every five years, the Environmental Protection Agency (EPA) develop a list of currently unregulated contaminants that may pose risks in drinking water and decide whether to regulate at least five of those contaminants. In March 1998, EPA published the first of these lists, known as the Drinking Water Contaminant Candidate List (CCL), of priority unregulated contaminants. Subsequently, EPA asked the National Research Council (NRC) for assistance in developing a process for setting priorities among the listed contaminants. This report responds to that request. Specifically, the report evaluates various existing schemes for set-

ting priorities among environmental contaminants and recommends a framework to guide EPA in deciding which contaminants on the CCL to regulate, which to monitor, and which to study further.

This report was written by the NRC's Committee on Drinking Water Contaminants. The committee was appointed in 1998 in response to EPA's request for assistance in establishing a priority-setting process for drinking water contaminants. EPA also requested help in establishing processes for identifying emerging drinking water contaminants and developing future CCLs. The committee consists of 14 volunteer experts in water treatment engineering, water chemistry, analytical chemistry, microbiology, toxicology, public health, epidemiology, risk assessment, and risk communication. This report's findings are based on a review of relevant technical literature, information gathered at two committee meetings, and the expertise of committee members. Future committee reports will advise EPA on emerging drinking water contaminants and establishment of future CCLs.

In reviewing this report, the reader should keep in mind that the committee was guided first and foremost by concerns about public health. The committee chose this perspective because public health is the basis for the SDWA; the SDWA directs the EPA administrator to consider ''contaminants for listing that, first, may have an adverse effect on the health of persons." Further, the report takes the position that scientific disagreements about the public health effects of contaminants and their relative severity are the norm and do not signal a deviation from sound science. Paradoxically, when data are sparse they often appear consistent and coherent (for example, when produced by one or a few laboratories), but data gaps become evident as the problem is examined more fully by different methods and from different perspectives. The EPA faces a challenging task in assessing the available scientific information about contaminant risks and, based on that assessment, making a risk management decision about which contaminants should be regulated. In this process, there is no replacement for policy judgments by EPA. The committee purposely declined to define what constitutes "sufficient" data for making decisions related to drinking water contaminants, because this is a matter of judgment that will vary with context.

Government agencies and private industries have developed a number of schemes that rank chemicals according to their importance as environmental contaminants. No equivalent schemes exist for microbial contaminants. The committee reviewed ten chemical prioritization schemes (see Table ES-1) to examine what methodological elements and data considerations in these schemes may be useful for prioritizing drinking water contaminants. All the schemes prioritize chemicals on the basis of risk to human health and/or the environment

by considering exposure and toxicity. Each scheme is unique, however, in its use of data and ranking criteria.

The committee concluded that a ranking process that attempts to sort contaminants in a specific order is not appropriate for the selection of drinking water contaminants from the CCL for regulation. In the absence of complete information, the output of prioritization schemes is so uncertain (though this uncertainty is generally not stated) that they are of limited use in making more than preliminary risk management decisions about drinking water contaminants. While the ranking schemes the committee evaluated presumably are useful for their intended purposes and may provide a quantitative means for screening and sorting large numbers of contaminants, their accuracy is not sufficient for prioritizing a relatively small number of contaminants, many of which may pose similar degrees of risk for drinking water. Simple quantitative ranking processes, such as those the committee examined, cannot substitute for policy judgments by EPA when moving toward final regulatory decisions. Furthermore, if enough information is available to determine that a contaminant occurs in drinking water at levels and frequencies that may pose a health risk, then the contaminant should be considered for regulation, without attempting to assign a priority to it. That said, however, the existing methods for ranking environmental contaminants may be useful for sorting large numbers of potential contaminants not yet on the CCL to determine which ones should be included on future CCLs. The committee will use its analysis of these schemes in providing guidance on CCL development in a future report.

The existing CCL is essentially an unprioritized list of research needs for drinking water contaminants. Additional research will need to be conducted for many, if not most, of the contaminants on the current CCL. At the same time, however, to be included on the list contaminants had to pass a relatively rigorous screening process involving assessment of existing contaminant occurrence and heath effects data by a group of experts (the National Drinking Water Advisory Council Working Group on Occurrence and Contaminant Selection). A variety of stakeholders, including representatives of the water utility industry and public interest groups, commented on the list, and the list was revised accordingly. Thus, the contaminants on the current CCL may have a much higher likelihood of posing risks in drinking water than would a randomly assembled list of unregulated chemicals and microorganisms. A key question for EPA is how to determine which contaminants can be moved off this research list and into the regulatory arena. Ideally, EPA will develop a process for making these determinations that will apply not only to the current CCL but also to future CCLs.

The committee recommends that EPA use a phased process, shown in simplified outline in Figure ES-1, for determining which contaminants on the CCL are appropriate candidates for regulatory action and which will require research. Figure ES-1 includes a recommended time line for completing each phase of the process. The time line is provided to help EPA allocate time and resources in order to meet the 1996 SDWA Amendments' requirement to publish regulatory determinations (i.e., regulate or not regulate) for at least five contaminants on the CCL by August 2001. However, the committee recognizes that almost one year of the originally allotted time (i.e., three and one-half years following publication of the first CCL) have already passed. Thus, the time line should not be strictly interpreted, but should serve as a guide for the relative amount of time to allocate to each step in the process. The suggested time line should be of more direct use following the publication of future CCLs.

Chapter 5 explains the details of the decision process shown in Figure ES-1. In brief outline, the process would proceed as follows:

• Within approximately one year of completion of the CCL, EPA should conduct a three-part assessment of each contaminant on the CCL. For each contaminant, the three parts consist of (1) a review of existing health effect data, (2) a review of existing exposure data, and (3) a review of existing data on treatment options and analytical methods. The first part of the assessment should consider data on the contaminant's effects on vulnerable subpopulations such as pregnant women, infants, the elderly, and those with compromised immune systems. While general guidelines for reviewing existing data are possible and are presented in Chapter 5, an important component of the reviews will be policy judgments by EPA about the significance of the data.
• After completion of the three-part assessment, EPA should conduct a preliminary risk assessment based on available data identified in the three-part assessment. The risk assessment, which integrates hazard and exposure analyses to estimate the public health implications of the contaminant, should be carried out even if there are data gaps; this will provide a basis for an initial decision about the disposition of the contaminant and guide research efforts, where needed. The preliminary risk assessment should not be overly detailed or resource intensive. EPA's usual approach to risk assessment is appropriate, and the committee does not see a need to create new procedures for this step.
• After completing the preliminary risk assessment, EPA should prepare a separate decision document for each contaminant that indicates whether the contaminant will be dropped from the CCL because it does not pose a risk, will be slated for additional research (e.g., on health effects, exposure, or risk reduction), or will be considered for regulation. The decision document should explain the reasoning for EPA's determination and should be publicly disseminated for com-

• ment. Decision documents for contaminants dropped from the CCL should specify the health and exposure data that EPA used to conclude that the contaminant poses little or no risk.
• When the three-part assessment or preliminary risk assessment identifies important information gaps, EPA should develop a research and monitoring plan to fill such gaps in time to serve as the basis for a revised assessment and decision document before the end of the three-and-a-half-year cycle required by Congress for evaluating contaminants on the CCL. In filling information gaps, EPA should solicit the voluntary participation of industry and others and should use its other authorities (such as those under the Toxic Substances Control Act and the Federal Insecticide, Fungicide, and Rodenticide Act) to help fill data gaps.
• Health advisories should be issued for all contaminants remaining on the CCL after completion of an initial set of decision documents. A health advisory is an informal technical guidance document that defines a nonregulatory (i.e., nonenforceable) concentration of a drinking water contaminant for which no adverse health effects would be anticipated over specific exposure durations. To provide the public with the best available information about the contaminant, EPA should develop a health advisory for any contaminant for which credible evidence of a risk in drinking water exists, even if existing data are insufficient to develop a full regulation. Contaminants subject to a health advisory may need additional research and monitoring even after completion of a revised assessment and decision document.

As indicated in Figure ES-1, decisions to drop a contaminant from the CCL, to issue a health advisory, or to proceed toward regulation should be based on health risk considerations only. However, EPA should fill data gaps in treatment technologies and analytical methods to avoid delaying regulatory action for contaminants for which current information on treatment and detection is inadequate.

In implementing this phased process, EPA should keep in mind that it should act immediately on any contaminant that meets the statutory tests of (1) a determination that the contaminant "may" adversely affect public health, (2) evidence that the contaminant is known or substantially likely to occur in public water systems with a frequency and at levels that pose a threat to public health, and (3) an indication that controlling the contaminant presents a meaningful opportunity for health risk reduction. Development of regulations for contaminants that meet these three requirements (which are specified in the SDWA amendments) should not be delayed by implementation of the phased approach. The ability to act quickly and short circuit the phased evaluation process is especially critical for newly discovered high-risk contaminants.

EPA should also keep in mind that a mechanistic, quantitative ranking approach—a system that would mechanically process all information about contaminants and produce a regulatory determination — is not appropriate for prioritizing contaminants on the CCL. While Chapter 5 of this report provides details

about data and criteria that EPA should consider in its three-part reviews of contaminants, it does not specify a mechanistic tool (one free of policy judgments) for assessing contaminants. The need for policy judgments by EPA cannot and should not be removed from this process. Ultimately, EPA is accountable to the public for the decisions it makes about regulating drinking water contaminants. In making these decisions, EPA should use common sense as a guide and should err on the side of public health protection.