ceptual two-step process (see Figure 1–3) for the creation of future CCLs that would take a much broader approach to contaminant selection than that used to create the 1998 CCL and, thus, would have a better potential to identify high-risk contaminants. At EPA’s request, the committee evaluated, expanded, and revised as necessary the conceptual approach and related conclusions and recommendations from its second report to form the majority of this report. Therefore, it is important to note that although the basic concept for the CCL development approach has not changed, many of the associated guidelines and recommendations for its design and implementation have necessarily been revised and expanded in accordance with this second phase of committee deliberations. The committee continues to recognize, however, that the need for policy judgments by EPA cannot and should not be removed from any CCL development process. Furthermore, in making decisions regarding the placement of a contaminant on a preliminary CCL (PCCL) or CCL, EPA should use common sense as a guide and err on the side of public health protection.

This chapter provides some initial guidance and recommendations for conducting the first step of the CCL development process. Consistent with the inclusive nature of the recommended process, the first step is to consider a broad universe of chemical, microbial, and other types of potential drinking water contaminants and contaminant groups. The total number of contaminants in this universe is likely to be on the order of tens of thousands of substances and microorganisms, given that the Toxic Substances Control Act inventory of commercial chemicals alone includes about 72,000 substances (NRC, 1999b). It is from this universe that contaminants will be selected first for inclusion on a PCCL—which may include a few thousand contaminants—and then on a corresponding CCL. Thus, the creation of a PCCL from the universe would entail an almost two-order-of-magnitude reduction of potential drinking water contaminants. The inherent difficulties associated with such a task are discussed in Chapter 2.

As noted earlier, the recommendation that EPA begin by identifying and assessing the universe of potential drinking water contaminants to arrive at a PCCL represents a dramatically larger set of substances to be considered initially in terms of types and numbers of contaminants than that used for creation of the 1998 CCL. Because of the proposed size of this universe, well-conceived screening criteria must be developed that can be rapidly and routinely applied in conjunction with expert judgment to cull the universe of contaminants to a much smaller PCCL. Thus, the PCCL may be thought of as a much more manageable and less concep-

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