Volume 44 Number 8
A Contaminant Candidate List: Prioritizing Drinking Water Contaminants
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In 1996, the Safe Drinking Water Act (SDWA) was amended requiring that the U.S. Environmental Protection Agency (USEPA) establish a list of contaminants of potential public concern that weren't currently regulated. This list is known as the Drinking Water Contaminant Candidate List (CCL)1 -- see http://www.epa.gov/safewater/ccl/cclfs.html -- and identifies priority contaminants for drinking water research, occurrence monitoring and guidance development, including health advisories.
The initial criteria for identifying contaminants for the CCL were based on whether or not the contaminant adversely affected public health and whether it's known or suspected to occur in public water systems at exposures potentially harmful to public health. Initially 391 contaminants, including 25 microorganisms, were listed as potential drinking water concerns. Limited resources and practical evaluation required that this list be reduced to more manageable numbers. Even when contaminants are recognized as waterborne and a public health threat, they may be difficult to regulate due to the lack of sensitive methods for their detection, information as to their harmful dose levels, and many other factors.
The first CCL
Although Table 1 lists all of the 1998 Candidate Contaminant List items, the USEPA recently proposed to eliminate nine items from regulatory consideration (see bolded text in column one of Table 1). Current data indicates that these nine items do not meet the requirements for contaminant regulation under the SDWA. The agency is soliciting public comment now on this decision (see www.epa.gov/safewater/ccl/pdf/prelimreg_fr.pdf).2
The first CCL -- a first draft of which was released in 1997 -- was an unranked list. With tens of thousands of potential candidates, choosing which contaminants of drinking water should be top priority for regulatory consideration isn't a simple process. In addition, the regulatory process isn't rapid, in part because a cost/benefit and risk management analysis must be performed so that the practicality, and relative need, of a new regulation can be assessed. In 1998, at the request of the USEPA Office of Ground Water and Drinking Water, the National Research Council's (NRC) Water Science and Technology Board and Board on Environmental Studies and Toxicology oversaw the formation of the Committee on Drinking Water Contaminants. The NRC committee's primary function was to provide advice on setting priorities for drinking water contaminants that would identify contaminants posing the greatest threat to public health.
Guidance from the NRC
Building on the first two reports, the third NRC report recommends a two-step process for developing future CCLs. In this process, a "universe" of drinking water contaminants is established including naturally occurring substances, water-associated microbes, chemicals, products of environmentally transformed chemicals, reaction by-products, metabolites in the environment, radionuclides, biological toxins and fibers. This broad list of contaminants is then reduced to a Preliminary CCL (PCCL) based on whether they are known or thought to be a health risk, and are known or thought to occur in drinking water. Each PCCL is then assessed using a classification tool and expert judgment to create the CCL. The expert judgment is important because occurrence and health effects data may not be known, even for some of the most harmful contaminants. These contaminants shouldn't be overlooked due to a lack of information. The CCL selection process should be repeated for each list development cycle to consider any new information that may have become available since the last CCL was finalized.
Problems with 1998 CCL
Some of the specific concerns of the committee were:
The USEPA was forthright in admitting there was a need to improve the process for the development of the next CCL in 2003.6
New approaches are needed to evaluate the harmful potential of microbes. The NRC committee suggested a method similar to that used for evaluation of chemicals. Quantitative structure activity relationships (QSARs) have been used for chemicals where the structures of new chemicals are compared to known chemical toxins, in an attempt to predict the new chemical's toxicity factor. Similarly, the virulence factor activity relationship (VFAR) -- the relationship between a known or potential microorganism and its real or potential ability to cause adverse health effects -- is suggested as a means of identifying microbial contaminants of drinking water.
The increasing advances in molecular methodologies have aided in the development of a genetic database of microbial pathogens. Genetic detection methods can help to identify the potential threat of a microbe since specific structural and biochemical characteristics (i.e., lipid structures, toxin production, surface proteins, attachment and invasion structures, etc.) are more likely associated with the ability of the organism to be harmful.
Other improvements suggested by the NRC committee to the USEPA included the need to recognize sensitive populations relative to drinking water contaminants. In the past, regulators have recognized that they must consider all consumers, including susceptible populations that are most affected by drinking water pollutants. Highly vulnerable populations include the very young, the elderly, the immunocompromised, the chronically ill, etc. Up to 20 percent of the U.S. population, a significant portion, is considered immunocompromised. The current SDWA legally mandates consideration of vulnerable subpopulations; however, the Committee on Drinking Water Contaminants recommends the category of susceptible populations be expanded to include women of child-bearing age, unborn fetuses, persons genetically predisposed to adverse reactions, the malnourished, and persons with individual sensitivities to specific contaminants.
* All three NRC reports are available to purchase or read online at www.nap.edu
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