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The Universe of Potential Contaminants to the Preliminary CCL

INTRODUCTION

As noted in Chapter 1, the Safe Drinking Water Act (SDWA) Amendments of 1996 require the U.S. Environmental Protection Agency (EPA) to periodically publish a list (the Drinking Water Contaminant Candidate List [CCL]) of unregulated contaminants and contaminant groups that are known or anticipated to occur in public water systems and which may require regulation (EPA, 1998b). The CCL is intended primarily to provide the basis for a mandated EPA decision whether to regulate or not at least five new (CCL) contaminants every five years (see Figure 1–1). More generally, each successive CCL is intended to be the source of priority contaminants for the agency’s drinking water program as a whole, including research, monitoring, and guidance development. The first CCL, published in March 1998, was created under very pressing time restraints by EPA with significant input from the National Drinking Water Advisory Council (NDWAC) Working Group on Occurrence and Contaminant Selection. The limitations associated with its rapid development were summarized in the committee’s second report (NRC, 1999b) and are described more fully in Chapter 2 of this report. To a large extent, it was the internal and external recognition of these limitations and the corresponding call for a more systematic, scientifically defensible, and transparent approach to the creation of future CCLs that led EPA to request the formation of this National Research Council (NRC) committee.

TWO-STEP APPROACH

In its second report (NRC, 1999b), the committee proposed a con-



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Classifying Drinking Water Contaminants for Regulatory Consideration 3 The Universe of Potential Contaminants to the Preliminary CCL INTRODUCTION As noted in Chapter 1, the Safe Drinking Water Act (SDWA) Amendments of 1996 require the U.S. Environmental Protection Agency (EPA) to periodically publish a list (the Drinking Water Contaminant Candidate List [CCL]) of unregulated contaminants and contaminant groups that are known or anticipated to occur in public water systems and which may require regulation (EPA, 1998b). The CCL is intended primarily to provide the basis for a mandated EPA decision whether to regulate or not at least five new (CCL) contaminants every five years (see Figure 1–1). More generally, each successive CCL is intended to be the source of priority contaminants for the agency’s drinking water program as a whole, including research, monitoring, and guidance development. The first CCL, published in March 1998, was created under very pressing time restraints by EPA with significant input from the National Drinking Water Advisory Council (NDWAC) Working Group on Occurrence and Contaminant Selection. The limitations associated with its rapid development were summarized in the committee’s second report (NRC, 1999b) and are described more fully in Chapter 2 of this report. To a large extent, it was the internal and external recognition of these limitations and the corresponding call for a more systematic, scientifically defensible, and transparent approach to the creation of future CCLs that led EPA to request the formation of this National Research Council (NRC) committee. TWO-STEP APPROACH In its second report (NRC, 1999b), the committee proposed a con-

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Classifying Drinking Water Contaminants for Regulatory Consideration 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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Classifying Drinking Water Contaminants for Regulatory Consideration tual list than the universe of potential contaminants. As such, a PCCL should contain individual substances and groups of related substances, including microorganisms, that merit further consideration for inclusion on the CCL. However, the committee continues to recommend that the preparation of a PCCL should not involve extensive analysis of data, nor should it directly drive EPA’s research or monitoring activities (NRC, 1999b). THE UNIVERSE OF POTENTIAL DRINKING WATER CONTAMINANTS Recognizing that no single comprehensive list of potential drinking water contaminants exists, the committee previously identified nine major categories (with twelve subcategories) of individual and related groups of substances and microorganisms that comprise the universe of potential drinking water contaminants (NRC, 1999b).1 These are listed in Table 3–1, along with examples of each category or subcategory. It is important to recognize that many contaminants (including the examples provided) could belong to more than one of these categories. The committee continues to recommend that EPA begin identifying the universe of potential contaminants by considering the categories and subcategories of potentialcontaminants listed in Table 3–1. Furthermore, EPA should start this task by relying on databases and lists that are currently available (see Tables 3–2 and 3–3 for chemicals and Table 3–4 for microorganisms) and under development along with readily available information to supplement them. However, while relevant databases and lists exist for many of these categories of potential drinking water contaminants, many have no lists or databases (e.g., products of environmental degradation). Thus, EPA should develop a strategy for filling the gaps and updating the existing databases and lists of contaminants (e.g., through involvement of the NDWAC or panels of experts) for future CCLs. This strategy should be developed with public, stakeholder, and scientific community input. In addition, to generally assist in identifying 1   According to SDWA Section 1401(6), “The term ‘contaminant’ refers to any physical, chemical, biological, or radiological substance in water.” This definition has not been revised since the inception of the SDWA in 1974 and, thus, includes nontoxic and potentially beneficial “contaminants.”

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Classifying Drinking Water Contaminants for Regulatory Consideration TABLE 3–1 Universe of Potential Drinking Water Contaminants Category Examplesa Naturally occurring substances Nitrates, humic acid, terpines, arsenic, lead, radon Microbial agents   Naturally occurring agents in water Legionella, toxic algae Agents associated with human feces Enteric viruses, coxsackie B viruses, rotavirus Agents associated with human and animal feces Enteric protozoa and bacteria, Cryptosporidium, Salmonella Agents associated with human and animal urine Nanobacteria, microsporidia Agents associated with water treatment and distribution systems Biofilms, Mycobacterium Chemical agents   Commercial chemicals Gasoline and additives, chlorinated solvents, trichloroethylene, 1,4-dioxane, cumene Pesticides Atrazine, malathion Pharmaceuticals Diclofenac (anti-inflammatory), acetaminophen (analgesic), ethynllestradiol (estrogen) Cosmetics Stearates, glycols Food additives Butylated hydroxyanisole, propylene glycol, dyes Water additives, including impurities Aluminum Water treatment and distribution system leachates and degradates Vinyl chloride, chloroform Products of environmental transformation of chemical agents Dichlorodiphenyldichloroethylene, trichloroacetic acid Reaction and combustion by-products Anthracene, benzopyrene, 2,3,7,8-tetrachlorodibenzodioxin, Metabolites in the environment Methylmercury, dimethylarsenic, dibutyltin, Radionuclides Radon, iodine-131, strontium-90 Biological toxins Endotoxin, aflatoxin Fibers Asbestos a Some examples can belong to more than one category of contaminants (e.g., enteric viruses might also be associated with animal feces). SOURCE: Adapted from NRC, 1999b.

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Classifying Drinking Water Contaminants for Regulatory Consideration TABLE 3–2 Examples of Existing and Planned Information Sources for Chemicals Demonstrated or Having the Potential to Cause Adverse Health Effectsa Name Responsible Agency or Organization Notes Endocrine Disruptor Priority-Setting Database EPA Health effects data on endocrine disruption collected from a variety of databases Everything added to Food in the United States Database U.S. Food and Drug Administration (FDA) Toxicologic information on 2,000 substances added to food Hazardous Substances Data Base National Library of Medicine (NLM) Summary of peer-reviewed health effect studies (about 2,000) Integrated Risk Information System EPA Official EPA summary of health effects information and reference doses or concentrations for approximately 600 compounds International Agency for Research on Cancer (IARC) IARC Expert group summaries of carcinogenic properties for a wide variety of substances and mixtures MEDLINE NLM Abstracts of peer-reviewed studies in medical literature National Research Council NRC Expert group publications summarizing health effects information, critical end points, and doses (e.g., arsenic, radon) Peer-reviewed published literature   Individual studies about health effects and related information (e.g., metabolism) Registry of Toxic Effects of Chemical Substances National Institute for Occupational Safety and Health Tabulation of effect levels for many substances reported in scientific literature

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Classifying Drinking Water Contaminants for Regulatory Consideration Name Responsible Agency or Organization Notes Toxic Substances Control Act Test Submissions—Health Effects EPA Information on unpublished health effects data for industrial chemicals TOXLINE NLM Abstracts of peer-reviewed toxicology-related studies a Includes acute and chronic health effects, such as genotoxicity, developmental toxicity, reproductive toxicity, immunotoxicity, and carcinogenicity. SOURCE: Adapted from EDSTAC, 1999; NRC, 1999b. the universe of potential contaminants and a PCCL, the committee recommends that EPA consider adding substances based on their commercial use, environmental location, or chemical characteristics as listed in Table 3–5. As an integral part of the developing future CCLs, the committee continues to recommend (NRC, 1999b) that the information used from any such lists be combined in a consolidated database to provide a consistent mechanism for recording and retrieving information on the contaminants under consideration. The database should be designed to accommodate a wide variety of chemicals, microorganisms, mixtures of agents, and other types of potential contaminants that are not necessarily defined by a unique chemical formula. Thus, the database could function as a “master list” that contains a detailed record of how the universe of potential contaminants was identified and how a particular PCCL and its corresponding CCL were subsequently selected. The design, creation, and implementation of such a database should be made in open cooperation with the public, stakeholders, and the scientific community, the importance of which is discussed in Chapter 2. Notably, this recommendation is similar to a recommendation from EPA’s Endocrine Disruptor Screening and Testing Advisory Committee (EDSTAC) for the development of a database for setting priorities for screening and testing various substances for endocrine disruption (EDSTAC, 1999). After its report was published, EPA began development of the Endocrine Disruptor Priority-Setting Database (EDPSD) in response to that recommendation (ERG-EPA, 2000). This database, now

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Classifying Drinking Water Contaminants for Regulatory Consideration TABLE 3–3 Examples of Existing and Planned Information Sources for Identifying Chemicals with Demonstrated or Potential Occurrence in Drinking Water Supplies Name Responsible Agency or Organization Notes Chemicals in Consumer and Cosmetic Products FDA Information on chemicals that have been registered voluntarily by manufacturers Comprehensive Environmental Response, Compensation, and Liability Act Information System EPA Contaminant data for Superfund sites Endocrine Disruptor Priority-Setting Database EPA Health effects on endocrine disruption collected from a variety of databases. Environmental Monitoring and Assessment Program EPA Monitoring information for air, groundwater, surface water, biota, and soil contaminants Food Quality Protection Act (FQPA) “Cumulative to Pesticides” List FDA List of chemicals satisfying FQPA statutory requirements of being cumulative to pesticides Generally Regarded as Safe Substances FDA Ingredients that can be added to food National Drinking Water Contaminant Occurrence Database EPA First release online in August 1999; superceded by current second release, which became operational in August 2000; see Chapter 1 for further information National Human Exposure Assessment Survey EPA Surveys designed to assess human exposure via multiple pathways (food, water, air, dust)

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Classifying Drinking Water Contaminants for Regulatory Consideration Name Responsible Agency or Organization Notes National Stream Quality Accounting Network U.S. Geological Survey (USGS) Water quality data for large subbasins of rivers National Water-Quality Assessment Program USGS Contaminant monitoring data for surface and ground water, some data available on-line Permit Compliance System EPA Information on municipal and industrial wastewater discharge Priority-Based Assessment of Food Additives (PAFA) Database EPA Contains administrative, chemical, and toxicological information on more than 2,000 substances added directly to food Toxic Substances Control Act Inventory and Updates EPA Production volumes and sites for industrial chemicals Toxics Release Inventory EPA Information about a select number of chemicals Unregulated Contaminant Monitoring Rule (UCMR) EPA First UCMR List published September 17, 1999; see Chapter 1 for further information   SOURCE: Adapted from EDSTAC, 1999; NRC, 1999b.

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Classifying Drinking Water Contaminants for Regulatory Consideration TABLE 3–4 Examples of Existing and Planned Information Sources for Occurrence and Health Effects of Water-Associated Microbial Agents Name Responsible Agency or Organization Notes FoodNet Centers for Disease Control and Prevention (CDC) Provides data on incidence of diseases associated with key enteric bacteria GenBank National Institutes of Health-National Library of Medicine Internet-based database with information on gene sequences for key microorganisms (see Chapter 6 for further information) Land use data and mapping (e.g., sewage discharge, number of farms or heads of livestock) USGS, states   National Ambulatory Medical Care Survey National Center for Health Statistics (NCHS) of the CDC Conducted in 1990, the survey provided data from office-based physicians through examination of patient records and gave an indication of the number of persons who seek a physician and are diagnosed National Animal Health Reporting System U.S. Department of Agriculture Animal and Plant Health Inspection Service, Veterinary Service, and Centers for Epidemiology and Animal Health Pilot project begun in March 1998, it will include all 50 states reporting on disease cases in commercial livestock (cattle, sheep, swine, poultry, fish) National Hospital Discharge Survey NCHS Begun in 1988, the survey assesses the number of patients treated in hospitals National Mortality Followback Survey NCHS Represents about 1% of U.S. resident deaths

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Classifying Drinking Water Contaminants for Regulatory Consideration Name Responsible Agency or Organization Notes National Notifiable Diseases Surveillance System NCHS Compiles U.S. statistics on diseases. Reported cases are summarized by type of disease, reported month, state, age, and race in some cases. The data represent only clinically identified cases and case ratios (cases to total population) or incidence rates that are most often reported annually National Waterborne Disease Outbreak Database CDC, EPA Catalogs reporting waterborne disease outbreaks since 1920 State department of health data By state Generally, state health departments report cases of disease by county   SOURCE: Adapted from NRC, 1999b. operational and undergoing advanced development and refinement, could serve as a starting point for EPA in the selection of chemicals for the PCCL (and CCL), or EPA could create a new database that might be used for multiple purposes. Indeed, in a demonstration of its recommended process for going from a PCCL to a CCL, the committee relied extensively on the EDPSD to help score several occurrence attributes for a wide variety of chemicals (see Chapters 4 and 5). In its present form, the EDPSD could be used to help provide EPA with information about chemical occurrence or potential occurrence in drinking water since most readily available data sources that would be useful for occurrence screening are already included in the database. However, obtaining information about health effects is somewhat more problematic. The EDSTAC database understandably focuses on obtaining information on health end points that are related to endocrine disruption rather than on the complete spectrum of health effects that drinking water contaminants may elicit. While expanding the EDPSD to include all types of potential drinking water contaminants and their health effects would require considerable effort and expenditure of resources, this may

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Classifying Drinking Water Contaminants for Regulatory Consideration TABLE 3–5 Additional Considerations for Identification of the Universe of Potential Drinking Water Contaminants Potential to Occur in Drinking Water Potential to Cause Adverse Health Effects Any gasoline additive, constituent, or contaminants of a petroleum product Any substance “routinely” stored in an underground storage tank Any halogenated hydrocarbon Any constituent found in a landfill leachate Any soluble component of “normal” soil (e.g., arsenic) Any disinfectant by-product Any constituent of wastewater treatment or septage Any chemical produced in “high volume” (use a cutoff) Any compound found in sludge leachate Any compound widely applied to land Any pharmaceutical excreted in urine or feces Any chemical routinely reported in a major biomonitoring program or study Any military munitions Any substance purposely intended to “affect” living systems Any carcinogen, mutagen, or teratogen Any compound on the Centers for Disease Control and Prevention’s “Bio-safety List” Any substance identified in an epidemiological study that is associated with an elevated measure of health risk Any hormonally active compounds Any enzyme inhibitor or inducer Any behavioral modifier Anything that perturbs gene function Anything “flagged” by structure-activity relationship, quantitative structure-activity relationship or virulence-factor activity relationshipa analysis a The feasibility of developing and applying virulence-factor activity relationships, or VFARs, for use in identifying potential waterborne pathogens for regulatory consideration is the subject of Chapter 6 of this report. prove less costly and more timely than creating an entirely new PCCL-CCL comprehensive database. DISTINGUISHING THE PCCL FROM THE UNIVERSE In the committee’s second report (NRC 1999b), contaminants from the universe are included on the PCCL if they (1) are known to have adverse health effects and the potential to occur in drinking water, (2) are known to occur in drinking water (unless known not to pose a significant health risk), or (3) are believed to pose potential drinking water risks as identified through occurrence criteria (e.g., release data, production data,

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Classifying Drinking Water Contaminants for Regulatory Consideration fecal loading) and health criteria (e.g., toxicity data, structure-activity relationships [SARs], clinical data). However, additional conceptualization and refinement of the PCCL have occurred since it was first introduced as a major step in the process of generating future CCLs. The committee now recommends the Venn diagram in Figure 3–1 as a useful way to view the PCCL as a subset of the universe of potential drinking water contaminants. The four shaded, intersection areas labeled I-IV represent the confluence of two major characteristics of a contaminant necessary for indicating whether it may cause a public health risk through exposure via drinking water. These two characteristics are a contaminant’s demonstrated or potential occurrence in drinking water and its intrinsic ability to produce adverse health effects in exposed persons. It is important to note that to FIGURE 3–1 Conceptual approach to identifying contaminants for inclusion on a PCCL through the intersection of their demonstrated and potential occurrence in drinking water and ability to cause adverse health effects. Note, the sizes of the intersections and rings in this Venn diagram are not drawn to scale and do not represent an estimate of the relative numbers of contaminants in each area.

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Classifying Drinking Water Contaminants for Regulatory Consideration effectively delineate the four “rings” for health effects and occurrence (and their intersections) from the universe of potential drinking water contaminants requires the development and use of screening criteria that define “demonstrated” versus “potential” for each characteristic (see more below). Otherwise, the use of a Venn diagram concept for the PCCL is meaningless. However, this conceptualization recognizes the importance of contaminants for which health effects or occurrence have not been demonstrated (but are possible), thus allowing for inclusion of emerging contaminants with limited data. The contaminants that fall in intersection I are the highest priorities for placement on a PCCL because they have demonstrated occurrence in drinking water and are demonstrated to cause adverse health effects. Contaminants that fall in intersections II and III are of equivalent importance and have medium priorities for inclusion on a PCCL since they have either demonstrated occurrence in drinking water or demonstrated adverse health effects in conjunction with a potential to cause adverse health effects or a potential to occur in drinking water, respectively. Lastly, contaminants that have the potential to occur in drinking water and the potential to cause adverse health effects fall into intersection IV and have the lowest priority for inclusion on a PCCL. The committee expects that the drinking water contaminants of intersection I will comprise the smallest assemblage of contaminants, while intersection IV will comprise the largest group. For examples of Venn diagram contaminants see Table 3–6. A significant remaining challenge lies in defining criteria that are to be used by EPA in conjunction with expert judgment to cull the universe of potential drinking water contaminants to a few thousand on the PCCL. Unfortunately, the committee was not able to deliberate extensively on the development of such criteria because sufficient time was not available to do so. The committee notes that this outcome was not unexpected, as reflected in the committee’s statement of task for this second phase of work (see preface to this report). Thus, the PCCL is still treated on a largely conceptual basis as it was in the committee’s second report (NRC, 1999b). The committee cautions that the Venn diagram in Figure 3–1 should not be interpreted to imply that a contaminant with limited or no data could not be included on a PCCL. Theoretically, a contaminant that is in the universe of potential drinking water contaminants and lying outside of all the intersections or even the rings in the diagram could be placed on a PCCL solely through the use of expert judgment. In this regard, the committee again notes that in formulating a PCCL, EPA should

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Classifying Drinking Water Contaminants for Regulatory Consideration TABLE 3–6 Examples of Venn Diagram Contaminants Area of Venn Diagram Chemical Microbial Area I Aldicarb Diethylhexyl phthalate Lead Vinyl chloride Campylobacter jejuni Cryptosporidium parvum Escherichia coli O157:H7 Area II n-Hexane Salicylate Cyclospora Helicobacter pylori Microsporidia Area III Dacthal degradates (DCPA) Pseudomonas aeruginosa Area IV Pyrethrin Triethylene glycol Cyanobacteria Nanobacteria Demonstrated occurrence ring Citric acid Phosphate Potassium ion Coliphage Potential occurrence ring Acetic acid Glucose Glycine Stachybotrys Demonstrated health effects ring Sodium pentothal Ebola virus Haemophilus influenzae Streptococcus pneumoniae West Nile virus Potential health effects ring Bilirubin Cyanophage (viruses for blue-green algae) In universe but not in any ring Inert gases (argon, helium) Methanobacter spp. NOTE: DCPA=(Dacthal) dimethyl-2,3,5,6-tetrachlorobenzene-1,4-dicarboxylate.

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Classifying Drinking Water Contaminants for Regulatory Consideration use common sense as a guide and err on the side of public health protection. GUIDELINES FOR DEVELOPING SCREENING CRITERIA Criteria for Potential and Demonstrated Health Effects The information that can be used to identify contaminants with potential and demonstrated health effects includes human data from epidemiological studies, clinical studies, and case reports; toxicological laboratory animal studies or field studies; and predictive biological activity or effects models (e.g., SARs). The feasibility of developing and using virulence-factor activity relationships, or VFARs, to identify emerging waterborne pathogens is discussed in Chapter 6. The term toxicological laboratory animal studies includes many types of studies such as whole studies, metabolic studies, and so forth. The committee recommends that human data and data on whole animals be used as indicators of demonstrated health effects and that other toxicological data and data from experimental models that predict biological activity be used as indicators of potential health effects. The committee has already provided general guidelines to evaluate and assess health effects data and prioritize the importance of their findings for deciding how to address contaminants already on a CCL (NRC, 1999a). However, these principles continue to apply and may be used to assist in evaluating potential drinking water contaminants for inclusion on a PCCL. Potency is an important characteristic of a substance’s ability to cause health effects. Of course, every substance can have some health effect, given a sufficiently high dose. However, the dose at which it causes adverse health effects, especially compared to the concentrations that may occur in drinking water, is an important consideration when selecting a substance for the PCCL. As the exposure to a substance from drinking water nears the dose that may cause health effects, it becomes more important to consider including that substance on the PCCL. Furthermore, substances can have a variety of health effects that range from minor and reversible to irreversible, and life threatening. In general, greater consideration should be given to including substances on the PCCL that cause serious, irreversible effects as opposed to those that cause less serious effects. The committee is not suggesting that less serious effects such as cholinesterase inhibition should be ignored; however, it recognizes that health effects such as cancer or birth defects may be

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Classifying Drinking Water Contaminants for Regulatory Consideration given greater weight. The issues of a contaminant’s potency and the severity of its health effects are discussed in greater detail in Chapter 4 for going from a PCCL to a CCL. Criteria for Potential and Demonstrated Occurrence A variety of metrics could be used to define occurrence of contaminants in drinking water. These are identified in a hierarchical framework in the committee’s first report (NRC, 1999a) and include (1) observations in tap water, (2) observations in distribution systems, (3) observations in finished water of water treatment plants, (4) observations in source water, (5) observations in watersheds and aquifers, (6) historical contaminant release data, and (7) chemical production data. The committee recommends that the first four of these should be used as indicators of demonstrated occurrence and information from items 5 through 7 should be used to determine potential occurrence. Recorded observations of contaminants in tap water, distribution water, finished water, or source water can be defined in terms of concentration, frequency of occurrence over time, and prevalence with respect to the number of facilities showing detects or people exposed. The committee believes that, generally, contaminant concentration alone should not be used as a relevant metric for culling from the universe of contaminants to those that will appear on the PCCL, although the committee recognizes that some consideration of concentration may be necessary as analytical procedures continue to reduce detection limits. EPA may want to consider binary data, such as found or not found in public water systems, for selecting chemicals for the PCCL from the universe. Also, the committee believes that frequency over time should not be used as the sole relevant metric for this step because it may place undue emphasis on contaminants that are found repeatedly and eliminate those that may have a significant impact but occur infrequently. The committee believes that prevalence at a large number of public water systems or prevalence at systems that serve large numbers of people is an important metric to determine inclusion into the demonstrated occurrence category. The appropriate threshold, in terms of number of facilities or number of people exposed, will have to be decided by EPA, which will also have to consider the manageability of the size of the resulting PCCL. Of the metrics that serve as indicators of potential occurrence, the committee recommends that EPA use production or release data, combined with physical properties, to serve as useful indicators of the poten-

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Classifying Drinking Water Contaminants for Regulatory Consideration tial for chemical occurrence in watersheds and aquifers. These properties include persistence and mobility in aquatic systems. For chemical contaminants, these may include aqueous solubility, octanol-water partition coefficient, Henry’s law constant, and recalcitrance. To screen initially for inclusion on the PCCL, which is appropriate at this stage, aqueous solubility could be used as the sole metric. For microbial contaminants, viability in ambient waters and particle charge or hydrophobicity are appropriate. The use of such properties reduces the reliance on recorded observations in watersheds or aquifers, data that are typically sparse. If necessary and in the absence of these data, contaminant properties could potentially be estimated using SARs and QSARs (quantitative structure-activity relationships) for chemicals or using VFARS in the future (see Chapter 6) for microorganisms. For chemicals, a binary approach would serve to categorize the universe of chemicals being produced commercially (i.e., it would not include by-products or chemicals formed in the environment) into four bins for potential occurrence as illustrated in Figure 3–2. The committee recommends that if such an approach were used for commercial chemicals, all chemicals except those with those with both low production volume and low water solubility should be considered for inclusion on a PCCL. Furthermore, as indicated by the number of “Xs,” such a categorization could also be used to roughly prioritize commercial chemicals for PCCL consideration. An example of a chemical in the high-water-solubility and high-production or release bin is MTBE (methyl-t-butyl ether). The aromatic hydrocarbon cumene is a good example of a chemical in the low-water-solubility and high-production or release bin. An example of a chemical in the high- water-solubility and low-production or release bin is the perchlorate ion. Finally, the steroid anti-inflammatory drug dexamethasone is an example of a low-production, low-water-solubility chemical that would not be expected to occur in high concentration in FIGURE 3–2 Categories for relative potential of chemicals to occur in drinking water based upon consideration of production volume and water solubility.

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Classifying Drinking Water Contaminants for Regulatory Consideration drinking water. (However, expert judgment in considering the potent biological effects of this compound might well place it on a PCCL.) In this case, the challenge for EPA is to determine scientifically defensible and transparent thresholds for defining “high” and “low” for the two metrics. Again, these thresholds should be chosen considering the manageability of the size of the resulting PCCL. It is important to note that several potential categories of drinking water contaminants would be missing after this initial screening exercise (see Tables 3–1 and 3–5). For example, EPA should review contaminants already included in the potential occurrence category (“ring”) to determine if they have any important environmental degradation products, production or reaction by-products, or metabolites in the environment that should also be considered for inclusion on the potential occurrence list. EPA should also review naturally occurring substances and fibers to determine whether any of them should be included on the potential occurrence list. EPA may also want to review data for specific watersheds and aquifers to determine if any other contaminants should be included on the potential occurrence list. The committee again notes that its recommendation for the development and use of a well-designed, comprehensive database would greatly support the task of developing future PCCLs. FATE OF THE PCCL As noted earlier, the committee recommends that an entirely new PCCL be created every five years as a precursor to the development of the corresponding CCL. This should encourage a more thorough and timely consideration of the contaminants that comprise the PCCL than may occur if new substances were simply added to the previous PCCL (excluding those reaching the CCL). Thus, current and past PCCLs would be retained and used in conjunction with (or in lieu of and acting as) a comprehensive relational database useful for recording and understanding the process used to select the PCCL and ultimately the CCL. The committee notes that it is entirely reasonable to expect that prior PCCLs will be reviewed and thereby utilized as an obvious starting point for the creation of subsequent PCCLs. However, a former constituent of a PCCL may not appear on a new PCCL for a variety of reasons, such as the availability of new data that indicate it does not occur in drinking water or does not cause adverse health effects. In keeping with its inclu-

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Classifying Drinking Water Contaminants for Regulatory Consideration sive nature, the PCCL should not be expected to maintain a more or less fixed number of potential drinking water contaminants. SUMMARY: CONCLUSIONS AND RECOMMENDATIONS The first CCL, published in March 1998, was prepared in a short time period by EPA with the assistance of NDWAC to meet the statutory requirements and mandated time line of the SDWA Amendments of 1996. The limitations associated with its necessarily rapid development have been summarized in the committee’s second report (NRC, 1999b) and are fully described in Chapter 2 of this report. While the contaminants included on the first CCL certainly merit regulatory and research consideration, a broader approach to contaminant selection could potentially identify higher-risk contaminants. As previously noted, the NRC Committee on Drinking Water Contaminants continues to recommend that EPA develop and use a two-step process for the creation of future CCLs along with several related recommendations (listed below), many of which were originally described in the committee’s second report (NRC, 1999b). However, several other associated guidelines and recommendations for CCL design and implementation have been revised and expanded as a result of the committee’s second phase of deliberations and are also listed below. The committee continues to recognize that the need for policy judgments by EPA cannot and should not be removed from any CCL (or PCCL) development process. In making these decisions, EPA should use common sense as a guide and err on the side of public health protection. Regarding the first step of this process (going from the universe of potential drinking water contaminants to a PCCL), the committee makes the following recommendations: EPA should begin by considering a broad universe of chemical, microbial, and other types of potential drinking water contaminants and contaminant groups (see Table 3–1). 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). This represents a dramatically larger set of substances to be considered initially in terms of types and numbers of contaminants than that used for the creation of the 1998 CCL.

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Classifying Drinking Water Contaminants for Regulatory Consideration EPA should rely on databases and lists that are currently available (see Tables 3–2 and 3–3 for chemicals and Table 3–4 for microorganisms) and under development, along with other readily available information to begin identifying the universe of potential contaminants that may be candidates for inclusion on the PCCL. For example, EPA should consider using the EDPSD to help develop future PCCLs (and perhaps CCLs). While relevant databases and lists exist for many “universe categories” of potential drinking water contaminants, others have no lists or databases (e.g., products of environmental degradation). Thus, EPA should initiate work on a strategy for filling the gaps and updating the existing databases and lists of contaminants (e.g., through involvement of the NDWAC or panels of experts) for future CCLs. This strategy should be developed with public, stakeholder, and scientific community input. As an integral part of the development process for future PCCLs and CCLs, all information from existing or created databases or lists used should be compiled in a consolidated database that would provide a consistent mechanism for recording and retrieving information on the contaminants under consideration. Such a database could function as a “master list” that contains a detailed record of how the universe of potential contaminants was identified and how a particular PCCL and its corresponding CCL were subsequently created. It would also serve as a powerful analytical tool for the development of future PCCLs and CCLs. As a starting point, the committee recommends that EPA review its developing EDPSD database to determine if it can be expanded and used as this consolidated database or can serve as a model for the subsequent development of such a database. Regardless, the (re)design, creation, and implementation of such a database should be made in open cooperation with the public, stakeholders, and the scientific community. To assist generally in the identification of the universe of potential contaminants and a PCCL, the committee recommends EPA consider substances based on their commercial use, environmental location, or physical characteristics as listed in Table 3–5. EPA should be as inclusive as possible in narrowing the universe of potential drinking water contaminants down to a PCCL. The committee envisions that a PCCL would contain on the order of a few thousand individual substances and groups of related substances, including microorganisms, for evaluation and prioritization to form a CCL. However, preparation of a PCCL should not involve extensive analysis of data, nor should it directly drive EPA’s research or monitoring activities.

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Classifying Drinking Water Contaminants for Regulatory Consideration The committee recommends the use of a Venn diagram approach (see Figure 3–1) to conceptually distinguish a PCCL from the universe of potential drinking water contaminants. However, because of the extremely large size of the universe of potential drinking water contaminants, well-conceived screening criteria remain to be developed that can be rapidly and routinely applied by EPA in conjunction with expert judgment to cull the universe to a much smaller PCCL. Thus, the PCCL should include those contaminants that have a combination of characteristics indicating that they are likely to pose a public health risk through their occurrence in drinking water. These characteristics are demonstrated or potential occurrence in drinking water and demonstrated or potential ability to cause adverse health effects. Regarding the development of screening criteria for health effects, the committee recommends that human data and data on whole animals be used as indicators of demonstrated health effects and that other toxicological data and data from experimental models that predict biological activity be used as indicators of potential health effects. A variety of metrics could be used to develop screening criteria for the occurrence of contaminants in drinking water. These are identified in a hierarchical framework in the committee’s first report (NRC, 1999a) and include (1) observations in tap water, (2) observations in distribution systems, (3) observations in finished water of water treatment plants, (4) observations in source water, (5) observations in watersheds and aquifers, (6) historical contaminant release data, and (7) chemical production data. The committee recommends that the first four of these should be used as indicators of demonstrated occurrence and information from items 5 through 7 should be used to determine potential occurrence. For commercial chemicals, their potential to occur in drinking water may be estimated using a combination of production volume information and water solubility (see Figure 3–2). Most likely occurrence would involve high-production-volume chemicals with high-water-solubility. A new PCCL should be generated for each CCL development cycle to account for new data and emerging contaminants. Each PCCL should be published and thereby serve as a useful record of past PCCL and CCL development and as a starting point for the development of future PCCLs. Development of the first PCCL should begin as soon as possible to support the development of the next (2003) CCL; each PCCL should

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Classifying Drinking Water Contaminants for Regulatory Consideration be available for public and other stakeholder input (especially through the Internet) and should undergo scientific review.