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Classifying Drinking Water Contaminants for Regulatory Consideration 4 PCCL to CCL: Attributes of Contaminants INTRODUCTION As described earlier in this report, the committee continues to recommend a two-step process for the creation of future Drinking Water Contaminant Candidate Lists (CCLs) (see Figure 1–3). This chapter provides some guidance and recommendations for conducting the second step of the CCL development process: selecting preliminary CCL (PCCL) contaminants for inclusion on the corresponding CCL through use of a prioritization tool in conjunction with expert judgment. In general, the preliminary CCL (PCCL) may be thought of as a much more manageable and less conceptual list than the universe of potential contaminants. As such, the PCCL is anticipated to contain up to a few thousand individual substances and groups of related substances, including microorganisms, that merit further consideration for inclusion on the CCL. However, it is anticipated that nearly all of the contaminants on a PCCL will have incomplete information on their potential occurrence and health effects. Thus, any process for selecting PCCL contaminants for inclusion on a CCL must recognize and overcome such limitations. Furthermore, the absence of information for a particular PCCL contaminant should not necessarily be an obstacle to its inclusion on the CCL, as recommended in the committee’s second report (NRC, 1999b). In all cases, some amount of expert judgment will be required for the assessment and promotion of each PCCL contaminant to its corresponding CCL. OVERVIEW OF CONTAMINANT ATTRIBUTES The committee’s recommended approach to this daunting problem
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Classifying Drinking Water Contaminants for Regulatory Consideration was to develop and use five attributes that contribute to the likelihood that a particular PCCL contaminant or group of related contaminants could occur in drinking water at levels and frequencies that pose a public health risk. In this regard, the committee devised a scoring system for each of these five attributes whereby the highest-priority PCCL contaminants are selected in conjunction with expert judgment for inclusion on a CCL. The five attributes are divided into health effect and occurrence categories. For health effects, the committee identified severity and potency as key predictive attributes. Prevalence, magnitude, and persistence-mobility comprise the occurrence attributes. It is important to note that the committee spent a great deal of time deliberating on the number and type of contaminant attributes that should be used in the prototype classification algorithm approach recommended for use (in conjunction with expert judgment) in the development of future CCLs. Ultimately, the committee decided that these five contaminant attributes constitute a reasonable starting point for the U.S. Environmental Protection Agency (EPA) to consider, especially since they were subsequently found to aptly demonstrate the utility of the recommended CCL development approach (see Chapter 5). Furthermore, the metrics and related considerations presented in this chapter for scoring each attribute should similarly be viewed as illustrative. Thus, the committee does not explicitly or implicitly recommend these five (or necessarily a total of five) attributes or the related scoring metrics as being ideally suited for direct adoption and use by EPA. Rather, should EPA choose to adopt a classification approach for the development of future CCLs, the committee recommends that options for developing and scoring contaminant attributes should be made available for public and other stakeholder input and undergo scientific review. Severity The question of severity may be stated simply as, How bad is the health effect? In terms of this report, severity can be scored using the most sensitive health end point for a particular contaminant (e.g., the health effect that occurs at the lowest dose compared to other health effects reportedly caused by the contaminant) and considering vulnerable subpopulations (see Chapter 2). In other words, for the most sensitive health end point, what is the anticipated clinical magnitude in affected individuals? The committee recommends that the assessment of severity
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Classifying Drinking Water Contaminants for Regulatory Consideration should be based, when feasible, on plausible exposures via drinking water. This information can be ascertained from clinical reports, from animal bioassay results, or by inference from effects of similar compounds. Information from epidemiological studies, structure-activity relationships, or future virulence-factors activity relationships (see Chapter 6) may also prove useful. For illustrative purposes, a 10-level hierarchy for scoring the severity of health effects may be defined as in Table 4–1. It should be noted that in developing this table, it was necessary to consider the chronicity of the health effect. An alternative approach to evaluating severity would be to perform a ranking using either quality adjusted or disability adjusted life-years (QALYs or DALYs), lost respectively, due to exposure to a contaminant. In this regard, various weighting scales can be used, each of which to some degree incorporates economic and social considerations of disease impact (Bowie et al., 1997; Havelaar et al., 2000; Hyder et al., 1998; Mauskopf and French, 1991). However, as noted in Chapter 2, the committee cautions that the use of approaches such as QALYs and DALYs may not adequately protect vulnerable subpopulations (Arneson and Nord, 1999). Thus, the committee recommends that EPA give consideration to different severity metrics. Potency Potency may be expressed simply as, How much of a contaminant does it take to cause illness? This is a relative scaling of the dose-response relationship. For carcinogens, an obvious metric for scaling is the cancer slope factor (q1), which is defined as the incremental risk divided by incremental dose in the low-dose region (EPA, 1999h). Databases of these potencies are available, such as EPA’s Integrated Risk Information System (see Table 3–2; EPA, 2000f). A method for scaling carcinogenic contaminants would be to use the percentile of the contaminant’s potency relative to the potencies of all contaminants being considered, including those drinking water contaminants with enforceable maximum contaminant levels (MCLs). The percentile (0–100) scale could then easily be converted to a 1 through 10 (decile) scale. For mutagens, a similar percentile ranking can be derived using bioassays results such as the Ames test. For noncarcinogenic contaminants, a logical basis of comparison is the benchmark dose (BMD10), which can be defined simply as the dose
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Classifying Drinking Water Contaminants for Regulatory Consideration TABLE 4–1 Scoring of Severity Attribute Severity Score Characteristic 0 No effect 1 Changes in organ weights with minimal clinical significance 2 Biochemical changes with minimal clinical significance 3 Pathology of minimum clinical importance (e.g., fluorosis, warts, common cold) 4 Cellular changes that could lead to disease; minimum functional change 5 Significant functional changes that are reversible (e.g., diarrhea) 6 Irreversible changes; treatable disease 7 Single organ system pathology and function loss 8 Multiple organ system pathology and function loss 9 Disease likely leading to death 10 Death corresponding to a 10 percent excess risk above background levels (Crump, 1995). These can be computed for continuous (Crump, 1995), quantal (Gaylor et al., 1998), and to a lesser extent, graded responses (Gibson et al., 1997). BMDs for a contaminant can then be scaled by percentile BMD for that contaminant relative to the BMDs of all contaminants being considered, including contaminants with MCLs; the percentile scale can similarly be converted to a 1 through 10 scale. Alternatively, the lowest observed adverse effect level (LOAEL) or the no observed adverse effect level (NOAEL) dose1 can be used. For infectious agents, a logical basis of comparison is the median infectious dose (N50), defined as the dose from a single exposure leading to infection of 50 percent of a population (Haas et al., 1999). This may be defined with reference to current MCLs.2 1 The lowest dose that results in a statistically or biologically significant increase in the frequency or severity of adverse health effects between an exposed group and an appropriate control group and the lowest dose that does not, respectively. 2 At the time of this writing, there are no MCLs for individual microbial pathogens—only treatment requirements. Thus, it may be appropriate to define the percentile with reference to pathogens for which treatment techniques have been set. At present, this would include Giardia lamblia and human enterovirus (both of which are regulated under the Surface Water Treatment Rule), Legionella pneumophila (which is regulated by implication under the Surface Water Treatment Rule), and Cryptosporidium parvum (which is to be regulated under the Enhanced Surface Water Treatment Rule).
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Classifying Drinking Water Contaminants for Regulatory Consideration If contaminants act via diverse health effect end points (e.g., a carcinogen that also has noncarcinogenic effects), then an appropriate potency score would be the worst (i.e., the highest) score of all of the relevant effects in order to err on the side of public health protection. When no experimental data are available to infer the potency of a chemical contaminant, the use of a (quantitative) structure-activity relationship should be considered. In the case of microbial agents, quantitative virulence-factor activity relationships (see Chapter 6) may become available in the future. Prevalence The attribute of prevalence may be defined as, How commonly does or would a contaminant occur in drinking water? Ideally, prevalence should be assessed based first on measurements in tap water, followed by measurements in distribution systems, finished water of water treatment plants, and source waters used for drinking water supply as discussed in the committee’s first report (NRC, 1999a) and earlier in this report (see Chapter 3). If such data are not available, inferences may have to be made from general watershed or aquifer measurements, historical contaminant release data, or even chemical production data. The focus of this attribute is the geographical and temporal range (i.e., sporadic or episodic versus frequent) of occurrence or anticipated occurrence. Thus, prevalence has both spatial and temporal aspects. For illustrative purposes, spatial and temporal prevalence can be consolidated into a single prevalence index using an approach such as that summarized in Table 4–2. Temporal prevalence represents the average fraction of time that a contaminant is found at a given locale (if multiple locations are sampled, the percentage of occurrence times at each site should be averaged among sites). Spatial (or geographical) prevalence represents the proportion of locales in which the contaminant would be found, such as all communities with public water systems for which contaminant data are available. For the purpose of assessing geographical prevalence, sites in the same watershed should be regarded as the same site. For example, if the temporal prevalence averaged 66 percent and the spatial prevalence averaged 80 percent, a score of 9 would be the result. There are two important questions to be resolved with respect to assessing prevalence. First, in the absence of data either on temporal prevalence (e.g., many sites each examined only at a single point) or on
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Classifying Drinking Water Contaminants for Regulatory Consideration TABLE 4–2 Overall Prevalence Score Related to Temporal and Spatial Prevalence Temporal Prevalence (%) Spatial Prevalence (%) <25 25–50 50–75 75–90 90–100 <25 1 2 3 4 6 25–50 2 4 5 7 8 50–75 3 5 8 9 9 75–90 4 7 9 10 10 90–100 6 8 9 10 10 spatial prevalence (e.g., one site examined many times), what score should be assigned? Second, prevalence may have to be defined with respect not to an absolute detection value (e.g., above the detection limit) but to a prevalence above some level likely to be of concern. If the latter is not done, the percentage prevalence on either a temporal or a spatial basis is anticipated to increase as analytical methods advance, since samples that would previously have scored as “nondetects” would be scored as “detects.” With respect to the first issue, if only temporal or only spatial prevalence information is available, then it may be appropriate to determine and use a default assumption in which the spatial prevalence and the temporal prevalence (in the absence of information on both factors) are assumed to be equal. In the second case, the committee believes that further consideration regarding the issue of decreasing detection limits is in order. Furthermore, the committee notes that in many cases (particularly where contaminants have been included on a PCCL on the basis of potential rather than demonstrated occurrence), insufficient information will be available to assess temporal or spatial prevalence (or both) directly. In such circumstances, this attribute must then go unscored. However, the absence of prevalence information, as described below, will not necessarily preclude PCCL contaminants from being included on the corresponding CCL.
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Classifying Drinking Water Contaminants for Regulatory Consideration Magnitude Magnitude can be defined as the concentration or expected concentration (e.g., based on chemical production) of a contaminant relative to a level that causes a perceived health effect. In lay terms, this might be stated, Is the level high enough to cause harm? However, the issue is not simply the absolute magnitude of a contaminant, but rather magnitude relative to potency. As in the case of prevalence, if a substance has been included on a PCCL on the basis of potential rather than demonstrated occurrence, then data would not generally be available to evaluate (even on a 10-point scale) the magnitude of expected concentration in drinking water. With substances for which at least some concentration measurements exist, a suitable metric for evaluating the magnitude would relate the median (as opposed to the mean) water concentrations to the potency. To score this attribute, the median water concentration and a measure of potency have to be combined in a simple and consistent manner to yield a numerical measure of scale. As noted previously, the committee recognizes that the available information with which to go from a PCCL to a CCL will almost always be imperfect and will often be poor. Thus, the following approach to scoring magnitude is suggested for illustrative purposes. First, the median water concentration for the contaminant under evaluation is ranked relative to the numerical values for median occurrence of contaminants with MCLs,3 on a decile basis (1–10 ranking, with 10 being compounds present at highest magnitude relative to the MCL medians4). This percentile is next multiplied by the converted decile score for potency and the square root is taken (e.g., if a compound is in the 9th decile for occurrence and its potency is given a score of 3, then 3 Because of obvious differences in measurement approaches, chemical contaminants and microbial contaminants should be ranked separately with respect to chemical and microbial MCL values. Due to the current lack of specific MCLs for microorganisms (except coliform organisms and heterotrophic plate count organisms), it may be appropriate to develop “apparent MCLs” for each of the microorganisms regulated under a treatment technique option (i.e., Giardia lamblia, human viruses, Legionella pneumophila, Cryptosporidium parvum). 4 Under the authority of the Safe Drinking Water Act Amendments of 1996, EPA has recently assembled expansive data on the concentrations of regulated compounds in public water supplies (EPA, 1999a).
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Classifying Drinking Water Contaminants for Regulatory Consideration the magnitude score is ). It is important to note that this calculation gives greater weight to contaminants with many large-magnitude occurrences and high relative potencies than to contaminants that have fewer but higher-magnitude occurrences but are of low relative potency. In addition, contaminants that have many low-magnitude occurrences but high relative potencies will also receive less weight. Persistence-Mobility The persistence-mobility attribute of a contaminant is intended to describe the likelihood that the contaminant would be found in the aquatic environment based solely on its physical properties. For PCCL contaminants that have demonstrated occurrence in water, the occurrence attributes of prevalence and magnitude should be scored and take precedence over their persistence-mobility scores. However, in the absence of data on occurrence, persistence and mobility should be used to assess the potential for significant occurrence of PCCL contaminants in drinking water. There are many chemical fate and persistence models that could be adapted for the current purpose. Recent examples include Bennett et al. (2000) and Gouin et al. (2000); however, most models require data on too many contaminant properties to be practical for screening perhaps thousands of chemicals on the PCCL. In addition, there appears to have been little prior effort to develop persistence-mobility metrics that are suitable for ranking chemicals and microorganisms together. To overcome these limitations, the committee recommends consideration of three general characteristics of contaminants that would foster their persistence and/or mobility in water systems: high potential for amplification by growth under ambient conditions (applies to microbial contaminants and not to chemicals); high solubility in water (applies primarily to chemicals), although the transportability of microorganisms may be assessed through sedimentation velocities and size and adsorption capabilities; and stability in water (i.e., resistance to degradation via mechanisms such as hydrolysis, photolysis, or biodegradation in the case of chemicals; death or the ability to produce nonculturable or resistant states [e.g., spores and cysts] in the case of microorganisms).
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Classifying Drinking Water Contaminants for Regulatory Consideration It should be possible to assess these characteristics for a large number of contaminants (both chemical and microbial) by applying a simple, semiquantitative scoring scheme such as that illustrated in Tables 4–3 to 4–5. In this system, a score of 1 represents a characteristic that is relatively unfavorable persistence-mobility in water (low amplification, solubility, or persistence), while a score of 3 represents a characteristic that is relatively favorable persistence in water (high amplification, solubility, or persistence). More specifically, any given chemical or group of related chemicals would be scored using Tables 4–4 and 4–5, whereas microorganisms would be scored primarily using Tables 4–3 and 4–5. If the individual characteristics are scored according to this scheme, an overall score for persistence-mobility can be obtained on a scale of 1 to 10 by taking the arithmetic average of the scores and multiplying by 10/3. TABLE 4–3 Scoring Contaminants for Microbial Amplification Subscore Doubling Time Under Environmental Conditions Low (1) >1 week Medium (2) 1 day-1 week High (3) <1 day TABLE 4–4 Scoring Contaminants for Solubility Subscore Solubility (mg/L) Low (1) 0.1 Medium (2) 0.1–10 High (3) >10 TABLE 4–5 Scoring Contaminants for Stability Subscore Half-Life (Combined, All Mechanisms) Low (1) <1 day Medium (2) Days to weeks High (3) >Weeks
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Classifying Drinking Water Contaminants for Regulatory Consideration LESSONS LEARNED IN APPLYING THESE CRITERIA The contaminant attributes and associated scoring criteria outlined in this chapter were applied to a panel of chemical and microbial validation test contaminants. The details of this demonstration are given in Chapter 5 to illustrate the utility of using such a classification approach for the creation of future CCLs. During the course of this demonstration, a number of procedural issues were discovered with respect to implementation of the approach. The following material summarizes the major factors identified during the scoring of various health effect and occurrence attributes. Monitoring Data Monitoring data are important in the scoring scheme, for both the prevalence and the magnitude attributes. For convenience in conducting this exercise, the committee relied extensively on EPA’s developing Endocrine Disruptor Priority-Setting Database (EDPSD) to obtain usable monitoring data for chemicals (ERG-EPA, 2000). The committee commends EPA for beginning to obtain and compile environmental monitoring data as exemplified by the creation and ongoing development of the EDPSD in addition to the Unregulated Contaminant Monitoring Regulation List and the National Drinking Water Contaminant Occurrence Database (NCOD) described in Chapter 1. It is important to note, however, that monitoring data in the EDPSD from water supplies are limited, both in the number of chemicals for which monitoring data exist and in the amount of sampling. Without additional monitoring data compiled in a comprehensive database that facilitates priority setting (as recommended in Chapter 3), EPA will be hampered in terms of the quality of its priority-setting exercise and the ease of completing that exercise. As noted earlier, some of the most relevant monitoring data for priority-setting activities in going from a PCCL to a CCL are data from water supplies. In keeping with this assertion, the committee relied, whenever possible, on drinking water monitoring data from the recently established NCOD as maintained in EDPSD. Earlier in this chapter, the committee described scoring criteria that use information on temporal and geographic occurrence. Because such data are not available in this particular database, however, the committee instead used information on
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Classifying Drinking Water Contaminants for Regulatory Consideration population exposed and number of analyses with detects for prevalence. Expanding on the recommendation in Chapter 3 that EPA review the EDPSD database to determine whether it can be used to help develop a PCCL and perhaps help select PCCL contaminants for inclusion on a CCL, the committee also recommends that EPA consider the possibility of including information on temporal and regional occurrence. However, the committee notes that the EDPSD does contain exposure information from the U.S. Geological Survey’s National Stream Quality Accounting Network and National Water-Quality Assessment Program, both of which include regional water quality information. Additionally, although information on the number of analyses with detects is included in the EDPSD and NCOD, information on the total number of analyses is not. Without denominator data, the usefulness of information on the number of detects is limited. The magnitude score uses information on potency and the average concentration. Although some information on median concentration is included in the occurrence data in EDPSD, the median concentration is calculated using only the detected concentrations; nondetected values are not included. The median thus does not provide a true picture of the median concentration of a chemical in drinking water if nondetected values are not included and especially if the majority of observations are nondetects. EPA should develop a method of expressing an “average concentration” value that uses as much of the data, including nondetects, as possible. The literature suggests several approaches to this issue (Gilliom and Helsel, 1986; Haas and Scheff, 1990; Helsel and Cohn, 1988). In addition, EPA may want to consider providing other measures of concentration in water supplies, such as the 95th percentile of contaminant concentration. The committee can envision other scoring schemes that might use monitoring data rather than relying on median values. For example, a chemical may occur in a relatively limited geographic area of the United States at high concentrations and not be found to a great extent in other areas of the country; under this circumstance, the chemical may still warrant inclusion on the CCL. Potency Compared to Monitoring Data The potency score for chemicals was calculated using LOAELs,
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Classifying Drinking Water Contaminants for Regulatory Consideration NOAELs, and RfDs5 from readily available health effects-related databases such as EPA’s Integrated Risk Information System (IRIS) (EPA, 2000f). Although the number of chemicals for which these data exist is limited, the quality of the data for scoring this attribute was often high. This contrasts to information contained in the monitoring database that was used extensively (EDPSD). The difference in the quality of these databases is not unexpected, however, given that IRIS has existed for more than a decade, while monitoring databases are in much earlier stages of development. Severity In this demonstration, severity was scored for chemicals based on the health effect associated with the LOAEL. In some cases, this could result in a fairly low severity score because the health effect associated with the LOAEL was relatively minor such as a decrease in body weight gain. It is important to note, however, that such a chemical could cause health effects associated with significantly higher severity scores (e.g., birth defects) at only a slightly higher dose. Severity and Potency For chemicals that caused both cancer and noncancer health effects, potency was scored for both types of effects and the higher one was used in the model. The severity score was then based on the corresponding health end point used (e.g., LOAEL or cancer). In a few cases, the potency score was higher for the noncancer end point, and the health end point associated with the LOAEL had a low severity score. Thus, potency was scored first and severity was based on the end point on which potency was based. Alternatively, severity could have been scored first and then potency based on the end point associated with the severity score. Another approach could have scored potency and severity independently of each other. If the scoring had been done differently, the modeling results are likely to have been different. This decision to score 5 An oral reference dose (or RfD) is an estimate of the concentration of a substance that is unlikely to cause appreciable risk of adverse health effects over a lifetime of exposure, including sensitive subgroups (Barnes and Dourson, 1988).
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Classifying Drinking Water Contaminants for Regulatory Consideration potency and then severity also affected the magnitude score since the potency score is used in calculating the magnitude score. Vulnerable subpopulations were considered, when readily feasible, in the scoring. For example, the health end point ultimately used for scoring the potency of nitrate (see Box 5–1) was methemoglobinemia in infants (i.e., an excess of methemoglobin—the oxidized form of hemoglobin—resulting in cyanosis). In this case, the severity and potency scores were based on the effect in this population, rather than for adults, and the committee considered this reasonable. Consistent with the recommendation in Chapter 2 that EPA consider expanding the current working definition of vulnerable subpopulations in the amended Safe Drinking Water Act, the committee deems it appropriate for EPA to consider using health end point data based on vulnerable subpopulations, especially those that comprise a “meaningful portion of the general population.” SUMMARY: CONCLUSIONS AND RECOMMENDATIONS The committee recommends that EPA develop and use a set of attributes to evaluate the likelihood that any particular PCCL contaminant or group of related contaminants could occur in drinking water at levels and frequencies that pose a public health risk. More specifically, these contaminant attributes should be used in a prototype classification algorithm approach, such as that described in Chapter 5, and in conjunction with expert judgment to help identify the highest-priority PCCL contaminants for inclusion on a CCL. In this chapter, the committee has presented a scoring system and related considerations for a total of five health effect and occurrence attributes. For health effects, the committee identified severity and potency as key predictive attributes; prevalence, magnitude, and persistence-mobility comprise the occurrence attributes. Although the committee spent a great deal of time deliberating on the number and type of contaminant attributes that should be used in the recommended CCL development approach, ultimately, it decided that these five attributes constitute a reasonable starting point for EPA to consider. Furthermore, the scoring metrics and related considerations for each attribute should be viewed as illustrative. Thus, the committee does not explicitly or implicitly recommend that these five (or that there should be five) attributes the related scoring metrics be adopted directly for use by EPA. If EPA chooses to adopt a prototype classification approach for the development of future CCLs, the committee recommends that options for developing and scoring contaminant attributes should be made available
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Classifying Drinking Water Contaminants for Regulatory Consideration for public and other stakeholder input and undergo scientific review. The committee also makes the following related recommendations: The assessment of severity should be based, when feasible, on plausible exposures via drinking water. The committee also recommends that EPA give consideration to different severity metrics such as a ranking through use of either quality adjusted or disability adjusted life-years lost due to exposure to a contaminant. Regarding the assessment of contaminant prevalence, in some cases (particularly where contaminants have been included on a PCCL on the basis of potential rather than demonstrated occurrence), information will often be insufficient to directly assess temporal or spatial prevalence (or both). Thus, EPA should consider the possibility of including information on temporal and regional occurrence to help determine (score PCCL) contaminant prevalence. When prevalence cannot be assessed, this attribute must then go unscored and the attribute of persistence-mobility used in its stead. The issue of changing (or incorporating) “thresholds” for contaminant detection, rather than relying on continually decreasing detection limits, is one that needs explicit attention and discussion by EPA and stakeholders. Existing and readily available databases may not be sufficient to rapidly and consistently score health effect and occurrence attributes for individual PCCL contaminants for promotion to a CCL. As recommended in Chapter 3, all information from existing or created databases or lists used in the development of a CCL and PCCL, should be compiled in a consolidated database that would provide a consistent mechanism for recording and retrieving information on the PCCL contaminants under consideration. As a starting point and as recommended in Chapter 3, EPA should review its developing EDPSD database to determine if it can be expanded and used (or serve as a model for the development of) such a consolidated database and to help develop future PCCLs and CCLs. Contaminant databases used in support of the development of future CCLs should report summary statistics on all data collected, not only the quantifiable observations. In this regard, EPA should formalize a process for reporting means and/or medians from data with large numbers of nondetect observations. In addition, EPA may want to consider providing other measures of concentration in water supplies such as the 95th percentile of contaminant concentration.
Representative terms from entire chapter: