6

Understanding the Risks

Although people commonly ask whether the actions they take are “safe,” with an implication that safety poses no risk of harm to human health, it is impossible to demonstrate such a definition of safety or indeed to achieve zero risk. It has previously been recommended (NRC, 1998) “that water agencies considering potable reuse fully evaluate the potential public health impacts from the microbial pathogens and chemical contaminants found or likely to be found in treated wastewater through special microbiological, chemical, toxicological, and epidemiological studies, monitoring programs, risk assessments, and system reliability assessments.” In other words, an evaluation of the adequacy of public health and ecological protection rests upon a holistic assessment of multiple lines of evidence, such as toxicology, epidemiology, chemical and microbial analysis, and risk assessment.

Major research efforts have attempted to refine our understanding of the human health risks of water reuse, particularly the risks of potable reuse, through toxicological and epidemiological studies (see Boxes 6-1 and 6-2; NRC, 1998).1 In the context of reclaimed water projects, epidemiological analyses of health outcomes are an imprecise method to quantify chronic health risks at levels generally regarded as acceptable. This is especially true when interpreting negative study results, which typically do not have the statistical power to detect the level of risks considered significant from a population-based perspective (e.g., an additional lifetime cancer risk of 1:10,000 to 1:1,000,000). Although epidemiology is invaluable as part of an evaluative suite of analytical tools assessing risk, epidemiology may be most useful at bounding the extent of risk, rather than actually determining the presence of risk at any level. Direct toxicological methods (Box 6-2) are intriguing, as indeed was noted in the National Research Council report on Issues in Potable Reuse (NRC, 1998), yet there remains insufficient development and knowledge for these methods to be broadly applied.

There will always be a need for human-specific data, and epidemiological studies will remain important to assessing and monitoring the occurrence of health impacts. However, today’s decisions as to health and environmental protection remain grounded in the measurement of chemical and microbiological parameters and the application of the formal process of risk assessment. Risk can be identified, quantified, and used by decision makers to assess whether the estimated likelihood of harm—no matter how small—is socially acceptable or whether it may be justified by other benefits. Risk assessment provides input to the overall decision process, which also includes consideration of financial costs and social and environmental benefits (discussed in Chapter 9).

The focus of this chapter is to present risk assess-

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1 Toxicological studies expose animals or organisms to a series of doses or dilutions of a single contaminant, complex mixtures, or actual concentrates of reclaimed water to predict adverse health effects (e.g., mortality, morphological changes, effects on reproduction, cancer occurrence). Toxicological tests on mammals often are used to identify doses associated with toxicity, and these dose-response data are subsequently used to estimate human health risks. Potential adverse human health effects are more difficult to predict based on studies in nonmammalian species or microorganisms; however, observed effects are considered cause for further investigation. Epidemiological studies examine patterns of human illness (morbidity) or death (mortality) at the population level to assess associated risks of exposure.



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6 Understanding the Risks Although people commonly ask whether the ac- projects, epidemiological analyses of health outcomes tions they take are “safe,” with an implication that safety are an imprecise method to quantify chronic health poses no risk of harm to human health, it is impossible risks at levels generally regarded as acceptable. This is to demonstrate such a definition of safety or indeed to especially true when interpreting negative study results, achieve zero risk. It has previously been recommended which typically do not have the statistical power to (NRC, 1998) “that water agencies considering potable detect the level of risks considered significant from a reuse fully evaluate the potential public health impacts population-based perspective (e.g., an additional life- from the microbial pathogens and chemical contami- time cancer risk of 1:10,000 to 1:1,000,000). Although nants found or likely to be found in treated wastewater epidemiology is invaluable as part of an evaluative suite through special microbiological, chemical, toxicologi- of analytical tools assessing risk, epidemiology may be cal, and epidemiological studies, monitoring programs, most useful at bounding the extent of risk, rather than risk assessments, and system reliability assessments.” In actually determining the presence of risk at any level. other words, an evaluation of the adequacy of public Direct toxicological methods (Box 6-2) are intriguing, health and ecological protection rests upon a holistic as indeed was noted in the National Research Council assessment of multiple lines of evidence, such as toxi- report on Issues in Potable Reuse (NRC, 1998), yet there cology, epidemiology, chemical and microbial analysis, remains insufficient development and knowledge for and risk assessment. these methods to be broadly applied. Major research efforts have attempted to refine our There will always be a need for human-specific understanding of the human health risks of water reuse, data, and epidemiological studies will remain important particularly the risks of potable reuse, through toxico- to assessing and monitoring the occurrence of health logical and epidemiological studies (see Boxes 6-1 and impacts. However, today’s decisions as to health and 6-2; NRC, 1998).1 In the context of reclaimed water environmental protection remain grounded in the measurement of chemical and microbiological param- 1 Toxicological studies expose animals or organisms to a series eters and the application of the formal process of risk of doses or dilutions of a single contaminant, complex mixtures, assessment. Risk can be identified, quantified, and used o r actual concentrates of reclaimed water to predict adverse by decision makers to assess whether the estimated health effects (e.g., mortality, morphological changes, effects on reproduction, cancer occurrence). Toxicological tests on mammals likelihood of harm—no matter how small—is socially often are used to identify doses associated with toxicity, and these acceptable or whether it may be justified by other dose-response data are subsequently used to estimate human health benefits. Risk assessment provides input to the overall risks. Potential adverse human health effects are more difficult to decision process, which also includes consideration of predict based on studies in nonmammalian species or microorgan- isms; however, observed effects are considered cause for further financial costs and social and environmental benefits investigation. Epidemiological studies examine patterns of human (discussed in Chapter 9). illness (morbidity) or death (mortality) at the population level to The focus of this chapter is to present risk assess- assess associated risks of exposure. 101

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102 WATER REUSE BOX 6-1 Water-Reuse–Specific Epidemiological Information NRC (1998) provided a comprehensive review of six toxicological and epidemiological studies of reuse systems. The epidemiological study findings from potable reuse applications are briefly summarized in this box. The results from several toxicology studies are summarized in Box 6-2. Windhoek, Namibia, is the first city to have implemented potable reuse without the use of an environmental buffer (sometimes called direct potable reuse; see Box 2-12). It has been doing so since 1968, especially during drought conditions, and the plant provides up to 35 percent of the potable water supply during normal periods. Epidemiological evaluations of the population have found no relationships between drinking water source and diarrheal disease, jaundice, or mortality (Isaacson et al., 1987; Isaacson and Sayed, 1988). Three sets of studies have been conducted for the Montebello Forebay Project in Los Angeles County, California: (1) a 1984 Health Effects Study, which evaluated mortality, morbidity, cancer incidence, and birth outcomes for the period 1962–1980; (2) a 1996 RAND study, which evalu- ated mortality, morbidity, and cancer incidence for the period 1987–1991; and (3) a 1999 RAND study, which evaluated adverse birth outcomes for the period 1982–1993, The first studies looked at two time periods (1969–1980 and 1987–1991) and characterized census tracts into four or five categories by 30-year average percentage of reclaimed water in the water supply. The annual maximum percentage of reclaimed water ranged from less than 4 percent to between 20 and 31 percent. The studies included 21 and 28 health outcome measures, respectively, including health outcomes related to cancer, mortality, and infectious disease incidence. Although some outcomes were more prevalent in the census tracts with a higher percentage of reclaimed water in the water supply, neither study observed consistently higher rate patterns or dose-response relationships (Frerichs et al., 1982; Frerichs, 1984; Sloss et al., 1996). Sloss et al. (1996) identified reclaimed water use and control areas so that comparisons could be made. Compared with the control areas, reclaimed water use areas had some statistically higher as well as lower rates of disease. After evaluating the overall patterns of disease, the authors concluded that the study results did not support the hypothesis of a causal relationship between reclaimed water and cancer, mortality, or infectious disease. Although assessment of a dose-response relationship was possible in the study design, none was identified for the excesses of disease seen. Since the NRC (1998) report, there have been only a few additional epidemiological studies of human health impacts of wastewater reuse. The largest and most comprehensive study was the third continuation of the Montebello Forebay study (Sloss et al., 1999). Sloss et al. (1999) included a health assessment utilizing administrative health data from 1987–1991 and birth outcomes from 1982–1993. They found some differ- ences between study groups but saw no pattern and concluded that the rates of adverse birth events were similar between the control group and the region receiving reclaimed water. The most recent study (Sinclair et al., 2010) compared the health status of residents in two housing developments: one with dual plumbing to support nonpotable reuse and a nearby development using a conventional water supply. The study assessed the rates that residents consulted with primary care physicians for gastroenteritis, respiratory complaints, and dermatological complaints (conditions that could be related to reclaimed water exposure) as well as two conditions unrelated to water reuse or waterborne disease exposure. Sinclair et al. (2010) reported no differences in consultation rates between the two groups. There were slight differences in the ratios of specific consultations (i.e., dermal versus respiratory), but the seasonal reporting patterns did not match the timing of reclaimed water exposure. Population-based studies, also called ecological studies, such as these face significant challenges such as short study periods for chronic disease outcomes, changing exposures over time, nonspecific disease outcomes with unknown attributable risks, and the inability to know actual water consumption rates. Their use for quantitative risk assessment is extremely limited. Such studies simply cannot have the statistical power to achieve detection of the risk expectations established in public water supply regulatory standards such as 10–5 or 10–6 lifetime cancer risk. Population-based studies are probably best viewed as “scoping” or hypothesis-forming exercises. They cannot prove that there is no adverse effect from the reuse of water in these areas (indeed no study can do so), but they can suggest an upper bound on the extent of the impact if one did exist. Two alternative study approaches could be considered for assessing the effects of reclaimed water on public health. Blinded-design household intervention studies could be used in which all households in the study receive point of use (POU) “treatment devices,” although the control group receives sham devices, and the occurrence of acute gastroenteritis illness is tracked. Most health concerns related to chemical exposures are chronic diseases that may take years to appear. To avoid the need for long observation periods, the household intervention approach could use human tissue chemical biomarkers rather than disease occurrences. Another methodology that is more passive but holds promise for assessing the health impacts of reclaimed water consumption is the “opportunistic natural experiment,” epidemiologically characterized as a community intervention study. These studies assess the incidence of acute gastrointestinal illness before and after scheduled changes in water sources or treatment processes. An example of such a study is a 1984–1987 Colorado Springs study of water reuse for public park irrigation. Three different sources of water (potable, nonpotable water of wastewater origin, and nonpotable water of runoff origin) were used to irrigate municipal parks, and randomly selected park users were surveyed for the occurrence of gastrointestinal disease. Wet grass conditions and elevated densities of indicator bacteria, but not exposure to nonpotable irrigation water per se, were associated with an increased rate of gastrointestinal illness. In- creased levels of disease and symptoms were observed when several different bacterial indicators exceeded 500/100 mL. These levels occurred most commonly with the nonpotable water of runoff origin (Durand and Schwebach, 1989). A well-designed case control study can also be used in select populations. Such studies in the context of ordinary potable water have been conducted by a number of authors (Payment et al., 1997; Aragón et al., 2003; Colford et al., 2005).

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103 UNDERSTANDING THE RISKS BOX 6-2 Potable Reuse Toxicological Testing In 1982, the National Research Council Committee on Quality Criteria for Reuse concluded that the potential health risks from reclaimed water should be evaluated via chronic toxicity studies in whole animals (NRC, 1982). Early studies in laboratory animals, most notably the Denver and Tampa Potable Water Reuse Demonstration Project studies, which used rats and mice exposed to concentrates of reclaimed water, failed to identify adverse health effects when tested in subchronic, reproductive, developmental, and chronic toxicity studies (Lauer et al., 1990; CH2M Hill, 1993; Condie et al., 1994; Hemmer et al., 1994; see also more comprehensive descriptions in NRC, 1998). The absence of adverse effects following repeated, long-term exposure to concentrates of reclaimed water was also confirmed in mice chronically exposed to 150 and 500× concentrates of reclaimed water from a Singapore reclamation plant (NEWater Expert Panel, 2002). Although data from the 24-month tests were planned for completion in 2002, the Singapore Water Reclamation Board did not reconvene the NEWater Expert Panel to evaluate the results or issue an updated final report. The Orange County Water District conducted online biomonitoring of Japanese Medaka fish exposed to effluent-dominated Santa Ana River water over 9 months and found no statistically significant differences in mortality, gross morphology, reproduction, or gender ratios (Schlenk et al., 2006). The Singapore Water Reclamation Board also exposed Japanese Medaka fish (Oryzias latipes) to reclaimed water over multiple generations and identified no estrogenic or carcinogenic effects in fish (Gong et al., 2008). However, the relevance of these findings to human health remains unclear. In addition to the in vivo studies described above, a number of in vitro genotoxicity studies have been conducted on samples of reclaimed water and/or concentrates of reclaimed water sampled from sites in Montebello, California, Tampa, Florida, San Diego, California, and Washington, DC (summarized in C. Rodriguez et al., 2009). These studies have identified a small number of positive results—a few tests showed mutagenic effects in the Ames assay in Salmonella typhimurium—although most in vitro and in vivo genotoxicity assays (e.g., mammalian cell transforma- tion, 6-thioguanine resistance, micronucleus, Ames, and sister chromatid assays) have been negative (Nellor et al., 1985; Thompson et al., 1992; Olivieri et al., 1996; CSDWD, 2005). Although in vitro assays are useful for identifying specific bioactivity and chemical modes of action, they are not likely to be used in isolation for the determination of human health risk. Such bioassays provide a high degree of specificity of response, but they generally cannot represent the actual situation in animals that includes metabolism, multicell signaling, and plasma protein binding, among others. In addition, some chemicals can be rapidly degraded during digestion and metabolism, whereas others are transformed into more toxic metabolites. At the same time, many limitations also plague the current in vivo testing paradigm in that interspecies and intraspecies variability can obfuscate the interpretation of animal testing results when applied to humans. For this reason, uncertainty factors are applied in an attempt to provide a conservative estimate of human health risk from animal models. The U.S. Environmental Protection Agency (EPA) and the National Toxicology Program continue to investigate modern in vitro, genomic, and proteomic methods for rapid screening of chemicals and mixtures and to better deduce the complex pathways leading to disease (NRC, 2007; Col - lins et al., 2008). Although high-throughput screening using in vitro tools will increase the knowledge on various modes of toxicity of chemicals, in vivo testing will remain an integral part of evaluation of human health consequences from chemical exposure. However, a powerful approach to screening waters can involve a battery of bioassays, each with different toxicological endpoints (Escher et al., 2005). ment methods for chemical and microbial contami- Rodriquez et al., 2007b, 2009; Huertas et al., 2008). nants that can be used to quantify health risks associ- Quantitative methods to assess potential human health ated with water reuse applications. In Chapter 7, these risks from chemical and microbial contaminants in methods are applied in a comparative analysis of several reclaimed water have evolved over the past 30 years reuse scenarios compared to a conventional drinking and are still being refined. Although EPA has extensive water source commonly viewed as safe. health effects data on regulated contaminants, potable reuse and de facto reuse involve some level of exposure to minute quantities of contaminants that are not INTRODUCTION TO THE regulated. Many of these classes of constituents may RISK FRAMEWORK require innovative approaches to assess health risks. With the limitations of toxicological testing and Challenges associated with assessing risks posed by population-level epidemiological studies, quantitative such contaminants include incomplete toxicological risk assessment methods become a critically important datasets, uncertainties associated with concomitant basis for assessing the acceptability of a reclaimed wa- low-level exposures to multiple chemical and biological ter project (NRC, 1998; Asano and Cotruvo, 2004; C. materials that may share similar modes of action; and

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104 WATER REUSE deficiencies in analytical methods to accurately identify of the assessment activities—with respect to agents, and quantify the presence of these contaminants in consequences, routes, and methodologies—should be reclaimed water (Snyder et al., 2009, 2010a; Drewes outlined. The nature of the management question to et al., 2010). be addressed should drive the nature of the assessment The contribution of water-associated risks to the activities. Examples of potential scoping questions rel- total U.S. disease burden is estimated to be relatively evant to water reuse include what is the risk from using small. However, water that is not treated to the ap- groundwater that has been mixed with reclaimed water propriate level for the end use can pose significant as a supplement to an existing surface water supply, or human health risks. These include chronic effects, what is the human health risk from the application of such as cancer or genetic mutations, or acute effects, undisinfected secondary effluent to fruit crops? • Stakeholder involvement: At all stages, there such as neurotoxicity or infectious diseases. These adverse outcomes may be caused by different agents, should be well-understood processes available for in- such as inorganic constituents, organic compounds, volvement of internal and external stakeholders. This and infectious agents. The impact of an agent may be is an important consequence of the fact that risk as- a function of the route of exposure (e.g., oral, dermal, sessment per se involves a number of trans-scientific inhalation, ocular). Rarely can an observed outcome be assumptions (Crump, 2003), and the involvement of ascribed to a particular agent and exposure route in a stakeholders at all stages promotes transparency to particular vehicle (such as reclaimed water). In water the process and, it is hoped, greater acceptance of the reuse considerations, there will invariably be multiple ultimate risk management decision. • Evaluation: Within the assessment phase itself, substances, types of effects, and modes of exposure that may be relevant. there is an explicit evaluation step to determine whether Historically, the paradigm for risk analysis has been the computations have produced results of sufficient divided into risk assessment (based on objective techni- utility in risk management and of the nature contem- cal considerations) and risk management, wherein more plated in problem formulation and scoping. If this is subjective aspects (e.g., cost, equity) are considered. not the case, further developed assessments should be Risk characterization served as the conduit between conducted. This recognizes that there are various levels the two activities, as introduced in NRC (1983; also of complexity that can be used in risk assessment with known as the “Red Book”). However, evolution in the a tradeoff between time and resources required for the use of risk to regulate human exposure has resulted in assessment and degree of uncertainty in the results. If substantial evolution of the framework. a risk management question can be addressed satis- Early in 2009, an updated risk framework, encap- factorily with a less intensive assessment process, such sulated in Figure 6-1, was developed (NRC, 2009b). an approach would be favorable inasmuch as it would This updated framework has a number of important enable a decision to be reached more expeditiously with revisions that are of particular relevance to the problem less resource expenditure. under consideration in this report. This framework shares a number of similarities with the 1983 Red Book There is also more explicit recognition (NRC 2009b) framework with respect to the central tasks of risk as- that risk management decisions will involve consider- sessment (i.e., hazard characterization, dose-response ation not only of the risk assessment results, but of is- assessment, exposure assessment, and risk character- sues of economics, equity, and law, which are discussed ization). However, it formally introduces several new in Chapters 9 and 10. aspects to the risk analysis and management process In the following sections, four core components of that are particularly germane to assessing and managing risk assessment are discussed with regard to a range of health risks from reclaimed water: water reuse applications: • Problem formulation: A t the outset, there 1. hazard identification, which includes a summary should be a problem formulation and scoping phase of chemical and microbiological agents of concern; in which the risk management question(s) to be an- 2. exposure assessment, which explains the route and swered should be explicitly framed, and the nature extent of exposure to contaminants in reclaimed water;

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105 UNDERSTANDING THE RISKS P has e I P has e II Problem Formulation and Planning and Conduct of P has e III Scoping Risk Assessment Risk Management Stage 1: P lanning -What are the - What -For the given decision context, what are the attributes of assessments necessary to characterize risks of relative health or problems are existing conditions and the effects on risk of proposed options? What level of uncertainty and variability analysis environmental associated with is appropriate? benefits of the existing proposed options? environmental -How are other conditions? decision-making - If existing Stage 2: R is k As s es s ment factors conditions (technologies, appear to pose costs) affected by a threat to Hazard Identi c ation the proposed human or -What adverse health or environmental effects are options? environmental R is k C haracterization associated with the agents of concern? -What is the health, what -What is the nature and Dos e-R es pons e As s es s ment decision, and its options exist for magnitude of risk -For each determining adverse effect, what is the justification, in altering those associated with existing relationship between dose and the probability of the light of benefits, conditions? conditions? occurrence of the adverse effect in the range of doses costs and - Under the -What risk decreases identified in the exposure assessment? uncertainties in given decision benefits are associated each option? context, what with each of the options? -How should the risk and other -Are any risks increased? decision be technical What are the significant E xpos ure As s es s ment communicated? assessments uncertainties? -What exposures doses are incurred by each population -Is it necessary to are necessary of interest under existing conditions? evaluate the to evaluate the -How does each option affect existing conditions and effectiveness of possible risk resulting exposures doses? the decision? management -If so, how should options? this be done? Stage 3: C on rmation of Utility NO YES -Does the assessment have the attributes called for in planning? -Does the assessment provide sufficient information to discriminate among risk management options? -Has the assessment been satisfactorily peer reviewed? F OR MAL P R OV IS ONS F OR INT E R NAL AND E XT E R NAL S T AK E HOL DE R INV OL V E ME NT AT AL L S T AG E S -The involvement of decision makers, specialists, and other stakeholders in all phases of the processes leading to decisions should in no way compromise the technical assessment risk, which carried out under its own standards and guidelines. FIGURE 6-1 Consensus risk paradigm. SOURCE: NRC (2009b) 3. dose-response assessment, which explains the re- and supply systems as well as wastewater collection lationship between the dose of agents of concern and and treatment systems. Despite much success across estimates of adverse health effects, and the developed world to consistently deliver safe wa- 4. risk characterization, in which the estimated risk ter, diseases associated with microorganisms in water under different scenarios is compiled. This may include continue to occur. Epidemiological investigations have a determination of relative risk (via the route under resulted in estimates of between 12 million and 19.5 consideration, e.g., reclaimed water) versus risks from million waterborne illnesses per year in the United the same contaminants via other routes (e.g., alternative States (Reynolds et al., 2008). Such illnesses are caused supplies). by exposure to bacteria, parasites, or viruses (Barzilay et al., 1999). Fortunately, in the United States these illnesses CONTEXT FOR UNDERSTANDING rarely result in death. On the other hand, death due to WATERBORNE ILLNESSES acute gastrointestinal illness, especially in the vulner- AND OUTBREAKS able young, is all too common in the developing world. As noted in Chapter 2, the early 20th century Most obvious to the public are the reported outbreaks brought significant public health improvements due of acute gastrointestinal illness largely due to patho- to the implementation of constructed water treatment gens in the water supply (Mac Kenzie et al., 1994).

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106 WATER REUSE Epidemiologists have been conducting surveillance for A review of 33 studies of incidence and prevalence of waterborne outbreaks for nearly 100 years and keeping acute gastrointestinal illness from all exposure sources statistics since 1920. The epidemiological investigation ranged from 0.1 to 3.5 episodes per adult per year, of these events has helped identify the vulnerabilities with child estimates higher (Roy, 2006). Roy (2006) in our drinking water delivery systems and led to many estimated 0.65 episode per person per year in the system improvements. From 1991 to 2002, an annual United States. Health effects from marine recreational average of 17 waterborne outbreaks were reported and exposures to microbial pathogens in water receiving investigated in the United States compared with an treated wastewater discharge (e.g., eye infection, ear annual average of 23 during 1920–1930 (Craun et al., and nose infections, wound infections, skin rashes) are 2006), while over the same period, the U.S. population also underreported (Turbow et al., 2003, 2008). increased by a factor of over 2.5. From 1991 to 2000 As illustrated above, many human illnesses have the there were 155 outbreaks recorded in the national epi- potential to be transmitted via water exposure. There demiological surveillance system. In 39 percent of the are few if any waterborne pathogens that are distinct reports, no causative agent was identified, and in 16 to reclaimed water, as opposed to other modes of in- percent, the cause was a chemical. These studies suggest troduction into the potable or nonpotable aquatic en- that the epidemiology of waterborne disease is complex vironments. Sometimes these other modes can result in and that outbreak surveillance is far from complete, large waterborne outbreaks. For example, an estimated with significant underreporting. Analyses from recent 400,000 cases of Cryptosporidium illness occurred in years have identified that deficiencies in the water dis- Milwaukee in 1993 caused by a failure in a filtration tribution system rather than failure in the treatment process at a water treatment plant (Mac Kenzie et al., process are increasingly the cause of outbreaks (Craun 1994), and an acute gastrointestinal illness outbreak in et al., 2006; NRC, 2006). Thus, water may be free Ohio affected over 1,500 people from microbial con- of contamination when it leaves the municipal water tamination of a groundwater supply (Fong et al., 2007). treatment plant but becomes recontaminated by the Therefore, although this chapter focuses on the risks time it reaches the household tap. The adequacy of of water reuse, potential waterborne hazards should be the distribution system may therefore provide a limit considered in the context of the full suite of possible to the degree of risk reduction even though treatment exposure routes. becomes more stringent. This also heightens the need for monitoring at the point of exposure (i.e., the tap) HAZARD IDENTIFICATION rather than relying solely on monitoring immediately after treatment. Data collected by the Centers for The first step in any risk assessment (microbial or Disease Control and Prevention’s Surveillance for chemical) is hazard identification, defined as “the pro- Waterborne Diseases and Outbreaks indicated that cess of determining whether exposure to an agent can Escherichia coli, norovirus, and unidentified microbial cause an increase in the incidence of a health condition” pathogens (likely viral) are the common causes of the (NRC, 1983) such as cancer, birth defects, or gastroen- waterborne disease outbreaks (Blackburn et al., 2004; teritis, and whether the health effect in humans or the ecosystem is likely to occur.2 Hazards of reclaimed wa- Liang et al., 2006; Yoder et al., 2008). Cases of men- ingitis and other infectious diseases also were reported ter may depend on factors such as its composition and during water recreation in virus-contaminated coastal source water (industrial and domestic sources), varying waters (Begier et al. 2008). removal effectiveness of different treatment processes, The record of waterborne disease outbreaks, how- the introduction of chemicals, and the creation of ever, is only the tip of the iceberg. Large numbers of transformation byproducts during the water treatment waterborne infectious diseases are undocumented. The process (NRC, 1998). It is important to remember that level of background endemic diseases associated with risk is a function of hazard and exposure, and where water and water supplies is not well understood. There there is no exposure, there is no risk. is no estimate of waterborne diseases by specific region 2 http://www.epa.gov/oswer/riskassessment/human_health_tox - or community or by water utility treatment modalities. icity.htm

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107 UNDERSTANDING THE RISKS Chemical and microbial contaminants constitute chemicals that are relevant to water reuse pose chronic two types of agents that may cause a spectrum of ad- health risks, where long periods of exposure to small verse health impacts, both acute and/or chronic. Acute doses of potentially hazardous chemicals can have a health effects are characterized by sudden and severe cumulative adverse effect on human health (Khan, illness after exposure to the substance. Acute illnesses 2010; see Chapter 10 for discussion of regulation of are common after exposure to pathogens, but acute drinking water contaminants). As noted in Box 6-1, health effects from exposure to regulated or unregu- epidemiological studies are seldom able to determine lated chemical contaminants found in drinking water which of the many chemicals typically present in the or reclaimed water are highly unlikely under anything water over time are associated with the chronic health but aberrant conditions due to system failures, chemical effects described. Box 6-3 provides a list of the biologi- spills, unrecognized cross connections with industrial cally plausible diseases investigated in the literature for waste streams, or accidental overfeeds of disinfection associations with water exposures as well as the organ agents. Chronic health effects are long-standing and systems most vulnerable to the contaminants present in are not easily or quickly resolved. They tend to occur wastewater (Sloss et al. 1996; NRC, 1998). after prolonged or repeated exposures over many days, As noted in Chapter 3, a large array of chemicals months, or years, and symptoms may not be immedi- are present at low concentrations in the nation’s source ately apparent. There is recently recognized concern waters and drinking water, including pharmaceuticals for effects arising via an epigenetic route wherein an and personal care products (see Table 3-3; Kolpin et agent alters aspects of gene translation or expression; al., 2002; Weber et al., 2006; Rodriquez et al., 2007a,b; such effects can be manifested in a variety of end points Snyder et al., 2010b; Bull et al., 2011). There is a grow- (Baccarelli and Bollati, 2009). ing public concern over potential health impacts from long-term ingestion of low concentrations of trace or- ganic contaminants (Snyder et al., 2009, 2010b; Drewes Chemical Hazards and Risks Health hazards from chemicals present in re- claimed water (discussed in Chapter 3) include poten- tial harmful effects from naturally occurring and syn- BOX 6-3 thetic organic chemicals, as well as inorganic chemicals. Biologically Plausible Possible Health Outcomes from Exposures to Chemicals Some of these chemicals, including the carcinogens Found in Wastewater N-nitrosodimethylamine (NDMA; see Box 3-2), and trihalomethanes (EAO, Inc., 2000), may be produced Cancer in the course of various treatment processes (e.g., dis- Bladdera Liver infection), rather than arising from the source water Colona Pancreas itself. Among the most studied of this latter class of Esophagus Rectuma chemicals are the chlorination disinfection byproducts, Kidney Stomach which have been associated with cancer as well as ad- Reproductive and Development Outcomes verse birth outcomes. Because of the need to disinfect wastewater, which may have comparatively higher or- Spontaneous abortiona Birth defectsa Low birth weight Preterm birth ganic content than typical drinking water sources, such treatment-related contaminants may be problematic in Target Organ Systems some reclaimed waters. Gastrointestinal organs Cardiovascular organs Multiple studies in the scientific literature have Kidney Cerebrovascular organs described associations between chemical contaminants Liver in drinking water and chronic disease such as cancer, chronic liver and kidney damage, neurotoxicity, and aMost consistently increased in epidemiological studies, adverse reproductive and developmental outcomes such especially those of trihalomethane disinfection byproducts. as fetal loss and birth defects (NRC, 1998). Most toxic

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108 WATER REUSE et al., 2010). In contrast to well-documented adverse pathogen infectious dose; the virulence factor; and the health effects associated with exposure to specific susceptibility of the human host. disinfection byproducts (such as trihalomethanes) in Bacterial pathogens in general are more sensitive municipal water systems, health hazards posed by long- to wastewater treatment than are viruses and proto- term, low-level environmental exposure to trace organic zoa; thus, few survive in disinfected water for reuse contaminants in reclaimed water or from de facto reuse (see Chapter 3, Table 3-2). Most bacterial pathogens scenarios are not well characterized, nor are their sub- (e.g., Vibrios) also have a high median infectious dose, sequent health risks known (NRC, 2008a; Khan, 2010; which requires ingestion of many cells for a likely es- Snyder et al., 2009, 2010b). Although chemicals cur- tablishment of infection in healthy adults (Nataro and rently regulated in drinking water have comparatively Levine, 1994). Other bacteria, such as Salmonella, can robust toxicological databases, many more chemicals constitute a likely human infection with 1 to 10 cells if present in water are unregulated and are missing critical consumed with high-fat-content food (Lehmacher et toxicological data important to understanding low-level al., 1995). Toxigenic E. coli O157:H7 with two potent chronic exposure impacts (Drewes et al., 2010). These toxins is also suspected of having a low median infec- same agents can be present in treated wastewater in tious dose (Teunis et al., 2004). concentrations not otherwise encountered in most In comparison with bacterial pathogens, protozoan public water supply sources. cysts and viruses are more resistant to inactivation in To date, epidemiological analyses of adverse health water. Protozoan cysts are resistant to low doses of effects likely to be associated with use of reclaimed chlorine, and high infection rates in water are associ- water have not identified any patterns from water reuse ated with suboptimal chlorine doses. Viruses can pass projects in the United States (Khan and Roser, 2007; the filtration system in water treatment plants because NRC, 1998; see Box 6-1). In laboratory animals and of their small size. Some viruses are also resistant to in vitro studies, there is a mixed picture, with more ultraviolet disinfection (see Chapter 4). Because they recent studies on genotoxicity, subchronic toxicity, have a low median infectious dose, viruses have the po- reproductive and developmental chronic toxicity, and tential to present a concern in water reuse applications. carcinogenicity showing negative results (summarized In addition to microbial characteristics, human in Nellor et al., 1985; Lauer et al., 1990; Condie et host susceptibility plays an essential role in microbial al., 1994; Sloss et al., 1999; Singapore Public Utilities hazards. Microbial agents that are benign to a healthy Board and Ministry of the Environment, 2002; R. A. population can lead to fatal infections in a susceptible Rodriguez et al., 2009; see also Box 6-2). Collectively, population. The growing numbers of immunocom- while these findings are insufficient to ensure complete promised individuals (e.g., organ transplant recipients, safety, these toxicological and epidemiological stud- those infected with HIV, cancer patients receiving che- ies provide supporting evidence that if there are any motherapy) are especially vulnerable to such infection. health risks associated with exposure to low levels of Because of their clinical status, infection is difficult to chemical substances in reclaimed water, they are likely treat and often becomes chronic. Infectious-agent dis- to be small. ease can also lead to chronic secondary diseases, such as hepatitis and kidney failure, and can contribute to ad- verse reproductive outcomes. The exacerbating factors Microbial Hazards are not unique to water reuse but apply to all exposure Most waterborne infections are acute and are the to infectious microorganisms via water, food, and other result of a single exposure. Disease outcomes associated vehicles. Table 3-1 lists the microbial agents that have with infection from waterborne pathogens include gas- been associated with waterborne disease outbreaks and troenteritis, hepatitis, skin infections, wound infections, also includes some agents in wastewater thought to conjunctivitis, and respiratory infections. Microbial pose significant risk. infection rates are determined by the survival ability of the pathogen in water; the physicochemical condi- tions of the water, including the level of treatment; the

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109 UNDERSTANDING THE RISKS WATER REUSE EXPOSURE ASSESSMENT For the purpose of human health risk assessments, exposure is defined as contact between a person and a chemical, physical, or biological agent. The amount of exposure (or dose) is a product of two variables: concentration of a substance in a medium (e.g., the concentration of trihalomethanes in reclaimed water) and the amount of that medium to which an individual is exposed (e.g., via ingestion or inhalation). For an ingested contaminant, the dose is the concentration in water multiplied by the amount of water ingested. Accurately assessing exposure to reclaimed water is a F IGURE 6-2 C ontinuum of water quality with use and critically important aspect of assessing health risks, treatment. because the likelihood of harm from exposure distin- NOTES: (1) Typical processes include coagulation-flocculation, sedi- guishes risk from hazard. mentation, filtration, and disinfection. (2) Processes include secondary treatment and disinfection. (3) Effluent discharged to environmental receiving water or Influence of Water Treatment on Potential reused. Exposures SOURCE: Adapted from McGauhey (1968); T. Asano, personal communication, 2010). Reclaimed wastewater that has undergone varying degrees of water treatment will have different levels of microbial and chemical contamination (see Table 3-2 and Appendix A). As discussed in Chapter 2, the ap- propriate end use of reclaimed water is dependent on reverse osmosis, high-energy ultraviolet light with the level of water treatment, with greater intensity hydrogen peroxide) is suitable for a greater number of of treatment more effectively reducing or removing nonpotable or potable uses, including uses that have a microbial and chemical contaminants as needed by higher degree of human exposure to the constituents particular applications (EPA, 2004; de Koning et al., in reclaimed water, such as food crop irrigation and 2008). The treatment and conveyance of waters of dif- groundwater recharge. In contrast, wastewater that ferent qualities is not novel and dates to the Roman has only undergone primary treatment (sedimentation imperial times (Robins, 1946). only), has no use as reclaimed water in the United Over the course of time, a unit volume of water un- States because of the likely chemical and microbial con- dergoes changes in quality (illustrated conceptually in tamination. It should be recognized that more extensive Figure 6-2). With use, a deterioration in quality occurs treatment generally is more cost- and energy-intensive, that may be reversed with treatment. Depending on the may have greater potential for byproducts to occur, and desired use, water may be abstracted at different loca- may have greater environmental footprints. Different tions along this continuum (i.e., at the right-hand side, applications of reclaimed water are also associated with increasing degrees of treatment will produce reclaimed different exposure scenarios, discussed in more detail wastewater suitable for increasingly stringent usages). later in this section. Reclaimed water that has undergone secondary treatment (biological oxidation or disinfection) has nu- Influence of Different Exposure Circumstances and merous nonpotable uses in applications with minimal Routes of Exposure on Dose human exposure potential, such as industrial cooling and nonfood crop irrigation (see Chapter 2). Second- Exposure to contaminants in reclaimed water oc- ary effluent that has undergone further treatment (e.g., curs not only through the ingestion of water that has chemical coagulation, disinfection, microfiltration, been designed for potable reuse applications but also

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110 WATER REUSE from food, skin and eye contact, accidental ingestion for ingestion, dermal contact, and inhalation of that during water recreation, and inhalation in other reuse chemical from reclaimed water. To assess the likeli- applications (Gray, 2008). Exposure can also result hood that adverse health effects may occur, the ADD from improper use of reclaimed water, improper opera- can be compared with a daily dose determined to be tion of a reclaimed water system, or inadvertent cross acceptable over a lifetime of exposure. See Appendix B connections between a potable water and a nonpotable for equations used for calculating each of these terms. water distribution system (see Box 6-4). This illustrates that regardless of the intended use, the assessment of Ingestion of Reclaimed Water risk should consider unintended but foreseeable plau- sible inappropriate uses of the reclaimed water. Ingested volumes of tap water vary with gender, A key component of a human health risk assess- age, pregnancy status (Burmaster, 1998; Roseberry and ment is the estimation of an individual’s average daily Burmaster, 1992), ethnicity (Williams et al., 2001), dose (ADD) of a chemical. The ADD of a chemical climate, and likely other factors. Also, the concentra- in reclaimed water represents the sum of the ADDs tion of contaminants in reclaimed water, which affects BOX 6-4 Cross Connections Several cross connections between nonpotable reclaimed water and potable water lines have been reported in the United States and elsewhere (e.g., Australia). Some of the cross connections existed for 1 year or longer prior to detection. Only a few cross connections involving reclaimed water have resulted in reported illnesses, and fewer still have been medically documented. Most cross connections that occur are accidental, although some are intentional by homeowners or others. Some examples of cross connection incidents reported in the literature are provided below: • In 1979, several people reportedly became ill as a result of a cross connection between potable water lines and a subsurface irrigation system that supplied reclaimed water for irrigation at a campground. Based on a survey of 162 persons who camped at the site, at least 57 campers reported symptoms of diarrheal illness (Starko et al., 1986). • In 2004, a cross connection in a large residential development with a dual-distribution system reportedly affected approximately 82 house- holds (Sydney Water, 2004). The cross connection resulted from unauthorized plumbing work during construction of a house in the development. • A meter reader discovered a cross connection in 1996 when he noticed that a water meter at a private residence was registering backwards, which indicated that reclaimed water was flowing into the public potable water system (University of Florida TREEO Center, 2011). The reclaimed water service had recently been connected to an existing irrigation system at the residence. The irrigation system had previously been supplied with potable water and was still connected to the potable system. A backflow prevention device was not installed at the potable water service con- nection, and it was estimated that about 50,000 gallons of reclaimed water backflowed into the public potable water system. • Homeowners reported illnesses (diarrhea and digestion and intestinal problems) resulting from a cross connection that occurred in 2002 between a reclaimed water line supplying reclaimed water to a golf course and a potable water line supplying water to more than 200 households. Contractors failed to sever a potable water line that previously provided irrigation water, which created a cross connection between the potable line and the reclaimed water irrigation system. Pressures in the reclaimed and potable systems were comparable, and when a higher demand was created on the potable system, water from the nonpotable reclaimed system was siphoned into the potable system (Bloom, 2003). • A cross connection between reclaimed (nonpotable) and drinking water lines was discovered at a business park in 2007. It was determined that occupants in 17 businesses at the business park had been drinking and washing their hands with reclaimed water for 2 years. The cross con- nection was found after the water district increased the percentage of reclaimed water in the nonpotable water line from 20 percent (the remaining 80 percent being potable water) to 100 percent, and occupants complained that the water tasted bad and had a odor and a yellowish tint (Krueger, 2007). Detailed information on cross-connection control measures is available in manuals published by the American Water Works Association (AWWA, 2009) and the EPA (2003a). Regulations often address cross-connection control by specifying requirements that reduce the potential for cross connections (see Box 10-5). However, effective as such programs are, 100 percent compliance has not been achievable.

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111 UNDERSTANDING THE RISKS end-user exposure, will differ according to the source wastewater such as nonylphenol (Snyder et al., 2001a) water, the level of treatment (see Chapter 4), and the and perfluorinated organic compounds (Plumlee et al., extent of dilution with other water sources. If the water 2008) have been shown to bioconcentrate in animals as is treated to levels intended for nonpotable uses, but it the result of water exposures. The potential for bioac- is inadvertently ingested (e.g., after a cross connection cumulation of chemicals and pathogenic microbes can of the delivery pipes), the exposure might be much occur, as well as decay of chemicals or microbes during greater than the ingestion of water intended for potable product cultivation. With long-term use of reclaimed consumption, depending on the level of treatment of water on agricultural land, attention should be paid to the reclaimed water (see Box 6-4). In terms of potential accumulation in food crops of persistent substances health risks, ingestion of reclaimed water is of greater such as perfluorinated chemicals and metals from re- importance than other reclaimed water uses because peated application of reclaimed water containing these exposure and estimation of potential health risks are substances. Limited data have suggested that certain assessed on the basis of the consumption of drink- compounds potentially present in reclaimed water may ing water, which most governments (including EPA be detectable in irrigated food crops (Boxall et al., 2006; and countries such as Australia) assume to be 2 L/d Redshaw et al., 2008). Thus, more research is needed (NRMMC/EPHC/NHMRC, 2008). to assess the importance of these indirect pathways of Aside from the consumption of reclaimed water exposure. for drinking water, other sources of ingestion exposure of reclaimed water—primarily from incidental expo- Inhalation and Dermal Exposures sures—would be less. Although more data are needed to define the variability of such exposures, Tanaka et Household uses of water can result in inhalation al. (1998) provide useful benchmarks for reclaimed and dermal exposure to chemicals from showering (Xu water ingestion exposures (see Table 6-1). Indirect and Weisel, 2003) and by volatilization (for volatile exposure pathways through ingestion of contaminants substances) from other water uses in household appli- in reclaimed water could potentially occur when re- ances, such as clothes washers and dryers (Shepherd claimed water is used for food crop irrigation, for fish and Corsi, 1996). Experimental studies in humans and or shellfish growing areas, or in recreational impound- in vitro test systems using skin samples indicate that ments that are used for fishing. In these cases, exposure certain classes of chemicals can be absorbed into the may occur from the accumulation of chemicals within body following inhalation or dermal exposure to water the particular food. Some compounds that occur in following bathing or showering. Research has exam- ined dermal and inhalation exposures to neutral, low- molecular-weight compounds, such as water disinfec- tion byproducts present in conventional water systems, TABLE 6-1 Illustration of Differential Water Ingestion including trihalomethanes (e.g., chloroform, bromo- Rates from Different Reclamation Uses form, bromodichloromethane, dibromochlorometh- ane) and haloketones, (e.g., 1,1-dichloropropanone, Amount of Water Ingested 1,1,1-trichloropropanone) (Weisel and Wan-Kuen, Application Risk Group Exposure in a Single 1996; Baker et al., 2000; Xu and Weisel, 2005). Levels Purposes Receptor Frequency Exposure, mL of these chemicals are not known to be higher in re- Scenario I, Golfer Twice per week 1 claimed water than in conventional water systems (see golf course irrigation Appendix A). As reliance on membrane processes in Scenario II, crop Consumer Everyday 10 reclaimed water increases (see Chapter 4), there will irrigation be a need to assess the potential exposure to neutral, Scenario III, Swimmer 40 days per 100 recreational year—summer low-molecular-weight organic compounds that could impoundment season only be present, such as 1,4-dioxane and dichloromethane. Scenario IV, Groundwater Everyday 1000 groundwater Consumer Use of reclaimed water in ornamental fountains, recharge landscape irrigation, and ecological enhancement may SOURCE: Tanaka et al. (1998).

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112 WATER REUSE result in inadvertent exposure via aerosolization, der- and cost-benefit analysis. These standards are intended mal contact, or ingestion from hand-to-mouth activity. to protect against adverse health effects such as cancer, Although these have not been studied with respect to birth defects, and specific organ toxicity, that occur reclaimed waters, there have been outbreaks or expres- after prolonged exposures and are generally established sions of concern from many of these exposure pathways using various margins of safety or acceptable risk levels fed by other waters (Benkel et al., 2000; Fernandez to protect humans, including sensitive subpopulations Escartin et al., 2002), and therefore the potential for (e.g., children, immunocompromised persons). such effects cannot be neglected. In instances where there is adequate information Chemical and justification to assess exposure following dermal and/or inhalation exposure to a contaminant in re- Dose-response assessment and the subsequent claimed water, an average daily dose for dermal and estimation of health risk from exposure to chemicals inhalation exposures can be computed analogously to has traditionally been performed in two different ways: that for ingestions as shown in Appendix B. linear methods to address cancer effects and nonlinear (or threshold) methods to address noncancer health effects. These different approaches have been used Recreational Exposures historically because cancer and noncancer health effects The storage of reclaimed water in recreational were thought to have different modes of action. Cancer impoundments or the conveyance through rivers used was thought to result from chemically induced DNA for recreational purposes may result in exposure via all mutations. Because a single chemical-DNA interac- three routes: oral, dermal, and inhalation. Frequently, tion in a single cell can cause a mutation that leads to for swimming, it is assumed that ingestion of 10–100 cancer, it has generally been accepted that any dose of mL per incident occurs (Tanaka et al., 1998; Heerden et chemical that causes mutations may carry some finite al., 2005), although direct estimation of this ingestion risk. Thus, in the absence of additional data on the rate is not common (Schets et al., 2008). mode-of-action, cancer risk is typically estimated using a linear, nonthreshold dose-response method. In con- trast nonlinear, threshold dose-response methods are DOSE-RESPONSE ASSESSMENTS typically used to estimate the risk of noncancer effects Dose-response assessment is “the process of char- becausemultiple chemical reactions within multiple acterizing the relation between the dose of an agent cells have been thought to be involved. administered or received and the incidence of an Dose-response assessment for chemicals is a two- adverse health effect in the exposed populations and step process. The first step involves an assessment of all estimating the incidence of the effect as a function available data (e.g., in vitro testing, toxicology experi- of human exposure to the agent” (NRC, 1983). The ments using laboratory animals, human epidemiologi- a ssessment includes consideration of factors that cal studies) that document the relationship(s) between influence dose-response relationships such as age, chemical dose and health effect responses over a range illness, patterns of exposure, and other variables, and of reported doses. In the second step, the available ob- it can involve extrapolation of response data (e.g., served data are extrapolated to estimate the risk at low high-dose responses extrapolated to low-doses animal doses, where the dose begins to cause adverse effects in responses extrapolated to humans) (NRC, 1994a,b). humans (EPA, 2010c; WHO, 2009). Upon considering Dose-response relationships form the basis for the all available studies, the significant adverse biological risk assessments used for establishing drinking water effect that occurs at the lowest exposure level is iden- regulatory standards. To protect public health, drink- tified as the critical health effect for risk assessment ing water standards are established at levels lower than (Barnes and Dourson, 1988). If the critical health effect those associated with known adverse health effects is prevented, it is assumed that no other health effects following analysis of a chemical’s dose-response curve of concern will occur (EPA, 2010c).

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113 UNDERSTANDING THE RISKS For both carcinogens and noncarcinogens, it is chemicals that can cause tumors by inducing muta- common practice to also include uncertainty factors to tions within a cell as well as chemicals whose mode of account for the strength of the underlying data, inter- action is unknown, the dose response is assumed to be species variation, and intraspecies variation. The effect linear, and the potency is expressed in terms of a cancer of these factors may be several orders of magnitude in slope factor (CSF, expressed in units of cancer risk per the estimated effect/no-effect level. dose; see Box 6-6). Cancer risk then is assumed to be linearly proportional to the level of exposure to the chemical, with the CSF defining the gradient of the Noncarcinogens/Threshold Chemicals dose-response relationship as a straight line projecting Chemicals that cause toxicity through mechanisms from zero exposure–zero risk (Khan, 2010). other than cancer are often thought to induce adverse Tumors that arise through a nongenotoxic mecha- effects through a threshold mechanism. For these nism and exhibit a nonlinear dose-response are quanti- chemicals, it is generally thought that multiple cells fied using an RfD-like method. Ideally, the risk is eval- must be injured before an adverse effect is experienced uated on the basis of a dose-response relationship for a and that an injury must occur at a rate that exceeds precursor effect considering the mode of action leading the rate of repair. For chemicals that are thought to to the tumor (EPA, 2005a; Donohue and Miller, 2007). induce adverse effects through a threshold mechanism, In the absence of specific mechanistic information the general approach for assessing health risks is to relating to how chemical interaction at the target site establish a health-based guidance value using animal is responsible for a physiological outcome or pathologi- or human data. These health-based guidance values, cal event, nonthreshold and threshold approaches are known as reference dose (RfD), acceptable daily intake generally employed when analyzing dose-responses for (ADI), or tolerable daily intake (TDI), are generally carcinogens and noncarcinogens, respectively. defined as a daily oral exposure to the human popula- tion (including sensitive subgroups) that is likely to Microbiological be free of appreciable health risks over a lifetime (see Box 6-5 for the derivation of RfDs). For pharma- Microbiological dose-response models serve as a ceuticals, maximum recommended therapeutic doses link between the estimate of exposed dose (number of (MRTDs) are generally derived from doses employed organisms ingested) and the likelihood of becoming in human clinical trials, and are estimated upper dose infected or ill. Infectivity has been used as an end point limits beyond which a drug’s efficacy is not increased in drinking water disinfection because of the potential and/or undesirable adverse effects begin to outweigh for secondary transmission (Regli et al., 1991; Soller beneficial effects. For a number of drug categories (e.g., et al., 2003). some chemotherapeutics and immunosuppressants), a From deliberate human trials (“feeding studies”), clinical effective dose may be accompanied by substan- such as for cryptosporidium (Dupont et al. 1995), tial adverse effects (Matthews et al. 2004). Matthews rotavirus (Ward et al. 1986), and other organisms, et al. (2004) analyzed FDA’s MRTD database and mechanistically derived dose response relationships found that the overwhelming majority of drugs do not (exponential and beta-Poisson) have been developed demonstrate efficacy or adverse effects at a dose ap- (Haas, 1983). It has also been possible to use outbreak proximately 1/10 the MRTD. data to develop dose response information, as in the case of E. coli O157:H7 (Strachan et al., 2005); how- ever, this will likely only be possible with agents in Carcinogens/Nonthreshold Chemicals foodborne outbreaks where exposure concentration A dose-response assessment for a carcinogen data are available. comprises a weight-of-evidence evaluation relating to In some cases, dose-response relationships relying the potential of a chemical to cause cancer in humans, on animal data must be used. It has generally been considering the mode of action (EPA, 2005a). For found that the ingested dose in animals from a single

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114 WATER REUSE BOX 6-5 Derivation of Reference Doses RfDs, ADIs, and TDIs can be derived from no-observed-adverse-effect levels (NOAELs) or lowest-observed-adverse-effect levels (LOAELs) in animal or human studies, or from benchmark doses (BMDs) that are statistically estimated from animal or human studies. The overall process associated with derivation of an RfD, ADI, or TDI is illustrated in the figure below, and the detailed equation is (NOAELCritical Effect or LOAELCritical Effect or BMDCritical Effect ) RfD = UFH × UFA × UFS × UFL × UFD Where NOAEL = The highest exposure level at which there are no biologically significant increases in the frequency or severity of adverse effect between the exposed population and its appropriate control. Some effects may be produced at this level, but they are not considered adverse or precursors of adverse effects. LOAEL = The lowest exposure level at which there are biologically significant increases in frequency or severity of adverse effects between the exposed population and the appropriate control group. BMD = A dose that produces a predetermined change in response rate of an adverse effect (called the benchmark response) compared with background. UFH = A factor of 1, 3, or 10 used to account for variation in sensitivity among members of the human population (intraspecies variation). UFA = A factor of 1, 3, or 10 used to account for uncertainty when extrapolation from valid results of long-term studies on experimental animals to humans (interspecies variation). UFS = A factor of 1, 3, or 10 used to account for the uncertainty involved in extrapolating from less-than-chronic NOAELs to chronic NOAELs. UFL = A factor of 1, 3, or 10 used to account for the uncertainty involved in extrapolating from LOAELs to NOAELs. UFD = A factor of 1, 3, or 10 used to account for the uncertainty associated with extrapolation from the critical study data when data on some of the key toxic end points are lacking, making the database incomplete (Donohue and Miller, 2007). Both the NOAEL approach and BMD approach involve use of uncertainty factors (UFs), which account for differences in human responses to toxicity, uncertainties in the extrapolation of toxicity data between humans and animals (if animal data are used), as well as other uncertainties associated with data extrapolation. The underlying basis of calculating an RfD, ADI, or TDI is the dose-response assessment, where critical health effects are identified for each spe- cies evaluated across a range of doses. The critical effect should be observed at the lowest doses tested and demonstrate a dose-related response to exposure presents the same risk as ingesting the same either positive deviations (due to sensitization) or nega- dose in humans; thus, there is not a need for interspe- tive deviations (due to immune system inactivation) cies “correction.” This has been shown, for example, could occur. Dose response experiments using multiple for Legionella (Armstrong and Haas, 2007), E. coli dose protocols would be necessary to further inform O157:H7 (Haas et al., 2000), and Giardia (Rose et al., this assessment. 1991). Depending on the agent, effects from exposure to While the one-time exposure to a pathogen carries pathogens can produce a spectrum of illnesses, from the possible risk of an adverse effect, multiple exposures mild to severe, either with acute or chronic effects. For (e.g., exposures on successive days) may enhance the some agents, particularly in sensitive subpopulations, risk. Very little is known about the description of risk mortality can occur. To determine public health conse- from multiple exposures to the same agent. As a default, quences, it is necessary to integrate across the spectrum multiple exposures are modeled as independent events of effects. This can be done using disability adjusted life (Haas, 1996), although it is biologically plausible that years (DALYs) or quality adjusted life years (QALYs) (see Box 10-4).

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115 UNDERSTANDING THE RISKS support the conclusion that the effect is due to the chemical in question (Donohue and Miller, 2007; Faustman and Omenn, 2008). The RfD, ADI, TDI, or MRTD can then be used as the basis for deriving an acceptable level of chemical contaminant in reclaimed water, using the following equation: Rfd × Body Weight × RSC Acceptable Level in Reclaimed Water Noncarcinogen/Threshold Chemical = e Drinking Water Intake where drinking water intake is assumed to equal 2 L/d, and the relative source contribution (RSC) equals the portion of total exposure contributed by reclaimed water (default is 20 percent). Extrapolated Observed Response Uncertainty Factor (UF) BMDLx UF LOAEL BMDx UF NOAEL RfD Dose Example RfD derivation for noncarcinogens or chemicals with a threshold effect. This figure shows graphically how various dose-response data are converted to an RfD, considering confidence intervals and various uncertainty factors. SOURCE: Adapted from Donohue and Orme-Zavaleta (2003). RISK CHARACTERIZATION When estimates or measures of exposure and po- tency (i.e., dose-response relationships) exist, risk can Risk characterization is the last stage of the risk be formally characterized in terms of expected cases of assessment process in which information from the types of illness (with uncertainties) resulting under a preceding steps of the risk assessment (i.e., hazard given scenario. For example, for a nonthreshold chemi- identification, dose response assessment, and expo- cal or microbial agent that has a linear dose-response sure assessment) are integrated and synthesized into relationship, the characterized risk from a uniform ex- an overall conclusion about risk. “In essence, a risk posure is the simple product of the potency multiplied characterization conveys the risk assessor’s judgment by the dose. The process is illustrated in Chapter 7. as to the nature and existence of (or lack of ) human There are a variety of summary measures of risk that health or ecological risks” (EPA, 2000). Ideally, a risk can be used (e.g., RfD, ADI, TDI, risk quotient [RQ; characterization outlines key findings and identifies i.e., the level of exposure in reclaimed water divided major assumptions and uncertainties, with results that by the risk-based action level, such as the maximum are transparent, clear, consistent, and reasonable. contaminant level or MCL]).

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116 WATER REUSE BOX 6-6 Derivation of Cancer Slope Factors CSFs can be derived using a multistage model of cancer (available through EPA’s Benchmark Dose Modeling software), where the quantal relationship of tumors to dose is plotted. A point of departure, or dose that falls at the lower end of a range of observation for a tumor response, is estimated, and a straight line is plotted from the lower bound to zero. The below figure illustrates a linear cancer risk assessment (Donohue and Orme-Zavaleta, 2003). The CSF is the slope of the line (cancer response/dose) and is the tumorigenic potency of a chemical. The CSF can be used as the basis for deriving an acceptable level of chemical contaminant in reclaimed water, using the following equation: Acceptable Risk Level × Body Weight × CSF Acceptable Level in Reclaimed Water (µg/L) = Drinking Water Intake where the acceptable risk level generally equals 10–6, and drinking water intake is assumed to be 2 L/d. Confidence interval on dose High-dose tumor Incidence (observed) Response Linear extrapolation Low-dose tumor Incidence (observed) MoE ED10 LED10 Dose level found in ambient environment Dose Example cancer risk extrapolation, using the linear dose-response model. The CSF is the slope of the line (i.e., cancer response/dose) and represents the tumorigenic potency of a chemical. NOTES: MoE = margin of exposure; ED10 = effective dose at 10 percent response; LED10 = lower 95th confidence interval of ED10. SOURCE: Adapted from Donohue and Orme-Zavaleta (2003) Risk Characterization Given Lack of Data address only one chemical at a time, leaving a gap in our understanding of the potential adverse effects of For many chemicals, dose-response information chronic, low-level exposure to a complex mixture of is unavailable. Nonetheless, communities still need to chemicals. A mixture of chemicals may result in toxicity make decisions on water reuse projects in the absence that is additive (i.e., reflecting the sum of the toxicity of of such data. In this section, frameworks for providing all individual components), antagonistic (i.e., toxicity is information on risk in absence of dose-response data less than that of an individual component), potentiated are discussed. (i.e., toxicity is greater than that of an individual com- Numerous organic and inorganic chemicals have ponent), or synergistic (i.e., with toxicity that is greater been identified in reclaimed water and waters that than additive). Of particular concern are chemicals that receive wastewater effluent discharges, and only a lim- are mutagenic or carcinogenic and share similar modes ited number of these chemicals are actually regulated of action. As with other types of exposures, in the case in water supplies. Current regulatory testing protocols of reclaimed water, multiple chemicals may be present

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117 UNDERSTANDING THE RISKS at the same time for prolonged exposure periods, and cal in reclaimed water. This risk quotient is known as a Margin of Safety (MOS), with values >1 indicating they may have a synergistic relationship. Due to the absence of a federal risk assessment that the presence of a chemical in reclaimed water is paradigm for evaluating health risks from trace con- unlikely to pose a significant risk of adverse health taminants in reclaimed water, private associations as effects. This is exampled in Chapter 7 for 24 organic well as states (particularly California) have embarked contaminants in reclaimed water. upon their own programs to use existing screening Benchmarks for unregulated chemicals without paradigms to assess health risks of contaminants in complete epidemiological or toxicological datasets reclaimed water (e.g., Rodriquez et al., 2007a; Bruce or risk values were evaluated by Rodriquez et al. et al., 2010; Drewes et al., 2010; Snyder et al., 2010b; (2007a,b) and Snyder et al. (2010b) using class-based Bull et al., 2011). Techniques to conduct such water risk assessment approaches, including the Threshold quality evaluations and subsequently perform exposure of Toxicological Concern (TTC), FDA’s Threshold and risk assessments are summarized in Khan (2010). of Regulation (TOR; see Box 6-7), or EPA’s Toxicity Rodriquez et al. (2007a,b, 2008) and Snyder et al. Equivalency Factor (TEF) approach. Rodriquez et al. (2010b) used these screening health risk assessment (2007a,b) used the TTC approach for both unregulated approaches to evaluate potential health risks from noncarcinogens and carcinogens without available chemicals in reclaimed water in Australia and the toxicity information, while Snyder et al. (2010b) used United States, respectively. In both evaluations, po- T TC for noncarcinogens and nongenotoxic carcino- tential health impacts of chemical contaminants were gens. The Toxicity Equivalency Factor (TEF)/Toxicity evaluated using a combination of approaches based on Equivalents (TEQ) approach was used by Rodriquez extrapolating health risks using actual health effects et al. (2008) to assess potential health risks from dioxin data on a specific contaminant, as well as chemical and dioxin-like compounds in Australian reclaimed class-based evaluation approaches in the absence of water used to augment drinking water supplies, based contaminant-specific data. For regulated chemicals, EPA MCLs, Australian drinking water guidelines, or WHO drinking water guideline values were used as benchmark risk values (or risk based action levels, BOX 6-7 RBALs), from which risk quotients can be evaluated Threshold of Regulation (TOR) (see also example in Appendix A). RBALs for unregu- One class-based approach is the Threshold of Regulation, lated chemicals with existing risk values can be based which was developed as a method to evaluate the potential tox- upon EPA reference doses (RfDs), WHO acceptable icity of carcinogens extracted from food contact substances. daily intakes (ADIs), lowest therapeutic doses for The TOR is a concentration of chemicals unlikely to pose a pharmaceuticals, or EPA cancer slope factors (CSFs), significant risk of adverse health effects, including cancer risk among other risk values. If existing risk values have (10–6) over a lifetime (FDA, 1995; Rulis, 1987, 1989). The FDA not been derived, it is possible to derive risk values derived a threshold value of 0.5 ppb for carcinogens in the diet based on carcinogenic potencies of 500 substances from 3500 for noncarcinogens or carcinogens using human or experiments of Gold et al.’s (1984, 1986, 1987) Carcinogenic laboratory animal datasets on the chemical under con- Potency Database. The distribution of chronic dose rates that sideration using methods described in Boxes 6-5 and would induce tumors in 50 percent of test animals (TD50s) 6-6. The selection of one risk value over another (e.g., was plotted. This distribution was extrapolated to a Virtually RfD vs. ADI) or selection of a specific epidemiological Safe Dose (10–6 lifetime risk of cancer) in humans and is or toxicological dataset used to derive a RBAL gener- equal to 0.5 μg chemicals/kg of food, or 1.5 μg/person/day (based on 3 kg food/drink consumed/day). This value can be ally should be based upon the critical health effect(s) extrapolated to a concentration in water intended for inges- identified for the specific chemical in the most sensitive tion, as follows: species. Potential health risks from the presence of a chemi- TOR: 0.5 μg/kg food/day x (3 kg food/day) cal in reclaimed water can be assessed by dividing a / (2 L water/day) = 0.75 μg/L. chemical’s RBAL by the concentration of that chemi-

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118 WATER REUSE on TEFs developed by the WHO. (For details on cal- BOX 6-8 culation of TEQs, see EPA, 2010c.) Thresholds of Toxicological Concern Although newer than traditional risk assessments, (TTCs) which are based upon chemical-specific data, these class-based values are widely used by regulatory author- For carcinogens, distributions of chronic dose rates from ities to assess health risks in the absence of complete lifetime animal cancer studies were statistically evaluated for substance-specific health effects datasets. The TTC more than 700 carcinogens to identify an extrapolated thresh- approach is used by the World Health Organization’s old value in humans unlikely to result in a significant risk of developing cancer over a lifetime of exposure (Cheeseman et Joint Expert Commission on Food Additives ( JECFA) al., 1999; Kroes et al., 2004; Barlow, 2005). This threshold to assess health risks from food additives present at value is equal to 1.5 μg/person/day. For noncarcinogens, low levels in the diet, and the U.S. Food and Drug analyses have been performed to identify human exposure Administration (FDA) uses the TOR approach when thresholds for chemicals falling into certain chemical classes. assessing health risk from indirect food additives (such One of the best known TTC evaluations is Munro et al. (1996)’s as chemicals in food contact articles; Box 6-7). evaluation of 613 organic chemicals that had been tested in noncancer oral toxicity studies in rodents and rabbits, where The TTC approach has evolved over the past 20 chemicals are grouped into three general toxicity classes years, starting from the FDA’s TOR concept (Rulis, based on the Cramer classification scheme (Cramer et al., 1987, 1989) and more recently developing into a tiered 1978): appear, where different threshold doses are established based on chemical structure and class (Munro, 1990; • Class I—Simple chemicals, efficient metabolism, low Munro et al., 1996; Kroes et al., 2004). The TTC oral toxicity • Class II—May contain reactive functional groups, approach is based on the existence of a threshold for slightly more toxic than Class I a toxic effect (e.g., cancer or a systemic toxicity end- • Class III—Substances that have structural features point such a liver toxicity), which is usually identified that permit no strong initial presumption of safety or may even through animal experiments. TTC values are statisti- suggest significant toxicity cally derived by analyzing toxicity data for hundreds of different chemicals, where doses in animal studies Human exposure thresholds (TTCs) of 1800, 540, and 90 μg/ person/day (30, 9, and 1.5 μg/kg body weight/day, respec- are extrapolated to doses that are unlikely to cause tively) were proposed for class I, II, and III chemicals using adverse health effects in humans. TTC values have the 5th percentile of the lowest No Observed Effect Level for been derived for carcinogens and noncarcinogens (see each group of chemicals, a human body weight of 60 kg, and a Box 6-8). safety/uncertainty factor of 100 (Munro et al., 1996). Using the Despite the utility of TTC, there are multiple above TTC human exposure thresholds, an acceptable level of classes of chemicals that cannot be screened using the each chemical in reclaimed water can be derived as follows: T TC approach, such as heavy metals, dioxins, endo- Acceptable Level In Reclaimed Water (μg/L) crine active chemicals, allergens, and high potency [X] μg/person/day x RSC carcinogens, which instead must be evaluated using = 2 L/person different risk assessment approaches (Kroes et al., 2004, Barlow, 2005, SCCP, 2008). Reasons for this Where X = 1800 μg/day for class I compounds, 540 μg/day for are primarily public health protective and include the class II compounds, and 90 μg/day for class III compounds; following factors: Relative Source Contribution (RSC) = 0.2 (assumed default), and drinking water intake = 2 L/day. Therefore, the TTC ap- proach assigns acceptable levels for these three classes of • Heavy metals and dioxins may bioaccumulate, chemicals in reclaimed water as follows:180 μg/L for Class I and safety factors used in derivation of TTC values compounds, 54 μg/L for Class II compounds, and 9 μg/L for may not be large enough to account for differences in Class III compounds. elimination of such chemicals in the human body com- pared to laboratory animals. In addition, the original databases used to develop TTC threshold values may not have included structurally similar chemicals.

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119 UNDERSTANDING THE RISKS • Endocrine active chemicals have limited datas- water were analyzed for chemical contaminants and ets relating at lower doses. compared against water samples from control wells • Allergens don’t always display a clear threshold, containing little or no reclaimed water. Health risks and may elicit adverse effects even at extremely low from contaminants of potential health concern were doses. estimated, and the datasets were compared. For both • High potency carcinogens, such as aflatoxin- types of groundwater samples, hazard indices were like, N-nitroso and azoxy compounds, are toxic even calculated representing the sum of potential noncan- at low levels. cer effects from exposure to the identified chemicals; cancer risks were assessed by estimating lifetime cancer The TTC approach is meant solely as a method risks associated with drinking water exposure to the to derive relatively rapid conservative estimation of chemicals present in wells. For both projects, hazard risk for compounds without detailed risk assessment indexes in the reclaimed and control water samples or with limited datasets. The screening approach was were below the threshold for potential health effects (i.e., <1). In the Chino Basin Groundwater Recharge not intended for detailed regulatory decision making. This tool also provides a means to prioritize attention Project, noncancer and cancer risks were judged to be to chemicals where complete toxicological relevance equivalent among the reclaimed water wells compared data are absent. The screening value also provides a with the control wells. In the Montebello Forebay means for analytical chemists to target meaningful Groundwater Recharge Project, noncancer and cancer method reporting limits based on health, rather than risks were equivalent among the reclaimed water wells simply relying on absolute maximum instrumental and compared to control wells, with the exception of risks method sensitivity. associated with arsenic. An analysis by the authors indi- cates that arsenic concentrations in water do not appear to be influenced by reclaimed water content, but rather Results of Screening-Level Analyses are caused by naturally occurring arsenic. Rodriquez et al. (2007b) evaluated a total of 134 chemicals, including volatile organic compounds, dis- CONSIDERATION OF UNCERTAINTY infection byproduct, metals, pesticides, hormones and pharmaceuticals, in water that had undergone advanced Many elements going into a risk characterization treatment (microfiltration or ultrafiltration followed contain elements of uncertainty and/or variability. by reverse osmosis) at the Australian Kwinana Water These terms are defined as (NRC, 2009b): Reclamation Plant (KWRP). Calculated risk quotients Uncertainty: Lack or incompleteness of informa- (RQ) were 10 to 100,000 times below 1 for all volatile organic compounds and all pharmaceuticals except tion. Quantitative uncertainty analysis attempts to cyclophosphamide (RQ=0.5). Risk quotients <1 indi- analyze and describe the degree to which a calculated cate that there is unlikely to be a significant health risk value may differ from the true value; it sometimes uses associated with exposure to a specific chemical. RQs probability distributions. Uncertainty depends on the for all metals were also <1. Rodriquez et al. (2007a) quality, quantity, and relevance of data and on the reli- concluded that there were no increased health risks ability and relevance of models and assumptions. Variability: Variability refers to true differences in from the KWRP reclaimed water destined for indirect potable reuse as evidenced by levels of contaminants attributes due to heterogeneity or diversity. Variability being well below benchmark values. is usually not reducible by further measurement or Soller and Nellor (2011a,b) performed quantita- study, although it can be better characterized. tive relative risk assessments of two different water reuse projects in Southern California: the Montebello The inputs to a risk characterization may have a Forebay and Chino Basin Groundwater Recharge number of sources of uncertainty and variability, and projects. In each project, water samples from wells that therefore, the final risk characterization has inevitable contained “relatively high proportions” of reclaimed

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120 WATER REUSE uncertainty as well. Some of these sources of uncer- characterization is to provide information in a form tainty and variability are useful to these groups), uncertainty can be captured and described in different ways (Patè-Cornell, 1996). • uncertainty from the use of animal species to The use of uncertainty or “safety” factors is perhaps derive effects data, the simplest. The quantitative factors contributing to • uncertainty from effects data based on single uncertainty (footnoted in the list above) can be charac- contaminants rather than mixtures, terized by probability distributions, and a Monte Carlo • variability in occurrence of contaminants and analysis can be performed to present the character- performance of treatment processes,3 ized risk as a probability distribution (Burmaster and • variability in response of populations Anderson, 1994). As the most intensive alternative, a (susceptibility),3 second-order or two-dimensional Monte Carlo analy- • variability in exposure to water with contami- sis (Burmaster and Wilson, 1996) may be performed nants (e.g., ingestion rates, inhalation rates),3 and in which elements of uncertainty (that could be reduc- • uncertainty in models (e.g., contaminant trans- ible if more information were obtained) are separated port; dose-response). from elements of variability (reflecting the intrinsic heterogeneity of the scenario). For the sake of con- Given the variability and uncertainty in the inputs to ciseness, the details of these various methods (beyond a risk characterization that may arise in both exposure the use of uncertainty factors) are not detailed in this assessment and dose-response assessment, any final report. However, formal uncertainty analysis can often characterization can never be known with absolute pre- be useful to decision makers (Finkel, 1990). Although cision and certainty. Therefore, the uncertainty in the safety factors and simple Monte Carlo analyses have risk assessment should be characterized. In speaking been performed in the context of reuse, the commit- about the level of analysis with which these facets are tee is not aware of use of the second-order methods in considered, NRC (2009b) makes the following state- this context. ment in the context of EPA decision making: An uncertainty analysis can also be used to assess the risks involved in excursions from usual process per- The characterization of uncertainty and variability in formance, accidents, or failure of one or more processes. a risk assessment should be planned and managed and Essentially the likelihood of such deviations and the matched to the needs of the stakeholders involved in risk-informed decisions. In evaluating the tradeoff impact on removal of contaminants are combined to between the higher level of effort needed to conduct assess the impact on overall risk on a per day (or per a more sophisticated analysis and the need to make year) basis. However, to perform such analyses, more timely decisions, EPA should take into account both data are needed on the process variability (including in the level of technical sophistication needed to identify the distribution system) and the risk of failure under the optimal course of action and the negative impacts long-term operations. The risk of a cross connection in that will result if the optimal course of action is in- correctly identified. If a relatively simple analysis of distribution systems (Box 6-4) is a special type of such uncertainty (for example, a non-probabilistic assess- risk that also should be considered, although a strong ment of bounds) is sufficient to identify one course quantitative database to estimate the frequency and of action as clearly better than all the others, there is impact of such occurrences is lacking. no need for further elucidation. In contrast, when the best choice is not so clear and the consequences of a wrong choice would be serious, EPA can proceed in an CONCLUSIONS AND RECOMMENDATIONS iterative manner, making the analysis more and more sophisticated until the optimal choice is sufficiently Health risks remain difficult to fully characterize clear. (NRC, 2009b) and quantify through epidemiological or toxicologi- cal studies, but well-established principles and pro- Depending on the preferences of the decision mak- cesses exist for estimating the risks of various water ers and stakeholders (recalling that the objective of risk reuse applications. Absolute safety is a laudable goal of society; however, in the evaluation of safety, some 3 Q uantitative factors contributing to uncertainty.

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121 UNDERSTANDING THE RISKS degree of risk must be considered acceptable (NAS, understanding of the typical performance of different 1975; NRC, 1977). To evaluate these risks, the prin- treatment processes exists, an improved understanding ciples of hazard identification, exposure assessment, of the duration and extent of any variations in perfor- dose-response assessment, and risk characterization can mance at removing contaminants is needed. When assessing risks associated with reclaimed be used. Although risk assessment will be an important water, the potential for unintended or inappropriate input in decision making, it forms only one of several uses should be assessed and mitigated. If the risk such inputs, and risk management decisions incorpo- rate a variety of other factors, such as cost; equitabil- is then deemed unacceptable, some combination of ity; social, legal, and regulatory factors; and qualitative more stringent treatment barriers or more stringent public preferences. controls against inappropriate uses would be necessary The occurrence of a contaminant at a detectable if the project is to proceed. Inadvertent cross connec- level does not necessarily pose a significant risk. tion of potable and nonpotable water lines represents Instead, only by using dose-response assessments (the one type of unintended outcome that poses significant second step of risk assessment), can a determination be human health risks from exposure to pathogens. To made of the significance of a detectable and quantifiable significantly reduce the risks associated with cross concentration. connections, particularly from exposure to pathogens, Risk assessment screening methods enable esti- nonpotable reclaimed water distributed to communi- mates of potential human health effects for circum- ties via dual distributions systems should be disinfected stances where dose-response data are lacking. Ap- to reduce microbial pathogens to low or undetectable proaches such as the threshold of toxicological concern levels. Enhanced surveillance during installation of and the toxicity equivalency factor may useful in this reclaimed water pipelines may be necessary for non- regard, although additional research in such approxi- potable reuse projects that distribute reclaimed water mate methods and assessment of their performance is that has not received a high degree of treatment and needed. disinfection. A better understanding and a database of the Guidance and user-friendly risk assessment tools performance of treatment processes and distribution would improve the understanding and application of systems are needed to quantify the uncertainty in risk these risk assessment methods. Although risk assess- assessments of potable and nonpotable water reuse ment is a useful tool to help prioritize efforts to protect projects. Failures in reliability of a water reuse treat- public health in the face of uncertainty, conducting a ment and distribution system may cause a short-term chemical or microbial risk assessment is complex and risk to those exposed, particularly for acute contami- resource-intensive. As the extent of water reclamation nants where a single exposure is needed to produce an increases around the United States, it may be desirable effect. Although there are many sources of uncertainty and appropriate for regulatory authorities (e.g., state, and variability, by using well-understood methods in federal) to prepare guidance or reference materials to risk assessment, the impact of such sources of variability facilitate understanding of these methods for water and uncertainty on estimated human health risk can reuse applications and to develop user-friendly tools be determined. To assess the overall risks of a system, for the use of more advanced assessment methods the performance (variability and uncertainty) of each that can be used by a greater number of utilities and of the steps needs to be understood. Although a good stakeholders.

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