3
Public Health Risk from Distribution System Contamination

One of the most challenging facets of reducing the risk of contaminated distribution systems is being able to quantify the existing risk. This is made complicated not only by the plethora of factors that can constitute public health risks, including a diversity of microbial pathogens and chemical compounds, but also by the varying response that a given individual will have when exposed to those factors. This chapter describes three primary mechanisms used to assess the acute public health risk of distribution system contamination, the limitations of these methods, and what conclusions can be derived from currently available data.

INTRODUCTION TO RISK

The process of risk assessment involves determining the likelihood and severity of different adverse impacts given exposure of a population to a hazard. Risk analysis includes the process of risk assessment, as well as risk management activities to decide what an acceptable risk level is and to take actions to reduce risk (NRC, 1983). Risk assessment requires the activities of hazard identification, exposure assessment, and dose-response (or exposure-response) assessment. Hazard identification is the determination of what adverse agents might be present and what adverse impacts they might cause. Exposure assessment is the quantitative determination of the levels of contaminants (in the case of environmental exposures) individuals may consume/inhale/contact over a specific time period. Dose-response assessment is the quantitative determination of the likelihood of an individual having a particular adverse effect from a given exposure. Alternatively, this can be viewed as the proportion of persons in a population who are expected to have the adverse effect were they to have the particular exposure.

Various federal agencies, including the U. S. Environmental Protection Agency (EPA), have developed specific guidelines and procedures for performing risk assessment, particularly for carcinogens and for substances that result in non-carcinogenic toxic effects. In the case of infectious agents (which are frequently the concern in drinking water), methodologies are at a developmental stage.

One of the goals of performing risk assessment within a regulatory framework is to develop regulatory guidance or standards (or decide not to undertake such action) based on the results. This process, which is part of risk analysis,



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Drinking Water Distribution Systems: Assessing and Reducing Risks 3 Public Health Risk from Distribution System Contamination One of the most challenging facets of reducing the risk of contaminated distribution systems is being able to quantify the existing risk. This is made complicated not only by the plethora of factors that can constitute public health risks, including a diversity of microbial pathogens and chemical compounds, but also by the varying response that a given individual will have when exposed to those factors. This chapter describes three primary mechanisms used to assess the acute public health risk of distribution system contamination, the limitations of these methods, and what conclusions can be derived from currently available data. INTRODUCTION TO RISK The process of risk assessment involves determining the likelihood and severity of different adverse impacts given exposure of a population to a hazard. Risk analysis includes the process of risk assessment, as well as risk management activities to decide what an acceptable risk level is and to take actions to reduce risk (NRC, 1983). Risk assessment requires the activities of hazard identification, exposure assessment, and dose-response (or exposure-response) assessment. Hazard identification is the determination of what adverse agents might be present and what adverse impacts they might cause. Exposure assessment is the quantitative determination of the levels of contaminants (in the case of environmental exposures) individuals may consume/inhale/contact over a specific time period. Dose-response assessment is the quantitative determination of the likelihood of an individual having a particular adverse effect from a given exposure. Alternatively, this can be viewed as the proportion of persons in a population who are expected to have the adverse effect were they to have the particular exposure. Various federal agencies, including the U. S. Environmental Protection Agency (EPA), have developed specific guidelines and procedures for performing risk assessment, particularly for carcinogens and for substances that result in non-carcinogenic toxic effects. In the case of infectious agents (which are frequently the concern in drinking water), methodologies are at a developmental stage. One of the goals of performing risk assessment within a regulatory framework is to develop regulatory guidance or standards (or decide not to undertake such action) based on the results. This process, which is part of risk analysis,

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Drinking Water Distribution Systems: Assessing and Reducing Risks requires additional considerations such as cost and equity. Under the Safe Drinking Water Act, EPA is required to set a maximum contaminant level goal (MCLG) for certain contaminants that is absolutely protective against all adverse health effects, given available risk assessment information. For most contaminants with MCLGs, a regulatory level is then established—a maximum contaminant level (MCL)—or a treatment technique is required, both of which incorporate considerations of feasibility (see Box 3-1). In determining a regulatory level such as an MCL, implicitly or explicitly the acceptable residual risk (after the implementation of any interventions) must be decided upon. The empirical evidence is that, for human carcinogens, EPA has regarded a window of residual lifetime risk of 1/1,000,000 to 1/10,000 to be acceptable (see Box 3-2 for an explanation of the origins of this value and its extension to infectious agents). In other words, a residual risk resulting in no more than 1 extra cancer in the lifetime of a population of 10,000 to 1,000,000 persons is regarded as being acceptable. Risks from Drinking Water Drinking water can serve as a transmission vehicle for a variety of hazardous agents: enteric microbial pathogens from human or animal fecal contamination (e.g., noroviruses, E. coli O157:H7, Cryptosporidium), aquatic microorganisms that can cause harmful infections in humans (e.g., nontuberculous mycobacteria, Legionella), toxins from aquatic microorganisms (such as cyanobacteria), and several classes of chemical contaminants (organic chemicals such as benzene, polychlorinated biphenyls, and various pesticides; inorganic chemicals such as arsenic and nitrates; metals such as lead and copper; disinfection byproducts or DBPs such as trihalomethanes; and radioactive compounds). Contaminants in drinking water can produce adverse effects in humans due to multiple routes of exposure. In addition to risk from ingestion, exposure can also occur from inhalation and dermal routes. For example, inhalation of droplets containing respiratory pathogens (such as Legionella or Mycobacterium) can result in illness. It is known that DBPs present in drinking water may volatilize resulting in inhalation risk, and these compounds (and likely other organics) may also be transported through the skin (after bathing or showering) into the bloodstream (Jo et al., 1990). Reaction of disinfectants in potable water with other materials in the household may also result in indoor air exposure of contaminants; for example Shepard et al. (1996) reported on release of volatile organics in indoor washing machines. Thus, multiple routes of exposure need to be considered when assessing the risk presented by contaminated distribution systems. It should be noted, however, that the report will not consider such indirect routes of exposure as (1) the loss of pressure and subsequent inadequate fire protection, (2) loss of water for hospitals and dialysis centers, and (3) leaks in household plumbing that lead to toxic mold growth.

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Drinking Water Distribution Systems: Assessing and Reducing Risks BOX 3-1 U.S. Code, Title 42(6A)(XIIB)§300g-1 (Safe Drinking Water Act as Amended) Maximum contaminant level goals. Each maximum contaminant level goal established under this subsection shall be set at the level at which no known or anticipated adverse effects on the health of persons occur and which allows an adequate margin of safety. Maximum contaminant levels. Except as provided in paragraphs (5) and (6)1, each national primary drinking water regulation for a contaminant for which a maximum contaminant level goal is established under this subsection shall specify a maximum contaminant level for such contaminant which is as close to the maximum contaminant level goal as is feasible. It has been recognized for some years that consumers face risk from multiple hazards, and that action to reduce the risk from one hazard may increase the risk from other hazards given the same exposure. There are prominent examples of this phenomenon in the drinking water arena that have greatly complicated efforts to reduce overall risk from distribution systems. Havelaar et al. (2000) assessed the relative changes in risk from switching to ozone treatment of drinking water in the Netherlands. In this case, there was a projected reduction in risk from waterborne infectious disease (such as Cryptosporidium) while there was a projected increase in risk from DBP formation (the primary one examined was bromate). To compare the net change in overall risk, it is necessary to place the multiple risks (with their different endpoints in terms of disease severity) on the same scale. Havelaar et al. (2000) did this comparison using the methodology of disability adjusted life years (DALY’s). In this approach, the severity of an adverse health effect is quantitatively weighted by an index (disability weight) reflecting the proportional degradation in health (a weight of 0 is reflective of absence of an effect, while a weight of 1 is reflected in total impairment); the integral of the years of diminished functioning multiplied by the disability weight is summed with the reduction in lifespan due to premature mortality to get the aggregate impact to a population. In principle, using such an approach one can optimize for the overall net reduction in risk, considering competing hazards. It is noted that the DALY framework has not been adopted for U.S. regulatory practice and remains controversial for a number of technical and policy reasons (including age equity) (Anand and Hanson, 1997). When risk is assessed for chemical or microbial exposure, it should be considered that not all segments of the population are at the same degree of risk. This may be due to differences in exposure in terms of either consumption (Gerba et al., 1996) or in concentrations (due to heterogeneity in the environ- 1 Paragraph (5) allows departure upwards from setting the MCL as close to the MCLG as feasible if doing so would result in an increase in risk from other contaminants, or would interfere with the performance of processes used to address other contaminants. Paragraph (6) allows departure upward from the “as close as feasible” criterion in certain circumstances if the benefits would not justify the cost of compliance at that standard.

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Drinking Water Distribution Systems: Assessing and Reducing Risks BOX 3-2 Origin of the 1/10,000 Acceptable Risk Level for Carcinogens and Infectious Agents EPA has been at the forefront of the issue of acceptable risk in virtually all of its programmatic areas, primarily as the result of court challenges to its regulations. In response to the 1987 Section 112 Clean Air Act decision (Natural Resources Defense Council vs. U.S. Environmental Protection Agency 824 F. 2nd 1146 [1987]), EPA decided it would base its regulatory decisions on quantitative risk assessments using the general policy that a lifetime added cancer risk for the most exposed person of 1 in 10,000 (1 × 10-4) might constitute acceptable risk and that the margin of safety required by statute and reinforced by the court should reduce the risk for the greatest number of persons to an added lifetime risk of no more than 1 in 1 million (1 × 10-6). However, EPA (along with the courts) has not viewed “safe” as the equivalent of risk-free and has determined that standards should protect against significant public health risks (EPA 49 Fed. Reg. 8386 [1984]; Rodricks et al. 1987; Industrial Union Department, AFL-CIO v. American Petroleum Institute et al. 448 U.S. 607 [1980]). EPA has repeatedly rejected the opinion that it can establish a universal (i.e., brightline) acceptable risk that should never be exceeded under any circumstances, and they maintain that guidance provided under one statute might have little relevance to others because of differing program goals. In practical terms, EPA almost never regulates at a theoretical risk below 1 × 10-6 (de minimis) and almost always regulates at a theoretical risk below 1 × 10-4 (de manifestis)” (NRC, 2004). Policy with respect to acceptable levels of risk from exposure to infectious agents is less well developed than for chemical carcinogens. However, in framing the Surface Water Treatment Rule (Federal Register, June 29, 1989, page 27486), the rule for reduction of risk from Giardia and viruses was set to achieve a residual estimated risk of infection below 1/10,000 per year. This number derived from the then average waterborne illness rate associated with reported waterborne outbreaks (Regli et al., 1991). However it is now recognized that the waterborne illness rate is substantially greater than this value—due to underreporting of outbreaks, as well as to substantial endemic illness. The use of infection rather than illness as an endpoint was intended to compensate for secondary cases and also for presumed heightened infectivity amongst sensitive subpopulations. The use by EPA of an acceptable risk window for microorganisms in the 10-6 to 10-4 range as one factor in setting standards continues. As recently as the promulgation of the Long Term 2 Enhanced Surface Water Treatment Rule (Federal Register, January 5, 2006), EPA has stated: “EPA and Advisory Committee deliberations focused on mean source water Cryptosporidium concentrations in the range of 0.01–0.1 oocysts/L as threshold levels for requiring additional treatment…these levels are estimated to result in an annual infection risk in the range of 1.7x10-4 − 6 x 10-3 … for a treatment plant achieving 3-log Cryptosporidium removal (the treatment efficiency estimated for conventional plants under existing regulations).” ment, e.g., in the distributed water), or to intrinsic differences in susceptibility (Balbus et al., 2000). Unfortunately, our ability to assess quantitative differences in intrinsic susceptibility remains poor, and therefore protection of susceptible subpopulations often relies upon the imposition of safety factors. Methods for Characterizing Human Health Risk Characterization of human health risks may be performed using an epide-

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Drinking Water Distribution Systems: Assessing and Reducing Risks miological approach or using a risk assessment approach. These methods are complementary and have different strengths and limitations, and each has been used for assessment of drinking water risks in various applications. Epidemiological approaches study the relationship between exposures and disease in actual populations and are descriptive, correlational, or analytic. In the descriptive study, population surveys or systematic disease surveillance (monitoring) describe disease patterns by various factors such as age, seasonality, and geographic location. Correlational (also called “ecologic”) studies collect population level data on disease rates and exposures and look for correlations. Analytical studies (whether experimental or observational) are those in which individual-level data is collected and the investigator tests a formal hypothesis about the association between exposure and disease. Risk assessment methods, on the other hand, follow the hazard identification, dose-response assessment, exposure assessment, and risk characterization paradigm noted above. Frequently, but not always, the dose-response assessment is based upon extrapolation from results of trials in animals (although results from human exposure may be used where available—for example, in human feeding trials of infectious agents or from studies in populations exposed in occupational or other settings to particular agents of concern). Epidemiological studies have the advantage of involving human populations, often experiencing the exposure of interest and representing a range of variability in susceptibility and behavior. However to detect a small increase in risk from the baseline, epidemiological studies require very large sample sizes, and thus considerable expense and effort. Epidemiological studies cannot provide direct information on the potential for risk reduction from a proposed change in treatment practice that has not yet been implemented since by definition there is not yet human exposure to conditions expected from the proposed change. However, epidemiological studies can be designed to measure the direct impact of a treatment intervention after it has been implemented. This is very powerful tool and it has provided the evidence base that changes in water treatment have had a positive impact on community health. For example, the recent meta-analysis by Fewtrell and Colford (2004) demonstrates the body of evidence linking improvements in community and household water quality to health. Risk assessment approaches have the advantage of being flexible in their application to potential (but not yet experienced) situations. A risk assessment can be performed even when the projected risk from a particular exposure or change of exposure is very small. They have the disadvantage of requiring extensive measurement or modeling to ascertain exposure, and also of the need for dose-response studies. Often these dose-response studies are in animals or at higher doses, thereby requiring extrapolation with respect to dose (via a formal mathematical dose-response curve) and/or between species. Generally, whether animal or human data are used to establish the dose-response relationship, the range in variability in susceptibility is small (compared to a full human popula-

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Drinking Water Distribution Systems: Assessing and Reducing Risks tion) and therefore some margin of safety may need to be explicitly used to account for more susceptible subpopulations. This chapter discusses what is known about the human health risks that derive from contamination of the distribution system, relying on three primary approaches: risk assessment methods that utilize data on pathogen occurrence measurements, outbreak surveillance data, and epidemiology studies. A special section is devoted to Legionella, for which all three types of activities have occurred, leading to greater understanding of the risks inherent from growth of this organism in distribution systems. Because the impetus for this study was revision of the Total Coliform Rule, the report focuses primarily on acute risks from microbial contamination of the distribution system. However, there are short-and long-term risks from chemicals that merit mention (particularly DBPs—lead and copper were outside the scope of the study). DBP concentrations in the distribution system can vary significantly depending on water residence time, the types of disinfectants used, and biological and chemical reactions, among many other factors (see Chapter 6). The concentrations of trihalomethanes in finished water tend to increase with increasing water age, while certain haloacetic acids tend to decrease in concentration over time (see Chapter 6; Arbuckle et al., 2002). A number of epidemiologic studies have examined the health significance of DBP exposure and have reported significantly increased risks of bladder, rectal, and/or colon cancers in some populations (King et al., 1996; Koivusalo et al., 1997; Doyle et al., 1997; Cantor et al., 1998; Yang et al., 1998; King et al., 2000) as well as adverse reproductive outcomes (Waller et al., 1998; Dodds et al., 1999; Klotz and Pyrch, 1999; King et al., 2000). However, determining and classifying DBP exposure in these studies has been extremely challenging and has made it difficult to interpret the findings of these studies (Arbuckle et al., 2002, Weinberg et al., 2006). Furthermore, the contribution of distribution systems to the reported risk, as opposed to drinking water treatment or other processes, has not been elucidated. Because epidemiological studies of DBP exposure have been extensively reviewed by others (Boorman et al., 1999; Nieuwenhuijsen et al., 2000; Graves et al., 2001), they are not reviewed here. EVIDENCE FROM PATHOGEN OCCURRENCE MEASUREMENTS The risk assessment approach relies on being able to measure or predict (e.g., by modeling) the concentration of an etiologic agent in the water supply. Certain microbial pathogens are indicative of distribution system contamination stemming from both internal and external sources. These include bacteria known to form biofilms—a physiological state in which organisms attach to and grow on a surface (Characklis and Marshall, 1990)—and bacteria that indicate an external contamination event such as intrusion. In distribution systems, the interior pipe walls, storage tanks, sediments, and other surfaces in contact with finished water are colonized by bacteria, which can survive, grow, and detach depending on local conditions. Other types of bacteria (such as coliforms) as

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Drinking Water Distribution Systems: Assessing and Reducing Risks well as enteric viruses and protozoa (Quignon et al., 1997; Piriou et al., 2000) are also found in biofilms. However, their presence can also be attributable to an external contamination event or break through of the treatment barrier. The microbiology of distribution systems can be influenced by a variety of factors (e.g., poor quality source water, inadequate treatment, unsanitary activity, backflow). Given this report’s assumption of adequate treatment, a discussion of all source water microbes and those that would be eliminated during treatment is not warranted. Furthermore, virtually any microorganism in close enough proximity to a vulnerable part of the distribution system (e.g., a cross connection, main break, or leak) could enter during an external contamination event. Control of these events—see Chapters 4 and 5—is important for reducing the risks of not only microbial pathogens but also chemicals that might enter distribution system. Because the complexity of microbes from such diverse sources is beyond the scope of this report, the following section focuses on those organisms most likely to indicate either internal or external contamination of the distribution system. The Microbiology of Bulk Water The microbiology of distribution systems essentially consists of two different environments—microorganisms in the bulk water column and those in biofilms attached to the surfaces of pipes, sediments, and other materials. Microorganisms in the bulk water column originate from either the source water, from bacterial growth within the treatment process (e.g., within the treatment filters), from biofilms within the distribution system, or from recontamination of the water from cross connections, intrusion, pipe breaks, or other external sources. Heterotrophic Bacteria Heterotrophic bacteria (a broad classification that takes into account all bacteria that utilize organic carbon) are commonly found in the bulk water of distribution systems because they readily form biofilms in such systems. They are measured by using heterotrophic plate counts (HPC). Heterotrophs have traditionally been divided into two primary groups based on their cell wall characteristics—Gram-negative and Gram-positive. The presence of a disinfectant residual in drinking water has a tremendous selective effect, particularly on Gram-negative bacteria, which are relatively sensitive to inactivation by disinfectants. Identification of bacteria using fatty acid analysis (Norton and LeChevallier, 2000) showed that chlorination resulted in a rapid shift from predominately Gram-negative bacteria (97 percent) in the raw water to mostly Gram-positive organisms (98 percent) in the chlorinated water (see Table 3-1). Bacteria in the raw water were diverse, with Acinetobacter

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Drinking Water Distribution Systems: Assessing and Reducing Risks TABLE 3-1 Bacterial Populations Isolated from the Water Column During Treatment Bacterial Identification Percentage of Population in Raw Water Percentage of Population in Ozone Contactor Percentage of Population in Filter Effluent Percentage of Population in Distribution System Influent Gram Negative         Acidovorax spp. 2   4 7 Acinetobacter spp. 29 6     Alcaligenes spp. 12 2 1   Alteromonas spp. 2       Comamonas spp. 1   3   Enterobacter spp. 2   5   Flavobacterium spp. 2   5   Hydrogenophaga spp. 8 3 1   Klebsiella spp. 10 1 3   Methylobacterium spp. 1   2   Pseudomonas spp. 14 53 22   Rhodobacter spp. 2 1     Sphingomonas spp. 2 2 19   Stenotrophomonas spp. 2 1 2   Xanthobacter spp. 3       Others* 2 1 5   Gram Positive         Bacillus spp.       7 Nocardia spp. 1 3 7 53 Rhodococcus spp.   16 4   Staphylococcus spp. 1 1     Others* 1 1 1   Unidentified 3 9 16 33 * Includes organisms isolated from only one site at a frequency of 1%. 100 isolates were identified from each site. SOURCE: Adapted from Norton and LeChevallier (2000). spp., Pseudomonas spp., and Klebsiella spp. predominate among the 20 genera identified. Ozonation of the raw water reduced the microbial diversity to 13 genera, dominated by Pseudomonas spp. and Rhodococcus spp. However, following biologically active granular activated carbon filtration, 19 genera were identified in the filter effluent, the majority of which (63 percent) matched isolates observed in the raw water. The predominant genera were Pseudomonas spp. and Sphingomonas spp., which are known to grow attached to the carbon fines of the filter while utilizing natural organic compounds found in the aquatic environment. Final chlorination of the filtered water resulted in a shift to Nocardia spp. as the water entered the pipe system. Nocardia spp. possess characteristic fatty acids that are closely related to Rhodococcus, Mycobacterium, and Corynebacterium. Its partially acid-fast cell wall and possession of the catalase

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Drinking Water Distribution Systems: Assessing and Reducing Risks enzyme, which breaks down hydrogen peroxide, are important factors that enable the organism to survive disinfection. Other Gram-positive bacteria found in chlorinated drinking water include Bacillus and Staphylococcus spp. Bacillus spp. form environmentally resistant spores that can withstand prolonged contact with chlorine. Some strains of Bacillus and Staphylococcus aureus can produce toxins when contaminated water is used in food preparation (LeChevallier and Seidler, 1980). Treated drinking water will include a mixture of Gram-negative and Gram-positive bacteria. In the absence of a disinfectant residual, Gram-negative bacteria will out grow Gram-positive bacteria and dominate the bacterial population. These organisms typically include Pseudomonas, Acinetobacter, Flavobacterium, and Sphingomonas spp. For the most part, these organisms have limited public health significance, except for Pseudomonas aeruginosa, which is a possible opportunistic pathogen in drinking water and in the biofilms of water systems. It is known to colonize point-of-use carbon filters in drinking water systems (de Victoria and Galvan, 2001; Chaidez and Gerba, 2004). Pseudomonas aeruginosa is of concern in bathing waters, especially in swimming pools and spas, where skin infections may result due to exposure. In the case of drinking water, there are a few studies that suggest a relationship between the presence of this organism in the water and disease. In one hospital setting, five of 17 patients with a Pseudomonas infection carried a genotype also detected in the tap water (Trautmann et al., 2001). In another outbreak of pediatric P. aeruginosa urinary tract infections, two isolates had genotypes similar to those in the water. The outbreak was resolved when the taps in the unit were changed (Ferroni et al., 1998). Despite these specific incidences, a workgroup recently convened by the World Health Organization (WHO) to address this issue concluded that HPC bacteria were not associated with any adverse health effect (Bartram et al., 2003). “Some epidemiological studies have been conducted into the relationship between HPC exposures from drinking water and human health effects. Other studies relevant to this issue include case studies, especially in clinical situations, and compromised animal challenges using heterotrophic bacteria obtained from drinking water distribution systems. The available body of evidence supports the conclusion that, in the absence of fecal contamination, there is no direct relationship between HPC values in ingested water and human health effects in the population at large. This conclusion is also supported indirectly by evidence from exposures to HPC in foodstuffs where there is no evidence for health effects link in the absence of pathogen contamination. There are a small number of studies that have examined possible links between HPC bacteria and non-intestinal outcomes in general populations. The conclusions of these studies do not support a [health] relationship” (WHO, 2002). One of the difficulties in interpreting the significance of HPC data is that test methods involve a wide variety of conditions that lead to a wide range of quantitative and qualitative results. For this reason, the EPA has not yet issued a health-based standard. However, the Surface Water Treatment Rule requires

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Drinking Water Distribution Systems: Assessing and Reducing Risks that distribution system locations without a detectable disinfectant residual maintain HPC levels at or below 500 colony forming units (CFU)/mL in at least 95 percent of the samples each month (EPA, 1989). Coliform Bacteria. Total coliform bacteria (a subset of Gram-negative bacteria) are used primarily as a measure of water treatment effectiveness and can occasionally be found in distribution systems. The origins of total coliform bacteria include untreated surface water and groundwater, vegetation, soils, insects, and animal and human fecal material. Typical coliform bacteria found in drinking water systems include Klebsiella pneumoniae, Enterobacter aerogenes, Enterobacter cloacae, and Citrobacter freundii. Other typical species and genera are shown in Table 3-2. Although most coliforms are not pathogenic, they can indicate the potential presence of fecal pathogens and thus in the absence of more specific data may be used as a surrogate measure of public health risk. Indeed, the presence of coliforms is the distribution system is usually interpreted to indicate an external contamination event, such as injured organism passage through treatment barriers or introduction via water line breaks, cross connections, or uncovered or poorly maintained finished water storage facilities (Geldreich et al., 1992; Clark et al., 1996). However, biofilms within distribution systems can support the growth and release of coliforms, even when physical integrity (i.e., breaches in the treatment plant or distribution system) and disinfectant residual have been maintained (Characklis, 1988; Haudidier et al., 1988; Smith et al., 1990), such that their presence may not necessarily indicate a recent external contamination event. Coliform regrowth in the distribution system is more likely during the summer months when temperatures are closer to the optimum growth temperatures of these bacteria. Thermotolerant coliforms (capable of growth at 44.5 °C), also termed “fecal coliforms” have a higher association with fecal pollution than total coliforms. And Escherichia coli is considered to be even more directly related to fecal pollution as it is commonly found in the intestinal track of warm-blooded animals. Although most fecal coliform and E. coli strains are not pathogenic, some strains are invasive for intestinal cells and can produce heat-labile or heat-stable toxins (AWWA, 1999). E. coli and most of the thermotolerant coliforms do not grow in biofilms, although they most likely can be trapped and retained within biofilms. TABLE 3-2 Coliform Isolates Typically Found in Drinking Water Citrobacter Enterobacter Escherichia Klebsiella C. freundii E. aerogenes E. coli K. pneumonia C. diversus E. agglomerans   K. oxytoca   E. cloacae   K. rhinoscleromatis       K. ozaena SOURCE: Adapted from Geldreich and LeChevallier (1999).

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Drinking Water Distribution Systems: Assessing and Reducing Risks Aeromonas. Aeromonas spp. are Gram-negative bacteria found in fresh and salt water and cause a wide variety of human infections including septicemia, wound infections, meningitis, pneumonia, respiratory infections, hemolytic uremic syndrome, and gastroenteritis (Carnahan and Altwegg, 1996; Alavandi et al., 1999). The ability of these microorganisms to grow at low temperatures and low nutrient conditions are important in their occurrence in drinking water supplies. Through the Unregulated Contaminant Monitoring Rule (see Chapter 2), EPA examined the occurrence of Aeromonas spp. in 308 drinking water systems and found detectable concentrations in 2.6 percent of 5,060 samples and in 13.6 percent of the systems. In a 16-month study conducted on the presence of A. hydrophila in drinking water in Indiana, 7.7 percent of the biofilm samples were positive for A. hydrophila (Chauret et al., 2001). The health significance of detecting aeromonads in drinking water is not well understood. Some countries (such as the Netherlands) have set standards for aeromonads in drinking water leaving the treatment plant (< 20 CFU/200 mL) and in the distribution system (< 200 CFU/100 mL). Mycobacteria. Organisms of the genus Mycobacteria are also found in drinking water. Of particular concern is the MAC, or Mycobacterium avium complex. Studies have detected M. avium complex organisms in drinking water distribution systems with concentrations ranging between 0.08 and 45,000 CFU/mL (Haas et al., 1983; duMoulin and Stottmeir, 1986; Carson et al., 1988; duMoulin et al., 1988; Fischeder et al., 1991; von Reyn et al., 1993; Glover et al., 1994; von Reyn et al., 1994; Covert et al., 1999). M. avium are resistant to disinfectants, especially free chlorine (Taylor et al., 2000). Indeed, it is postulated that they may in fact be selected for in distribution systems as a result of their resistance to chlorine (Collins et al., 1984; Schulze-Robbecke and Fischeder, 1989; Briganti and Wacker, 1995). However, there is also evidence that MAC are susceptible to chlorine dioxide and chloramine (Vaerewijck et al., 2005). Falkinham et al. (2001) examined eight, well characterized drinking water systems and reported that 20 percent of the water isolates and 64 percent of the biofilm isolates were identified as M. avium or M. intracellulare. Additionally, 8 percent of the water isolates were identified as M. kansasii. Most of these isolates were detected in raw water samples, with M. avium complex organisms detected in five of six surface water sites ranging from 6 to 35 percent of the organisms isolated. M. avium complex organisms were not detected in any plant or well effluent sample, but were occasionally detected at low levels (< 1 CFU/mL) in drinking water systems. However, M. avium and M. intracellulare were recovered frequently from drinking water biofilm samples, indicating that M. avium levels were increasing in the distribution system. Increases in M. avium levels in drinking water were correlated to levels of AOC (r2 = 0.65, p = 0.029) and BDOC (r2 = 0.64, p = 0.031) (Falkinham et al., 2001; LeChevallier, 2004).

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Drinking Water Distribution Systems: Assessing and Reducing Risks tem provides the best available evidence of public health risks associated with distribution systems in the United States. These data suggest that about one-third to one-half of reported waterborne disease outbreaks are associated with distribution system problems. To date, only one epidemiological study (the second Laval study) has been specifically designed to examine the contribution of the distribution system to endemic disease occurrence. Until better data are available from these three approaches, it will not be possible to accurately assess the magnitude of the health impacts resulting from distribution system deficiencies. The following conclusions and recommendations are made. The distribution system is the remaining component of public water supplies yet to be adequately addressed in national efforts to eradicate waterborne disease. This is evident from data indicating that although the number of waterborne disease outbreaks including those attributable to distribution systems is decreasing, the proportion of outbreaks attributable to distribution systems is increasing. Most of the reported outbreaks associated with distribution systems have involved contamination from cross-connections and backsiphonage. Furthermore, Legionella appears to be a continuing risk and is the single most common etiologic agent associated with outbreaks involving drinking water. Initial studies suggest that the use of chloramine as a residual disinfectant may reduce the occurrence of Legionella, but additional research is necessary to determine the relationship between disinfectant usage and the risks of Legionella and other pathogenic microorganisms. Distribution system ecology is poorly understood. There is very little information available about the types, activities, and distribution of microorganisms in distribution systems. Limited HPC data are available for some systems, but these data are not routinely collected, they underestimate the numbers of organisms present, and they include many organisms that do not necessarily present a health risk. To more adequately assess risk, more information on the microbial ecology of distribution systems, including premise plumbing, is needed. There is inadequate investigation of waterborne disease outbreaks associated with distribution systems, especially in premise plumbing. Legionella has only recently been added to the outbreak surveillance system. Existing data on outbreaks due to other etiologic agents would rarely implicate premise plumbing because backflow and regrowth events likely would not be recognized and reported unless an institutional building with large numbers of people was affected. The Centers for Disease Control and Prevention are commended for revising the format used to report waterborne disease outbreaks to the surveillance system such that outbreaks arising from events in premise plumbing are now more clearly identified.

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Drinking Water Distribution Systems: Assessing and Reducing Risks Epidemiology studies that specifically target the distribution system component of waterborne disease are needed. Recently completed epidemiological studies have either not focused on the specific contribution of distribution system contamination to gastrointestinal illness, or they have been unable to detect any link between illness and drinking water. Epidemiological studies of the risk of endemic disease associated with drinking water distribution systems need to be performed and must be designed with sufficient power and resources to adequately address the deficiencies of previous studies. This chapter highlights the lack of information available to assess the public health risk of contaminated distribution systems. One of the consequences of this fact is that the committee was forced to rely heavily on its best professional judgment to prioritize contamination events into high, medium, and low priority (see Appendix A). Better public health data, including data on waterborne outbreaks, from epidemiological studies, and on distribution system water quality, could help refine distribution system risks and provide additional justification for the rankings. The following three chapters consider the roles of physical, hydraulic, and water quality integrity. Protection of public health requires that water professionals incorporate approaches that combine all three into a comprehensive program of best practices to maintain the highest level of water quality. REFERENCES Abbaszadegan, M., M. LeChevallier, and C. Gerba. 2003. Occurrence of viruses in U.S. groundwaters. J Amer. Water Works Assoc. 95(9):107–120. Alavandi, S. V., M. S. Subashini, and S. Ananthan. 1999. Occurrence of haemolytic and cytotoxic Aeromonas species in domestic water supplies in Chennai. Indian J. Med. Res. 110:50–55. Amann, R. I., B. J. Binder, R. J. Olson, S. W. Chisholm, R. Devereux, and D. A. Stahl. 1990. Combination of 16S rRNA-targeted oligonucleotide probes with flow cytometry for analyzing mixed microbial populations. Appl. Environ. Microbiol. 56:1919–1925. Amann, R. I., W. Ludwig, and K.-H. Schleifer. 1995. Phylogenetic identification and in situ detection of individual microbial cells without cultivation. Microbiol. Rev. 59:143–169. American Water Works Association (AWWA). 1999. Waterborne pathogens: manual of water supply practices—M48, First Edition. Denver, CO: AWWA. Anand, S., and K. Hanson. 1997. Disability-adjusted life years: a critical review. J. Health Econ. 16(6):685–702. Arbuckle, T. E., S. E. Hrudey, S. W. Krasner, J. R. Nuckols, S. D. Richardson, P. Singer, P. Mendola, L. Dodds, C. Weisel, D. L. Ashley, K. L. Froese, R. A. Pegram, I. R. Schultz, J. Reif, A. M. Bachand, F. M. Benoit, M. Lynberg, C. Poole, and K. Waller. 2002. Assessing exposure in epidemiologic studies to disinfection by-products in drinking water: Report from an international workshop. Environ. Health Perspect. 110 (Suppl 1):53–60.

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