Drinking Water Distribution Systems Assessing and Reducing Risks (2006) / Chapter Skim
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3 Public Health Risk from Distribution System Contamination
3 Public Health Risk from Distribution System Contamination
Pages 87-141
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From page 87... ...
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.
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From page 88... ...
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)
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From page 89... ...
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.
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From page 90... ...
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 0.010.1 oocysts/L as threshold levels for requiring additional treatment...these levels are estimated to result in an annual infection risk in the range of 1.7x104 6 x 103 ...
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From page 91... ...
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.
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From page 92... ...
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.
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From page 93... ...
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)
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From page 94... ...
matched isolates observed in the raw water. The predominant genera were Pseudomonas spp.
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From page 95... ...
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.
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From page 96... ...
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.
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From page 97... ...
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)
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From page 98... ...
However, cysts have also been isolated from drinking water distribution systems in France (Jacquemin et al., 1981; Geldreich, 1996)
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From page 99... ...
. Outside of specialized research studies, potable water supplies are not routinely tested for fungi.
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From page 100... ...
. The purpose of these studies was not to indicate that cer tain pipe materials are preferred over another, but to demonstrate the importance of considering the type of materials that come into contact with potable water.
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From page 101... ...
Diagnostic kits are unreliable for many heterotrophic bacteria because the methodology often requires the analyst to perform a Gram stain, which is difficult because of the slow growth and acid-fast or partially acid-fast nature of bacteria surviving in disinfected drinking water. An alternative method includes fatty acid profiling.
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From page 102... ...
. For most of these microbes, methods do not exist for routine testing of drinking water supplies, and basic research is needed on their occurrence, survival, and importance in potable water.
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From page 103... ...
Although this is believed to be an underestimate of the true number of outbreaks that occurred during this period, the information collected in this surveillance system has been extremely valuable for improving our understanding of the agents that cause waterborne disease and the risk factors involved in waterborne disease outbreaks. The data collected in this surveillance system includes: · Type of exposure (drinking water or recreational water)
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From page 104... ...
The overall number of reported waterborne disease outbreaks associated with drinking water is declining from a peak of over 50 reported outbreaks in 1980 to eight reported outbreaks in 2002.
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From page 105... ...
, or chemical. Indeed, surveillance data on waterborne disease outbreaks associated with drinking water in the United States from 2001 to 2002 indicate that almost 30 percent of reported outbreaks were due to bacterial agents, 16 percent were due to protozoa, 16 percent were due to viral agents, 16 percent were due to chemical contaminants, and 23 percent had an unidentified etiology.
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From page 106... ...
The large number of waterborne disease outbreaks associated with protozoa in the early 1980s was mostly caused by Giardia and was greatly reduced by the implementation of the Surface Water Treatment Rule in 1989 (Barwick et al., 2000)
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From page 107... ...
. Outbreaks Associated With Groundwater Systems In recent years, as treatment of surface water supplies has improved, waterborne outbreaks have increasingly involved groundwater supplies (Figure 3-3)
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From page 108... ...
. Other epidemiological and outbreak investigations conducted in the last five years suggest that a substantial proportion of waterborne disease outbreaks, both microbial and chemical, is attributable to problems within distribution systems (Craun and Calderon, 2001; Blackburn et al., 2004)
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From page 109... ...
The Extent of Underestimation The number of identified waterborne disease outbreaks is considered an underestimate because not all outbreaks are recognized, investigated, or reported to health authorities (Blackburn et al., 2004)
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From page 110... ...
of the 143 reported drinking water outbreaks were associated with individual homeowner water systems, suggesting that there may be increased recognition and reporting of these smaller outbreaks in the past ten years of surveillance. Underreporting of Outbreaks Involving Premise Plumbing Outbreaks associated with premise plumbing are not specifically identified in the CDC surveillance reports.
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From page 111... ...
The detection, investigation, and reporting of waterborne disease outbreaks linked to chemical exposures are not as well established as the methods for dealing with outbreaks associated with infectious agents. Finally, many physicians may have difficulty recognizing and diagnosing chemical poisonings unless they have had additional training in this area (Barwick et al., 2000)
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From page 112... ...
Finally, CDC is moving toward a web-based system for reporting outbreaks and developing a public access database on waterborne disease outbreaks that will allow investigators to examine and analyze these data. EPIDEMIOLOGY STUDIES Three basic epidemiological study designs can be used to assess the public health risk of contaminated water supplies (Steenland and Moe, 2005)
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From page 113... ...
Analytical studies can be experimental, such as a clinical trial where some households are given bottled water to drink and other households are asked to drink tap water, and then disease rates between the two study groups are compared to determine the risk of disease attributable to drinking water. In these clinical trials, study participants are randomly assigned to a study group in order to ensure that other potential risk factors for disease are equally distributed among the study groups.
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From page 114... ...
. Descriptive Studies of Endemic Waterborne Disease The risk of endemic waterborne disease (sporadic cases)
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(1999) estimated the incidence of gastrointestinal illness to be 0.79 episodes/person/year.
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From page 116... ...
Differences in gastroenteritis rates between groups 1 and 2 versus group 3 was assumed to be due to changes in water quality that occurred between the time the water left the treatment plant and the time the water reached the household. The water ingested by group 1 represented tap water that had gone through the distribution system and also had residence time in the household plumbing.
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From page 117... ...
(1997) concluded that the distribution system played a role in waterborne disease because the rates of HCGI were similar for group 3 (ingested purified bottled water)
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From page 118... ...
The rates of HCGI ranged from 0.79/person/year for those with functional treatment units and 0.82/person/year with the sham devices. The study concluded that the water was not a source of measurable gastrointestinal disease (the ratio of illness rates between the group drinking treated water compared to the normal tap water was 0.99, with a 95 percent confidence interval of 0.85 1.15; p = 0.85)
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From page 119... ...
To address this mandate, EPA scientists conducted several epidemiological studies of waterborne disease, and EPA funded several studies by external investigators, including the pilot study and full-scale study in Davenport, Iowa. As a preliminary trial to the subsequent epidemiology study, a randomized, triple-blinded, home drinking water intervention trial of 77 households was conducted for four months in Contra Costa County, California (Colford et al., 2002)
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From page 120... ...
. This study was part of a larger case-control study of risk factors associated with sporadic cryptosporidiosis and was not specifically designed to study waterborne disease.
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From page 121... ...
Although there had previously been concern about possible health risks from pressure loss and pathogen intrusion in water distribution systems (LeChevallier et al., 2003) , this was the first study to provide solid evidence of that risk, with policy implications for how to manage low pressure events in public water supplies.
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From page 122... ...
, may be reasons why the Davenport study did not detect a significant risk of waterborne illness. TABLE 3-5 Comparison of Population Parameters from the Epidemiology Studies Study Laval Laval Melbourne Davenport 1988-1989 1993-1994 1997-1999 2000-2002 # households in 307 346 (tap water)
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From page 123... ...
All of the studies monitored water quality at the treatment plant, but there was a wide range in the amount of sampling and analyses of water in the distribution system. For example, monitoring in the Davenport study was extensive, with tap water samples and treatment device samples collected from about one-fourth of the study households at three times during the study.
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From page 124... ...
rate expressed as episodes/person/year The conflicting results of these epidemiological studies raise a number of questions. The fact that these were carefully conducted studies by research teams with considerable experience implies that there are detectable elevated risks of waterborne disease associated with some water systems and not others.
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From page 125... ...
. Tap water drinkers had elevated risk of HCGI compared to those who ingested bottled water from the treatment plant or purified bottled water, suggesting that water in the distribution system posed an increased health risk (although routine water quality monitoring of the distribution system did not provide evidence of compromised quality)
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From page 126... ...
The researchers would provide purified bottled water to half of the study households, and ask the other half of the study population to drink tap water. All study households would be asked to record health symptoms in a health diary.
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From page 127... ...
A B C D First 6 Purified Bottled Purified Bottled Bottled Plant Bottled DS months Water Water Water Water Last 6 Bottled Plant Bottled DS Purified Bottled Purified Bottled months Water Water Water Water 176 households per group (average) ; Total 704 Power estimates: GI risk due to source water quality and treatment efficacy: 91% GI risk due to distribution system: 86% Assumptions: 20% attrition 20% variance inflation due to clustering FIGURE 3-6 Study Design to Examine Risks from Water Quality in the Distribution System: Method 2 Cross-over Study.
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From page 128... ...
The researchers intend to model the illness rate in the study population as a function of household pathogen concentration using dose-response models where incidence of acute gastrointestinal illness in the study population is a function of the pathogen dose in the household water (calculated as concentration of virus in the volume of water ingested over a defined period of time)
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From page 129... ...
However, there is little evidence that filtration and disinfection of surface water prevents the growth of Legionella species in distribution system plumbing. In fact, since Legionella was incorporated into the waterborne disease outbreak surveillance system starting in 2001, several outbreaks have been attributed to the microorganism.
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From page 130... ...
. CONCLUSIONS AND RECOMMENDATIONS Accurate estimates are not yet available for the prevalence of adverse health effects attributable to deficiencies in distribution systems from pathogen occurrence measurements, waterborne disease outbreak surveillance, or epidemiological studies.
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From page 131... ...
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.
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From page 132... ...
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.
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From page 133... ...
2004. Surveillance for waterborne-disease outbreaks associated with drinking water -- United States, 20012002.
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2001. Waterborne disease outbreaks caused by distri bution system deficiencies.
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From page 135... ...
2001. Factors influencing numbers of Mycobacterium avium, Mycobacterium intracellulare, and other myco bacteria in drinking water distribution systems.
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From page 136... ...
2001. A randomized, blinded, controlled trial investigating the gastrointestinal health effects of drinking water quality.
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From page 137... ...
2003. The potential for health risks from intrusion of contaminants into distribution sys tems from pressure transients.
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From page 138... ...
2006. Surveillance for Waterborne Disease and Outbreaks Associated with Drinking Water and Water not Intended for Drink ing -- United States, 20032004.
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1982. Actinomycetes and fungi in surface waters and in potable water.
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2001. A prospective study of rural drink ing water quality and acute gastrointestinal illness.
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1985. Effect of non-legionellaceae bacteria on the multiplication of Legionella pneumophila in potable water.
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