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 evaluated 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 differences 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. Increased 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).