Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter.
Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 101
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
OCR for page 102
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).
OCR for page 103
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
OCR for page 104
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;
OCR for page 105
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).
OCR for page 106
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
OCR for page 107
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
OCR for page 108
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
OCR for page 109
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
OCR for page 110
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.
OCR for page 111
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).
OCR for page 112
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).
OCR for page 113
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
OCR for page 114
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).
OCR for page 115
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]).
OCR for page 116
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
OCR for page 117
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-
OCR for page 118
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.
OCR for page 119
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
OCR for page 120
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.
OCR for page 121
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.
OCR for page 122
