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© IWA Publishing 2011 Water Quality Research Journal of Canada
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2011
Application of risk assessment tools to small
drinking water systems in British Columbia
Ian Michael Summerscales and Edward A. McBean
ABSTRACT
A number of risk assessment tools have been developed for drinking water systems, but there is a
lack of published independent evaluation of how well the tools incorporate the multiple barrier
approach to drinking water safety. Selected risk assessment tools were evaluated by applying the
tools to five small drinking water systems serving residential developments in British Columbia. The
selected risk assessment tools generally identified hazards and vulnerabilities in the source,
Ian Michael Summerscales (corresponding
author)
Edward A. McBean
University of Guelph,
50 Stone Road East,
Guelph,
ON N1G 2W1,
Canada
E-mail: [email protected]
distribution, storage and monitoring barriers of the water systems. The risk assessment tools had
varying levels of success identifying vulnerabilities in the treatment barrier. In some cases, the
existing tools consider the presence or absence of a water system barrier, such as a disinfection
process or a monitoring procedure, but do not consider how effective or appropriate that barrier is.
A common shortcoming of the risk assessment tools is the failure to identify the need for multiple
treatment processes capable of removing or inactivating pathogens, which is particularly important
for surface water supplies. In addition to not incorporating the multiple barrier approach into the
evaluation of the treatment barrier, none of the risk assessment tools successfully reflected the
interconnected nature of the water system barriers.
Key words
| British Columbia, drinking water, multiple barrier approach, risk assessment
ABBREVIATIONS
INTRODUCTION
B.C. Screening British Columbia drinking water source-
Pathogen contamination of drinking water has been ident-
Tool
to-tap screening tool
ified as the most significant human health risk on an
BWA
boil water advisory
international scale (Ritter et al. ). To protect public
DWO
drinking water officer
health, many jurisdictions have developed more strict drink-
GLUMRB
Great Lakes-Upper Mississippi River Board
ing water legislation, such as the Total Coliform Rule in the
of State and Provincial Public Health and
United States, which established monitoring requirements
Environmental anagers
and legal limits for coliform bacteria in drinking water.
groundwater under the direct influence of
Nevertheless, legislation based on monitoring finished
surface water
water quality alone has not been sufficient to prevent water-
T
hydraulic residence time
borne disease outbreaks in the United States and other
MRA Tool
microbial risk assessment ranking tool
developed nations. These outbreaks have shown that coli-
PWS Tool
public water supplies
form bacteria are not good indicators of the presence of
WQA
water quality advisory
waterborne pathogens, including protozoa (Craun et al. )
GUDI
doi: 10.2166/wqrjc.2011.102
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I. M. Summerscales & E. A. McBean
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Evaluation of drinking water risk assessment tools
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and enteric viruses (Borchardt et al. ). The failure of
whether the tools could capture the risks and vulnerabilities
monitoring programs based on total coliform bacteria is
observed by an inspector, and how well the tools incorporate
of even greater concern for small water systems, which
the multiple barrier approach to drinking water safety.
generally have reduced sampling frequencies, and hence
are less likely to detect intermittent contamination events
(Craun et al. ). For instance, private water supplies in
METHODS
England and Wales, which supply only 0.5% of consumers
in those countries, were implicated in 36% of waterborne
Case studies
disease outbreaks over the period of 1970–2000 (Said
et al. ).
All five of the British Columbia water systems that were vis-
The failure of compliance strategies based on monitor-
ited were considered to be ‘small systems’ because they serve
ing to prevent waterborne disease outbreaks in communities
fewer than 500 persons (Ministry of Health Services ).
served by public water systems has emphasized the need
There are over 3,300 water systems in the Province of British
for better methods of identifying vulnerable water systems
Columbia (B.C.). The 96 municipally owned water systems
(The Groundwater Foundation ). As a result, a
in the province serve 90% of the population; small public
number of jurisdictions have moved towards a risk-based
and private water systems serve the remaining 10% of the
drinking water management framework. The Australian
population (Halliday ). A study of annual disease rates
Drinking Water Guidelines 2004 and the Drinking-water
in a BC community for the period 1996 to 2005 showed
Standards for New Zealand 2008 and their respective
that disease rates were significantly higher for individuals
supporting documents include risk assessment components
with private wells when compared with individuals living
that make reference to the multiple barrier approach to
in the same area that were served by a municipal water
drinking water safety. In both cases, the water supplier is
supply (Uhlmann et al. ).
responsible for identifying the hazards and risks that are appli-
The water systems were visited over the course of a four
cable to their water system. However, owing to a lack of
day period in July 2009. The site visit to each water system
resources and expertise in risk assessment, it may not be
lasted between 3 and 5 hours, and included the opportunity
realistic for small water systems to perform risk assessments
to interview the operator or other water system representa-
and develop plans to manage those risks on an individual
tive(s), review available design documents and monitoring
basis (World Health Organization ).
data, and complete an inspection of the water system
Risk assessment tools have been developed specifically
source, treatment, and storage and distribution infrastruc-
for small water systems. Unfortunately, there is a lack of
ture. These site visits allowed for the collection of data
independent published data where existing risk assessment
necessary to complete the risk assessment tools.
tools are applied to small water systems in order to evaluate
To maintain confidentiality, the small water systems
how well these tools identify the hazards and vulnerabilities
have been assigned generic names based on the type of
that could result in microbial contamination of the water
water supply.
system, or how well they incorporate the multiple barrier
approach to drinking water safety.
Groundwater system A
This paper builds on the results of previous research by
Summerscales & McBean (), which used historic water-
Groundwater system A was brought into service in 2005 to
borne disease outbreaks as case studies to determine
provide a new subdivision of 36 residential lots with water
whether selected risk assessment tools were capable of iden-
for potable use and firefighting purposes. At the time of
tifying the hazards and vulnerabilities that led to the
the inspection, approximately half of the lots had been
outbreaks. In this paper, the selected risk assessment tools
developed.
were applied to five small water systems serving residential
Groundwater system A is supplied by two wells: a pri-
developments in British Columbia in order to determine
mary water supply well and a back-up well. Raw water
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I. M. Summerscales & E. A. McBean
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Evaluation of drinking water risk assessment tools
from both wells enters the treatment building through the
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2011
Groundwater system C
same PVC forcemain. A 12 wt% sodium hypochlorite solution is injected into the raw water before it enters a
The development served by groundwater system C is along
456 m3 underground reservoir designed to provide contact
the east shore of a lake. Groundwater system C has over
time and storage for firefighting purposes. The reservoir is
100 service connections, including a large resort complex.
designed such that chlorinated water can enter one of two
The majority of the service connections, including the
parallel reservoir cells through a baffled inlet. Both cells dis-
resort, are seasonal.
charge to the wet cell, which the distribution system lift
Groundwater system C is supplied by a single ground-
pumps and fire pump draw from. The submersible pumps
water well. The well is constructed in a belowground
in the supply wells are controlled by adjustable level
chamber that is located in a wooded area, approximately
switches in the reservoir.
25 m from a creek and 250 m from the lake. The submers-
A Boil Water Advisory (BWA) was issued for ground-
ible well pump is controlled by level switches in a 114 m3
water system A at one point, after a distribution sample
underground reservoir that supplies the distribution system
tested positive for Escherichia coli. The operator attributed
with water by gravity flow. Raw water from the well enters
this result to fecal contamination at the residential sample
a ductile iron distribution water main, which carries water
tap. For the period between the positive result and the
to the reservoir and throughout the distribution system.
inspection, all samples for microbiological analysis have
Drinking water receives no treatment prior to entering the
been collected from a sink in the treatment building. None
distribution system.
of those samples was positive for microbial parameters.
A BWA was issued for groundwater system C approximately eight years before the inspection based on a water
Groundwater system B
sample that had been collected shortly after the reservoir
was cleaned. There had been no adverse results in the two
Groundwater system B serves approximately 75 properties,
years prior to the inspection.
including a community centre, a medical clinic and a
restaurant. Most of the water system infrastructure was
Surface water system D
approximately 35 years old at the time of the inspection.
Groundwater system B is supplied by two wells: a 300
Surface water system D is located along the west shore of a
millimetre (mm) well that acts as the primary water supply
lake. Surface water system D had been in operation for
and a 150 mm diameter well that acts as the secondary
approximately 20 years at the time of the inspection. The
supply. Both wells are constructed in below ground well
water system serves approximately 40 residential lots,
chambers that are located approximately 40 m from the
roughly half of which are developed. Approximately one-
bank of a river. The submersible well pumps are controlled
quarter of residences are occupied year-round.
by the water level in a 98 m3 underground reservoir that
Surface water system D is supplied with raw water by a
supplies the distribution system with water by gravity flow.
submersible pump located in the lake. Raw water enters the
Raw water from both wells enters a 200 mm asbestos-
pump house, which is located on the shore of the lake, where
cement distribution water main, which carries water to the
it is treated by ultraviolet (UV) irradiation in order to achieve
reservoir and throughout the distribution system. Drinking
primary disinfection. The submersible pump is controlled by
water receives no treatment prior to entering the distribution
level switches in a concrete reservoir with an operating
system.
capacity of 145 m3, which provides the village with treated
Based on the available records, two distribution samples
water for domestic and firefighting purposes by gravity flow.
collected from groundwater system B had tested positive for
Treated water is transported to the reservoir, and throughout
total coliform bacteria in the 10 year period prior to the
the distribution system, by a 150 mm PVC water main.
inspection. There had been no adverse water quality results
in the calendar year prior to the inspection.
A Water Quality Advisory (WQA) was issued for surface water system D in 2008 after two distribution
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I. M. Summerscales & E. A. McBean
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Evaluation of drinking water risk assessment tools
samples collected for microbial analysis on the same day
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2011
Risk assessment tools
were overgrown such that the total coliform count could
not be determined accurately. These samples were col-
For the purpose of this paper, the British Columbia Drinking
lected shortly after the annual chlorination of the storage
Water Source-to-Tap Screening Tool (B.C. Screening Tool),
reservoir and flushing of the distribution system. The distri-
the Montana Water Center Microbial Risk Assessment rank-
bution samples collected five days later during the follow-
ing tool (MRA Tool), and the risk assessment forms in the
up sampling event were also positive for total coliform
Scottish Private Water Supplies Technical Manual (PWS
bacteria, but there had been no additional adverse results
Tool) were applied to the aforementioned five small drinking
between that time and the inspection. The WQA was
water systems. These tools were selected because they
lifted early in 2009, upon completion of a site inspection
include risk assessment surveys with specific questions that
by a public health inspector.
address different aspects of the water system, and do not
rely on the user to identify all of the risks and vulnerabilities
Surface water system E
applicable to the water system. As a result, they are more suitable tools for use at small water systems, where the system
Surface water system E is located along the west shore of a
owners or operators are less likely to have sufficient
lake. The water system serves 26 residential lots, some of
resources or expertise to identify all of the vulnerabilities or
which are seasonal.
risks that are relevant to the water system (World Health
Surface water system E is supplied by two submersible
Organization
).
Although
the
tools
have some
pumps located in the lake in separate intake lines. Only
common survey questions, they have different formats, use
one pump is in use at a given time. Raw water is pumped
different scoring systems to identify and rank risks, and
uphill to the treatment building. Filtration is provided by
require different levels of involvement from regulatory auth-
two sets of filter housings that are installed in parallel,
orities. Relevant features of the selected risk assessment tools
which are typically outfitted with 10, 5, and 1-μm nominally
are summarized in Table 1. Additional background infor-
rated bag filters in series in order to remove sediment from
mation regarding the selected risk assessment tools was
the raw water prior to disinfection. Chlorination is provided
previously published by Summerscales & McBean ().
by two chemical metering pumps, which have separate
This study focuses on the output from the tools with
injection points so both pumps can be in operation at the
respect to microbial risks and vulnerability to microbial
same time, if necessary. A commercial bleach solution is
contamination. The decision to focus on microbial contami-
used for chlorination. Chlorinated water enters a 150 mm
nation was made partly because the MRA Tool does not
PVC distribution water main that carries water through
address chemical hazards, but primarily because pathogenic
the lower branch of the distribution system to a 133 m3 con-
contamination represents the greatest risk to drinking water
crete reservoir. Designated residents turn the raw water
safety (Hrudey & Hrudey ).
pumps and treatment system on and off manually based
on the level in the reservoir. When the lake pumps are not
in operation, a booster station pumps water from the reservoir to the upper branch of the distribution system, and
RESULTS AND DISCUSSION
the lower branch of the distribution system receives treated
water from the reservoir by gravity flow.
A distribution sample collected from surface water
The discussion of the results of the site inspections and the
completion of the risk assessment tools is organized in terms
system E in 2008 tested positive for total coliform bacteria.
of the barriers to drinking water contamination. The five bar-
There were no positive results for microbial parameters
riers were summarized as follows by Hrudey & Hrudey ():
between that time and the inspection. There was a historical
BWA for surface water system E, although that occurred
before upgrades were made to the water treatment system.
•
Source protection measures to maintain raw water quality and reduce the risk of contamination entering the
water supply
336
Table 1
I. M. Summerscales & E. A. McBean
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Evaluation of drinking water risk assessment tools
Water Quality Research Journal of Canada
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2011
Summary of selected risk assessment tools
Publication year
B.C. Screening Tool
MRA Tool
2004
2004
PWS Tool
2006
®
Format
Paper checklist
Microsoft Excel spreadsheet
Paper checklist
User
Owner/operator
Owner/operator
Local health authority
Regulatory role
Drinking water officer (DWO)
reviews the completed tool
N/A
Local authority completes the risk
assessment
Hazards
assessed
Chemical and microbial
Microbial
Chemical and microbial
Scoring/ranking
system
N/A
Semiquantitative microbial risk scores
(scale of 0 to 1) are assigned to
each survey question based on user
input; scores are aggregated to
assign a total score to each survey
and the water system
Qualitative risk characterization scores
(i.e. low to high) are assigned to each
survey question, survey and the water
system based on user input.
Semiquantitative hazard assessment
scores (scale of 1 to 256) are
calculated for survey questions based
on user input
Output
DWO reviews the completed tool
and identifies aspects of the water
system that represent a risk. The
DWO may order comprehensive
assessment if necessary
Microbial risk scores are summarized
in tables and charts. The
spreadsheet also generates
comments regarding survey
questions that received high risk
scores
Priorities for corrective action are
selected based on hazard assessment
scores
References
Ministry of Health Services and
Ministry of Water, Land, and Air
Protection (, )
Butterfield and Camper ()
Scottish Executive ()
•
•
•
•
Treatment processes to remove or inactivate pathogens
procedures, if such procedures were in place, would be
Distribution system security to prevent the contamination
effective. Owing to the lack of relevant information avail-
of treated water, and maintenance of a disinfection
able for the inspected water systems, the selected risk
residual to ensure delivery of safe water to consumers
assessment tools were not evaluated with respect to how
Monitoring activities to ensure that treatment processes
well they incorporate the response barrier of drinking
are reliable, and to detect contamination
water safety.
Response activities to address adverse conditions
The selected risk assessment tools are more focused on
Results for the source protection barrier
the first four barriers, which address the physical infrastructure of the water system and its performance. With respect
The three selected risk assessment tools generally identified
to response procedures, the B.C. Tool and PWS Tool include
hazards located near a groundwater well, such as the
questions regarding the preparation of an emergency
septic systems serving individual residences in each of the
response or emergency action plan, but do not address the
communities with a groundwater supply. Survey questions
details of these documents in sufficient depth to determine
regarding these potential sources of microbial contami-
whether they would be effective in the event of an emer-
nation received relatively high risk scores for both the
gency. Although a BWA had been issued for three of the
MRA and PWS tools. With respect to the two communities
case study water systems in the past, none of the water sys-
served by surface water supplies, the three risk assessment
tems was under a BWA at the time of inspection. As a result,
tools identified sources of contamination in the watershed
it would be difficult to determine whether existing response
including upstream discharges from municipal wastewater
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plants, septic systems serving the residents of the commu-
GUDI based on the input to the PWS Tool. The B.C. Tool
nity, and wildlife. The survey questions regarding upstream
collects information regarding the construction and depth
discharges in both the MRA and PWS tools also received
of the well, the setback from surface water bodies, the
relatively high scores.
water supply aquifer and the presence of a confining layer,
It is important for the risk assessment tools to identify
assuming that information is available. It would then be
vulnerabilities in the construction or current condition of
the responsibility of the drinking water officer (DWO) to
the wells, given the strong correlation between well con-
interpret the input to the B.C. Tool to determine whether
struction and the likelihood of contamination (Ritter et al.
the available information suggests that the supply well is vul-
). Both the MRA and PWS tools identified unscreened
nerable and requires further assessment. Only the MRA Tool
vents on the water supply wells at groundwater system A
addresses the GUDI concept directly, and includes a survey
as potential points of entry for vermin. The water supply
question regarding the results of the GUDI assessment, if
wells for groundwater systems B and C were constructed
one has been performed. The maximum risk score for that
in below-ground well chambers that were not watertight.
survey question would be assigned if the well has been ident-
All three risk assessment tools recognized the well chamber
ified as GUDI, or if there is no information available
construction as being vulnerable, as surface water could
regarding the supply aquifer.
enter the chamber and pool in the area around the wells
If the user changes the input to the MRA Tool source
during snowmelt or severe storm events, potentially submer-
survey question regarding aquifer type such that the water
ging the well casing.
supply is an unconfined GUDI well instead of a confined
The risk assessment tools had varying levels of success
non-GUDI well, there is a noticeable increase in the risk
when determining whether a groundwater supply was
score calculated for the source survey as a result. However,
‘groundwater under the direct influence of surface water’
the change in the supply aquifer type does not have an
(GUDI). This information is particularly important to the
impact on the treatment survey questions or their relative
risk assessment process, as GUDI supplies are generally
importance, despite the fact that chlorination, which can
considered to be more vulnerable to contamination by
typically provide the required log inactivation of viruses
pathogens that are associated with surface contamination,
given a relatively short contact time, does not effectively
such as Cryptosporidium oocysts and Giardia cysts. As a
inactivate Cryptosporidium oocysts (Ministry of the Environ-
result, GUDI supplies have far more stringent treatment
ment ). In order to account for the interdependent
standards than groundwater supplies in some jurisdictions,
nature of the water system barriers, such as the effect that
in order to ensure that these pathogens are effectively
variations in raw water quality due to surface contamination
removed or inactivated (Ministry of the Environment
can have on the performance of the treatment barrier, the
). A review of waterborne disease outbreaks in the US
input to the source survey regarding the vulnerability of
for the period of 1991 to 1998 identified a number of cryp-
the source should be taken into account when assessing
tosporidiosis and giardiasis outbreaks that occurred in
the downstream barriers. At this time, none of the selected
communities with groundwater supplies (Craun et al.
risk assessment tools incorporates rules that automatically
), which emphasizes the importance of assessing
increase the relative importance of the treatment barrier or
whether a groundwater supply is vulnerable to surface
survey questions regarding the performance of the treatment
water and sewage contamination.
barrier in the event that the source is classified as GUDI.
The PWS risk assessment form for boreholes does not
collect any information regarding the local geology, the
Results for the treatment barrier
water supply aquifer, or the presence of nearby surface
water bodies with the exception of a survey question that
With the exception of two survey questions regarding the
addresses evidence of ‘poor drainage causing stagnant/
operation and maintenance of point of entry or point of
standing water’ (Scottish Executive ). As a result, it is
use water treatment systems, the PWS risk assessment
difficult to determine whether a given water supply is
forms do not address treatment. As a result, the PWS tool
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Evaluation of drinking water risk assessment tools
will not be discussed further with respect to the treatment
barrier.
The treatment survey of the B.C. Tool identifies the disinfection process used to treat a water supply, but does not
collect any information regarding the design or performance
of the disinfection process. Of the selected risk assessment
Table 2
Water Quality Research Journal of Canada
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Design parameter
416
Based on the capacity of the
duty lift pump, assumes one
of two pumps in operation
Water use
Peak flow rate
(L/min)
Storage tank volume
(m3)
98
a water system is calculated by multiplying the effective contact time provided by a chemical disinfection process,
Contact volume (m3)
69
T (min)
165
Baffling factor
Average
T10/T
0.5
hydraulic residence time, T. The T10/T ratio for a water treat-
T10 (min)
382
ment process can be estimated based on the design and
Water quality
construction of the reservoir providing contact time.
pH
measured in minutes, by the concentration of the disinfec-
‘the length of time during which not more than 10 [percent]
of influent water’ will pass through that treatment process
10
Based on a groundwater source
with a partially buried
reservoir
CT requirement for
virus inactivation
6
4 log removal/inactivation at
pH ¼ 6–9, T ¼ 10 C (Ministry
of the Environment )
Calculated free
chlorine residual
(mg/L)
0.08
Temperature ( C)
whether the existing chemical dosing equipment has suffiflow rate. However, only the surface water treatment surveys
consider the frequency with which the treatment system
CT calculations
W
meets CT requirements. The MRA Tool treatment survey
questions for groundwater systems do not consider the main
performance indicator for chemical disinfection systems,
Based on a reservoir with
baffled inlets, and a three cell
layout that provides internal
baffling
Based on a historical result for
the primary well
W
cient capacity to meet CT requirements at the system design
Based on the water level at the
well no. 3 ‘on’ switch
7.4
The MRA Tool surveys for both groundwater and surface
water treatment systems include a question to determine
2011
Assumptions
Disinfection process
(Ministry of the Environment ), is used instead of the
|
Value
cept when assessing the treatment system. The CT value for
milligrams per litre (mg/L). The T10 value, which represents
46.4
CT calculations for groundwater system A
tools, the MRA Tool is the only one that refers to the CT con-
tant in water as it exits that process, measured in
|
and as a result, place more importance on the presence of a
treatment barrier than in the performance and operation of
that barrier. The chlorine dosing rate selected by the water
system operators can have a significant impact on the quality
the storage reservoir effluent is necessary to achieve 4 log
of treated water, as illustrated by the waterborne disease out-
inactivation of viruses, which is the level of disinfection
break in Walkerton, Ontario. Walkerton Public Utilities
that a DWO should consider when issuing a construction
Commission operators consistently ‘added significantly less
or operating permit for a new water system in British Columbia
chlorine than was required’ (O’Connor ). If a free
(Drinking Water Leadership Council ). Based on infor-
chlorine residual of 0.5 mg/L after 30 min of residence time
mation collected during the site inspection, the treatment
had been maintained in treated water throughout the contami-
equipment installed at groundwater system A was capable
nation incident that led to the waterborne disease outbreak,
of providing the CT necessary to achieve primary disinfection
bacteria present in the raw water would have been inactivated
for the full range of flow rates for domestic use. However,
prior to entering the distribution system (O’Connor ),
the free chlorine residual in treated water samples collected
which would have limited the scope of the outbreak.
from the treatment building sink was often below the 0.5 mg/L
CT calculations for groundwater system A in Table 2
detection limit of the chlorine test swabs used for monitoring
indicate that a free chlorine concentration of 0.08 mg/L in
purposes. If the operator cannot accurately measure the
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free chlorine residual, then the CT value for the water
per square centimetre (mJ/cm2) at the design flow rate
system cannot be calculated, and there is no guarantee that
(GLUMRB ). The MRA Tool was the only risk assess-
primary disinfection has been achieved. Also, if the free
ment tool that identified a number of design flaws in the
chlorine residual is below the detection limit under
UV system that would likely prevent it from consistently
normal operating conditions, the water system operator
achieving the 40 mJ/cm2 UV dose. Despite the fact that it
would not be able to detect changes in treated water
identified design flaws in the only barrier capable of inacti-
quality in the event that the storage reservoir became
vating pathogens present in the water supply of surface
contaminated.
water system D, the MRA Tool did not make any recommen-
If the water system does not provide treatment prior to
dations regarding the installation of a filtration system
distribution, as was the case for groundwater systems B
capable of removing surface water pathogens prior to the
and C at the time of the inspection, the MRA Tool assigns
UV treatment. When issuing a construction or operating
a risk score of zero to the treatment survey for that system.
permit for a small system treating a surface water or GUDI
This appears to understate the level of risk associated with
supply in British Columbia, guidance documents suggest
distributing untreated groundwater. A German study
that the DWO consider including a requirement for two
showed that areas where the majority of residents were sup-
treatment processes that, in combination, are capable of
plied with treated surface water had significantly lower
meeting the recommended log inactivation or removal
incidence rates of gastroenteritis compared with areas
requirements (Drinking Water Leadership Council ).
where most residents were supplied with groundwater,
Another issue with the MRA Tool is that it does not
which was attributed to the low proportion of groundwater
address the full range of treatment technologies used by
supplies that were disinfected (Dangendorf et al. ). Dis-
small water systems, such as the bag filters that provide sedi-
infection is typically the last barrier that is capable of
ment removal prior to chlorination for surface water system
removing or inactivating pathogens before the water enters
E. The B.C. Screening Tool is somewhat more flexible with
the distribution system, and in the case of most groundwater
respect to treatment processes. To complete the B.C. Tool
supplies, may be the only barrier capable of removing or
treatment survey, the water supplier only needs to indicate
inactivating pathogens. In summary, a risk assessment tool
whether the raw water is filtered, and whether the treatment
that assigns a risk score of zero to the treatment survey
system includes filtration or other processes capable of
when there is no treatment present, does not accurately rep-
removing pathogens and their ‘carriers’, such as turbidity
resent the risk associated with distributing untreated water.
(Ministry of Health Services and Ministry of Water, Land
The treatment systems installed at surface water systems
and Air Protection ). Turbidity is widely used as a surro-
D and E and the output from the selected risk assessment
gate parameter for pathogen removal in filtration and
tools for these treatment systems demonstrate some of the
removal processes. However, filtration equipment, such as
problems with the incorporation of the multiple barrier
the bag filters installed at surface water system E, may be
approach for these tools. For surface water system D, the
capable of reducing turbidity without effectively removing
only treatment that surface water received prior to entering
pathogens. As a result, the user input for this survey question
the distribution system was UV irradiation. The B.C. Tool
may be somewhat misleading, and may result in the DWO
treatment survey recognizes that surface water system D is
having false confidence in the robustness of the filtration
treated by UV disinfection alone, but does not collect any
barrier in place for the small water system.
information regarding the design or performance of the
The need for an effective filtration barrier that is capable
UV system. The Recommended Standards for Water
of removing surface water pathogens is especially important
Works, prepared by the Great Lakes – Upper Mississippi
for surface water system E, as chlorination cannot effec-
River Board of State and Provincial Public Health and
tively inactivate Cryptosporidium oocysts (Ministry of the
Environmental Managers (GLUMRB), states that UV sys-
Environment ). Historical waterborne outbreaks of
tems must undergo validation testing to demonstrate that
cryptosporidiosis have occurred in a number of commu-
they are capable of providing a UV dose of 40 millijoules
nities
served
by
water
treatment
plants
that
used
340
I. M. Summerscales & E. A. McBean
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Evaluation of drinking water risk assessment tools
Water Quality Research Journal of Canada
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46.4
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2011
chlorination to achieve primary disinfection, but did not
control program in place. The distribution survey questions
have a filtration process capable of removing Cryptospori-
regarding
dium oocysts (Hrudey & Hrudey ). The chlorination
received relatively high risk scores for the remainder of
system installed at surface water system E was also unlikely
the water systems.
cross-connections
and
backflow prevention
to meet the CT requirements for 3-log inactivation of Giar-
The risk assessment tools also identified a number of vul-
dia cysts, which is the level of disinfection that a DWO
nerabilities in the construction or current condition of the
should consider when issuing a construction or operating
water storage infrastructure. The storage facilities serving
permit for a new water system (Drinking Water Leadership
the five water systems were either completely or partially
Council ). The reservoir that stores treated water at sur-
buried tanks constructed from concrete or metal. All three
face water system E was not designed to enhance contact
risk assessment tools identified issues with unscreened sto-
time; a single pipe was both the inlet to and outlet from
rage tank overflow lines or vents at groundwater system B
the reservoir, and there were no baffles or recirculation
and surface water systems D and E. The MRA and PWS
equipment inside the reservoir to promote mixing. As a
tools also identified issues with access hatch construction at
result, the effective contact time provided by the reservoir
groundwater systems A and B and surface water system E,
was minimal. Even if the reservoir was designed to enhance
such as hatches that were not vermin- or waterproof, or did
mixing, the reservoir was located such that the lower branch
not have sufficient clearance above either the top of the
of the distribution system conveyed water to the reservoir as
tank or ground level to prevent the hatch from being buried
well as supplying a number of residences with potable water.
or submerged. If vulnerabilities were present in the storage
If these residents consumed water while the lake pump was
tank construction, the relevant survey questions in both the
on and the reservoir was being filled, they would consume
MRA and PWS tools received relatively high scores.
water directly from the water treatment plant, which has
The B.C. Tool and MRA Tool identified low free chlor-
received minimal contact time after chlorination. The
ine residual levels in the distribution system as a weakness
output from the MRA Tool for surface water system E
in the distribution system barrier for all five water systems.
reflects the high risk associated with distributing water
Groundwater system A and surface water system E were
that rarely meets CT requirements by assigning the highest
the only systems that used chlorination to provide disinfec-
risk score for the treatment survey to the survey question
tion, but at the time of the inspection, both had recent
regarding disinfected water quality.
distribution system monitoring results where the free chlorine concentration was below either the target concentration
Results for the distribution and storage barrier
(listed as 0.2 mg/L in the MRA distribution survey) or the
detection limit of the chlorine monitoring equipment being
The distribution system is the barrier that is capable of pre-
used. As a result, the MRA Tool distribution survey question
serving the quality of treated water from the time it leaves
regarding residual disinfection received a relatively high
the treatment plant until it is delivered to the user
score for both groundwater system A and surface water
(Geldreich & Singley ). As a result, it is critical that
system E, although not as high as for the water systems
drinking water risk assessment tools identify vulnerabilities
that did not provide residual disinfection. The free chlorine
and risks that could allow microbial contamination of the
residual does not necessarily provide effective disinfection
water within the distribution system.
capacity in the event that microbial contamination enters
The selected risk assessment tools were generally suc-
the distribution system (Hrudey & Hrudey ), but a sig-
cessful in identifying physical vulnerabilities in the
nificant decrease in the free chlorine residual does provide
distribution system infrastructure. All of the selected risk
an indication of a major contamination event. The combi-
assessment tools included survey questions regarding the
nation of chlorination to maintain a free chlorine residual
presence of either backflow prevention devices or an
and a thorough free chlorine monitoring program in the dis-
enforced cross-connection program. Of the five water sys-
tribution system could have reduced the scope and duration
tems, only groundwater system C had a cross-connection
of a number of documented waterborne disease outbreaks,
341
I. M. Summerscales & E. A. McBean
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Evaluation of drinking water risk assessment tools
including the previously mentioned Walkerton outbreak
(Hrudey & Hrudey ).
Water Quality Research Journal of Canada
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2011
The B.C. Screening Tool storage survey includes a question regarding design features that encourage water
The storage survey of the MRA Tool includes a question
circulation and prevent chlorine decay. This is the only
regarding excessive drawdown or stagnation in the storage
one of the selected risk assessment tools that includes
reservoir. The hydraulic residence time (T) for the storage
a survey question that addresses the internal design of
reservoirs was calculated based on the operating volume
the water storage infrastructure directly. None of the risk
of the individual reservoirs and the calculated average day
assessment tools considers the design of water storage
water demand for each system based on the number of resi-
infrastructure in terms of providing or maximizing contact
dences occupied during high and low use periods, assuming
time. This is likely because reservoirs that provide contact
water use of 1,000 litres per day per dwelling. As seen in
time, often referred to as clear wells, are generally con-
Table 3, all of the reservoirs with the exception of the reser-
sidered to be part of the treatment plant and not part of
voir for groundwater system B had a T of greater than 2 days
the distribution system (Kirmeyer ). Regardless, the
for at least part of the year, and, as a result, were identified
design of these facilities remains important, as it can have
as having excessive storage capacity by the MRA Tool sto-
a significant impact on CT calculations.
rage survey.
Water systems that use potable water for firefighting
Results for the monitoring barrier
purposes generally require greater storage volumes and a
larger distribution water main in order to meet fire flow
The frequency with which distribution samples were col-
requirements. The increased residence time in storage
lected and submitted to the local health authority for
facilities and the distribution system increases the time
microbial analysis ranged from two samples per week to
between drinking water treatment and consumption. Resi-
one sample per quarter, depending on the water system.
dence time can be further increased if there are extended
More frequent sampling was generally required for water
periods of reduced water demand, either because a rela-
systems using surface water sources.
tively large number of the serviced lots are occupied on a
All of the selected risk assessment tools include ques-
seasonal basis, as seen at groundwater system C, or
tions regarding recent monitoring results. The MRA Tool
because a relatively large number of lots are undeveloped,
includes the most specific questions regarding sampling
as was the case for groundwater system A. The resulting
locations and practices, including a question in the distri-
increase in residence time in storage facilities and the dis-
bution survey regarding the collection of water samples for
tribution system further increases the potential for chlorine
microbial and free chlorine analyses near dead ends and
low flow areas of the distribution system. A relatively high
decay.
risk score was assigned if no samples had been collected
in these areas, which applied to groundwater systems A
Table 3
|
and C, or if a recent sample collected in these areas had
Estimated T in drinking water storage reservoirs
tested positive for total coliform bacteria, which applied to
T
Operating
3
surface water systems D and E. A risk score of zero was
Water system
volume (m )
Operating scenario
(d)
Groundwater system A
388
All connections
Developed properties
10.8
18.5
Groundwater system B
90
All connections
1.2
near the six distribution system dead ends on a rotating
Groundwater system C
103
All connections
Year-round users
1.0
9.3
basis, and none of these samples tested positive for total coli-
3.6
7.3
14.5
However, as groundwater system B collects one sample for
Surface water system D
Surface water system E
145
116
All connections
Developed properties
Year-round users
All connections
4.5
assigned to this survey question for groundwater system B,
as samples for microbial analysis are collected at locations
form bacteria in the three years prior to the inspection.
microbial analysis per quarter, current sampling practices
are unlikely to accurately reflect variations in water quality
throughout the distribution system, as a water sample will
342
I. M. Summerscales & E. A. McBean
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Evaluation of drinking water risk assessment tools
Water Quality Research Journal of Canada
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46.4
|
2011
be collected near each dead end approximately once every
vulnerabilities in the design or current condition of the
18 months. This illustrates a common problem with risk
source, distribution, storage and monitoring infrastructure.
assessment tools and other surveys, where risk scores are
Of the selected tools, the MRA Tool requires the greatest
assigned to survey questions based on the presence or
amount of input regarding the design and performance of
absence of a piece of equipment, an operating procedure,
the existing treatment infrastructure. As a result, the MRA
or an aspect of the water system, but the survey question
Tool identified some vulnerabilities in the treatment barrier
does not account for whether that aspect of the water
that were not identified by the other two risk assessment tools.
system is suitable or effective.
One shortcoming that all of the selected risk assessment
The B.C. Screening Tool includes a survey question
tools share is the failure to reflect the interconnected nature
regarding the use of alarms to notify operators in the event
of the water system barriers. With respect to the source
that monitoring equipment detects a low free chlorine
barrier, it is important for a risk assessment tool to deter-
residual, or other conditions that could arise at the treat-
mine whether the source is vulnerable to surface water
ment plant and have a negative impact on water quality
contamination. If surface water pathogens can enter the
(Ministry of Health Services and Ministry of Water, Land
raw water supply, then the downstream treatment barrier
and Air Protection ). The MRA Tool treatment surveys
must be capable of removing or inactivating those patho-
also include questions that address the need for continuous
gens.
monitoring equipment for both disinfection processes and
emphasized the need for additional treatment barriers in
turbidity to be connected to alarms. The risk scores assigned
the event that a groundwater source was identified as
to these questions take the type of alarm into account,
being GUDI based on user input regarding the source aqui-
whether it is an audible or visual alarm located at the treat-
fer or well construction.
None
of
the
selected
risk
assessment
tools
ment plant, or an alarm message sent to an on-call operator.
The multiple barrier approach should also be incorpor-
If the alarm message can be sent to an on-call operator at a
ated within the treatment barrier, such that there are
remote location, the MRA Tool assigns a risk score of zero
multiple processes capable of removing or inactivating the
to the survey question. The ability to immediately notify an
pathogens of concern. Both surface water systems D and E
on-call operator of a critical water system failure is particu-
had a single treatment barrier that was not capable of consist-
larly important for small water systems, which are unlikely
ently achieving the recommended log removal levels for
to staff the treatment plant 24 h per day.
pathogens. However, none of the risk assessment tools
Of the five small water systems, only groundwater
emphasized the need for a filtration process capable of
system A has continuous monitoring equipment installed.
removing pathogens to be installed prior to the disinfection
A continuous chlorine analyser is installed in the treatment
process in order to improve treated water quality and protect
building such that it can monitor the free chlorine residual
the health of water system users. As a result, the development
of water leaving the reservoir; however the monitoring
of new risk assessment tools should focus on incorporating
equipment was not in use at the time of the inspection. As
the multiple barrier approach, such that the need for robust
a result, the MRA Tool treatment survey question regarding
downstream treatment barriers is emphasized in the event
monitoring and alarms for disinfection equipment received
that the source barrier is vulnerable to contamination.
a relatively high risk score, similar to the results for surface
water systems D and E.
This study identified flaws in the B.C. Tool and MRA
Tool, which collected information regarding the presence
of a treatment process (i.e. the chlorination equipment at
groundwater system A) or a monitoring procedure (i.e. col-
CONCLUSIONS
lection of samples near the distribution system dead ends
at groundwater system B), but did not consider the how
Based on the five small water systems presented as case
well the barrier had performed or how appropriate the pro-
studies in this paper, the selected risk assessment tools had
cedure was for the water system. Poor operation and
varying levels of success in identifying hazards and
performance of treatment barriers that were capable of
343
I. M. Summerscales & E. A. McBean
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Evaluation of drinking water risk assessment tools
achieving pathogen removal has contributed to a number of
historical drinking water outbreaks, including the Walkerton
outbreak (Hrudey & Hrudey ). To accurately reflect
the current level of risk associated with a water system, a
risk assessment tool must consider the performance of the
water system barriers, not just the design of the barriers that
are in place.
ACKNOWLEDGEMENTS
This research was financially supported by a research grant
from the Canadian Water Network and the NSERC strategic
research program. The time and information provided by the
representatives of the small water systems that were visited
as part of this study was greatly appreciated. Denny RossSmith of the Small Water Users Association of British
Columbia was also incredibly helpful in selecting and
providing contact information for the small water systems
that were included in this study.
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