Investigating the relationship between drinking water and gastroenteritis in Edmonton: 1993 - 1998

1: Introduction

1.1 Background

Contaminated municipal water supplies have been linked to numerous gastrointestinal outbreaks. Recent events in North America have highlighted the far-reaching impact of waterborne outbreaks on the community (Stirling et al, 2001; Grey Bruce Health Unit, 2000; MacKenzie et al, 1994). While outbreak investigations strive to estimate the health impact within a defined time interval, less is known about the effect of water quality on the endemic rate of illness within a community. Advances in analytical methodologies have enabled researchers to derive quantitative estimates of these impacts. By applying these techniques, several recent studies have identified links between water quality and endemic gastroenteritis (Aramini et al, 2000; Schwartz et al, 2000; Morris et al, 1998; Schwartz et al, 1997).

The current investigation is part of a series of national multi -centric studies being led by Health Canada to investigate risk factors and the burden of illness associated with drinking water. Since this investigation is an extension of the original study that was conducted in Vancouver (Aramini et al, 2000)², a detailed account of the historic background and rationale for this study is not repeated in this report. A comprehensive review of common waterborne pathogens and their associated risk to public health is also provided in the Vancouver report.

The Vancouver study identified strong relationships between drinking water quality (measured by the level of turbidity in the water received at home) and gastroenteritis. At the time of the investigation (1993-1998), filtration procedures were not part of the water purification process in Vancouver. The primary objective of this study was to apply similar methodologies to determine if Edmonton's municipal water supply influenced endemic gastroenteritis in Edmonton, between 1993 to 1998.

1.2 Edmonton's Water Supply

EPCOR Water Services Inc., which is a subsidiary of EPCOR Utilities Inc., supplies water to the city of Edmonton and its surrounding region. An extensive pipe distribution system spanning approximately 3100 kilometres enables access to more than 40 communities. Twelve reservoirs interspersed throughout the city have a total holding capacity of 808 million litres, which is equivalent to a three-day reserve supply. The majority of consumers within the city of Edmonton receive water within two days of leaving the water treatment facilities (per. comm., EPCOR). This distribution system is depicted in Figure 1.

Figure 1: Regional water supply in Edmonton

During the year 2000, EPCOR distributed water to 658,000 Edmonton residents, and 187,000 persons in surrounding communities . With a diverse portfolio, only half of the water consumption comes from residential and multi-family dwellings. Other consumer types include commercial, industrial, wholesale and regional users.

1.2 a) Water treatment plants

Edmonton and many of the surrounding communities in the region are serviced by two water treatment plants located within the city: the Rossdale plant and the E.L. Smith plant. Using Geographic Information Systems application software, (Arc View 3.2, Environment Systems Research Institute, Inc.), water service areas for each plant were digitally mapped, and are shown in Figure 2. Regions where the water source could not be uniquely identified are shaded in grey.

The Rossdale plant consists of two treatment facilities - the first was built in 1947, and the second in 1956. Combined, they have the capacity to treat 275 million litres of water per day. The E.L. Smith plant was subsequently built in 1976 to meet the demands of an expanding population. With a treatment capacity of 240 million litres per day, this latter plant was built away from the downtown core, upstream of the Rossdale facilities. Both plants draw in water from the North Saskatchewan River, which is within the North Saskatchewan River Basin.

Figure 2: Water service areas for the Rossdale and E.L. Smith plants, Edmonton.

1.2 b) Watershed characteristics

The North Saskatchewan River originates from the foot of the Columbia Icefields in the Rocky Mountains south-west of Edmonton, and flows directly through Edmonton. En route, the river is fed by numerous rivers and creeks.

In contrast to the watersheds that serve as Vancouver's water source, the North Saskatchewan River Basin is not protected from human and agricultural influences. Covering 28,000 square kilometres, this watershed contains mountains, forests, several communities and farm lands.

The upper half of the watershed is mountainous and forested, with little human activity. In contrast, the lower watershed is flatter, more inhabited, a nd supports agricultural activities. The watershed contains a thriving livestock industry, with approximately 290,000 cattle. It also supports an extensive wildlife population that includes deer, elk, beaver, and moose. Approximately 76,000 people reside in the upstream basin in a number of small towns and hamlets. Four continuous discharge sewage treatment plants and 16 sewage treatment lagoons discharge waste to the North Saskatchewan River upstream of the city of Edmonton. The continuous discharge sewage treatment plants utilise secondary treatment to process waste materials. There is limited industry in the upstream basin apart from forestry, and oil and gas extraction. This latter activity includes many pipelines that collect at refineries in Edmonton (per. comm., EPCOR).

The soils in the watershed are primarily glacial clays, and result in very high turbidities during high river flow situations. The mean yearly flow in the North Saskatchewan River is just over 200 m³ per second, but peak flows can exceed 800 m ³ per second. The river has stable flows during the winter season under ice cover, but is susceptible to rapidly changing flows during the spring season when snowmelt and spring run-off situations occur. Heavy rainfalls during the summer also increase the river flow rate (per. comm., EPCOR).

1.2 c) Water purification procedures

The Rossdale and E.L. Smith plants utilise a multi -step approach to treat water drawn-in from the North Saskatchewan River. The process begins with intake pipes that draw-in water from the deepest part of the river. Screens at the end of these pipes prevent debris and fish from being drawn into the system. Coagulant chemicals are mixed with the raw water to achieve flocculation. During sedimentation, the aggregate particles settle to the bottom, and the clear water above is passed on to the next stage. Lime is then added, and the precipitated hardness is removed in a further sedmintation process. Carbon dioxide is added to reduce the pH after softening. Chlorine is added to the clarified water for disinfection, then ammonia is added to convert the remaining free chlorine to chloramine. Then the water is filtered through an almost 1-metre deep bed of fine anthracite coal and sand (E.L. Smith) or monomedia sand (Rossdale). Activated carbon is added during runoff periods to reduce the presence of organ ic material and to improve the taste and odour of the water. At the end of this process, the water is pumped out into the city to the customer, either directly, or through additional storage reservoirs located across the city.

1.3 Study Objectives

The primary objective of this investigation was to determine if endemic gastroenteritis within the city of Edmonton was influenced by the municipal water supply between January 1993 to December 1998. Using the level of turbidity in finished water as the primary measure of drinking water quality, a significant association was identified with endemic gastroenteritis in Vancouver. At the time of that investigation, filtration was not part of the water purification process in Vancouver. Other investigators have also found similar relationships using similar methodologies (Morris et al, 1998; Schwartz et al, 1997).

Geographic differences between the two Edmonton water treatment plants with respect to their point of access on the North Saskatchewan River warrant comparison in the levels of illness among their respectively serviced populations. The E.L. Smith plant is located upstream from the Rossdale plant, which in contrast, is situated in the downtown core (Figure 2). While the E.L. Smith plant is unaffected by city storm sewer runoff, the Rossdale plants are susceptible to runoff from 80 stormsewers upstream. In December of 1997, the Rossdale intake pipe was moved away from the shoreline towards the centre of the river in an attempt to minimise the impacts of stormsewer run-off on the quality of raw water. Concurrent with this event was the introduction of the use of particle counters at the plant, which allowed much better optimisation of the filters to minimise the effect of filter-to-waste periods, filter spikes, and particle increases at the end of filter runs.

Another objective of this investigation was to compare and contrast the utility of various water quality indicators for identifying potential relationships with gastroenteritis. In addition to finished water turbidity, particle counts in finished water were examined. Raw water indicators, such as turbidity and coliform counts, were also considered.

Finally, the effect of environmental parameters including temperature and precipitation were assessed. Heavy rainfall has been implicated as an inciting factor in several waterborne gastroenteritis outbreaks (Curriero et al, 2001; Grey Bruce Health Unit, 2000). In Edmonton, run - off potentially carries animal waste and irrigation chemicals into the North Saskatchewan River, which may then compromise the water purification process.

Achieving the objectives listed for this investigation allowed for an assessment of the nature of the relationship, if any, between Edmonton's municipal water supply and endemic gastroenteritis.