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

Executive Summary

The risk of microbial disease associated with drinking water is presently a priority concern among North American water jurisdictions. Numerous past outbreaks, together with recent studies suggesting that drinking water may be a substantial contributor to endemic (non-outbreak related) gastroenteritis, demonstrate the vulnerability of many North American cities to waterborne diseases. These findings have fuelled ongoing debates in Canada and the United States, and highlight the need for stricter water quality guidelines, changes in watershed management policies, and the need for additional water treatment.

EPCOR Water Services Inc. supplies water to the city of Edmonton and its surrounding region through two water treatment plants. The Rossdale plant has been upgraded several times since its initial construction during the 1940's, while the E.L. Smith treatment plant was built in 1976. Both plants draw raw water from the North Saskatchewan River, and utilise clarification, softening, recarbonation, filtration, and disinfection in the water treatment process. Downstream of the E.L. Smith plant and upstream of the Rossdale plant, are 85 stormsewers that discharge into the North Saskatchewan River. Covering 28,000 square kilometres, the North Saskatchewan River watershed has many potential points of pathogen introduction: recreational sites, treated wastewater discharge from upstream communities, private septic systems, and agricultural operations. An event of particular interest in this study, was the relocation of the Rossdale raw water intake site on December 10, 1997. The site was moved towards the centre of the river in an attempt to improve raw water quality. Also concurrent with this event was the introduction of particle counters at the Rossdale plant, which allowed for the optimisation of filter performance.

The primary objective of this investigation was to determine if endemic gastroenteritis among Edmonton residents was influenced by the municipal water supply between 1993 to 1998. Several analytic techniques were used to achieve the study objectives. Multi variate logistic regression using generalised linear (GLM) and generalised additive models (GAM) was used to determine if differences in risk for endemic gastroenteritis (as assessed by hospital admissions, emergency room visits, physician visits, and long-term care visits) existed among the water service areas (Rossdale vs. E.L. Smith), both prior and subsequent to the implementation of changes at the Rossdale plant on December 10, 1997 (relocation of the intake pipe and the introduction of particle counte rs).

A time series approach using generalised additive models (GAM) was used to investigate and quantify the temporal association between gastrointestinal-related health outcomes on a specific day (as assessed by hospital admissions, emergency room visits, physician visits, and long-term care visits) and environmental and water quality parameters (primarily finished water turbidity). Observed values for these parameters from 0 to 40 days prior to a health outcome event were assessed for their impact on the measured health outcomes. In addition to daily readings of finished water turbidity, the utility of particle counts data were assessed, together with raw water indicators including turbidity and coliform counts. Environmental parameters, including daily precipitation and maximum and minimum temperatures, were also investigated.

As anticipated, descriptive analyses and summary statistics of the raw water parameters indicated better quality of water entering the E.L. Smith plant in comparison to the Rossdale plant, both before and after December 10, 1997. These differences are most pronounced in daily total and faecal coliforms. Following December 10, 1997, the differences in raw water parameters between the plants are less prominent. Of primary interest for the time series analysis was the daily finished water turbidity. Prior to December 10, 1997, finished water turbidity values (mean and median) were slightly lower for the E.L. Smith plant. Following December 10, 1997, finished water turbidity values were equivalent between the two plants. Mean and median particle counts from the E.L. Smith plant were marginally lower compared to those of the Rossdale plant. However, particle count data were only available for 1998. Throughout the period of investigation (1993-1998), finished water turbidity levels for both plants were well within federal (Health Canada, 1996) and provincial (Alberta Environment Protection, 1997) drinking water turbidity guidelines. Results of the multivariate logistic model which fit a categorical variable for water source (GLM), suggest a slight decrease in the risk of gastroenteritis among residents supplied with Rossdale water following the changes that were implemented on December 10, 1997. The interpretation of the logistic regression results should, however, be made with caution as the magnitude of the effects were relatively small (odds ratios near one). Although it is plausible that this effect was due to the improvements at the Rossdale plant, other combinations of events could have resulted in a similar relationship. An obvious spatial pattern in gastroenteritis risk with respect to water plant service area was not identified using spatial regression models (GAM). This latter observation further underlies the tenuous nature of this causal hypothesis.

Given the descriptive findings, and the results of the multivariate logistic modelling, time series analysis was restricted to households supplied by the Rossdale water treatment plant prior to December 10, 1997. It was anticipated that if a temporal relationship between daily water quality and gastroenteritis existed, it would be most likely identified in this subset. Nevertheless, no significant lags were identified between daily finished water turbidity and gastroenteritis among Rossdale serviced residents (prior to December 10, 1997) using time series analysis. Lagged water quality parameters from 0 to 40 days prior to health outcome events were assessed for statistical significance and biological relevance using a time series approach. Finished water turbidity provided the best fit to the data. None of the raw water nor environmental parameters were adequate predictors of health in the final model. Similar to the spatial analysis, seasonal and temporal effects were adjusted for use in the final model. Despite limiting the analysis to the water supply posing the greatest potential risk, the time series analysis failed to identify temporal relationships between the finished water turbidity from the Rossdale plant prior to December 10, 1997 and endemic gastroenteritis.

In comparison to the results of the Vancouver time series study¹, evidence in the present study suggests very little impact, if any, of drinking water on the level of endemic gastroenteritis among Edmonton residents. Despite markedly poorer raw water quality in Edmonton compared to Vancouver during the respective study periods, no significant temporal relationships were identified between water quality parameters and the risk of gastroenteritis. The lack of a temporal relationship supports the opinion that the high quality of Edmonton's drinking water adequately minimises the risk of waterborne endemic gastroenteritis. Furthermore, the lack of a significant relationship between raw water turbidity and faecal coliform counts and endemic gastroenteritis supports the effectiveness of the treatment processes utilised.

Under the multivariate logistic regression model, the suggestion of an overall decrease in risk of endemic gastr oenteritis among Rossdale serviced residents following improvements implemented at the plant on December 10, 1997 is consistent with the observed improvements in raw and finished water quality parameters. The lack of an obvious spatial relationship between endemic gastroenteritis and water service area (using spatial regression) together with the lack of a significant temporal relationship between endemic gastroenteritis and finished water turbidity (using time series analysis), suggest that this apparent decrease in risk is minor, and the relationship tenuous. Nevertheless, the lack of an obvious decrease in the risk of endemic gastroenteritis following the improvements at the Rossdale plant despite marked improvements in raw water quality, together with the lack of an obvious spatial relationship among the service areas, further supports the effectiveness of the operation and treatment processes utilised in Edmonton.