Three-dimensional Reservoir Modelling and Quantitative Microbial Risk Assessment of Recreational Access to a Drinking Water Reservoir

The major drinking water reservoirs and catchments supplying many of Australia’s capital cities have been protected from significant levels of public access since their construction (up to 140 years ago). In addition to the primary intended initial benefit, (protecting drinking water quality to prevent typhoid and similar disease outbreaks), additional benefits included reduced flooding, improved and more stable yield, reduced water treatment costs, and protection of native ecosystems. In relation to the latter benefit, over that period, much of the Australian landscape has been modified for various forms of development and recreational activities, leaving these water catchments as some of the last broad areas of remnant habitat for vulnerable and endangered ecosystems and species. Despite these widely appreciated and well-understood benefits, there has been continuous pressure from a diverse range of interest groups to open these areas. As government-owned organisations, the Australian water utilities consider all such requests from the community. Among the interest groups that make representations to access drinking water sources are a wide range of recreators. Pressure from such groups is increasing as populations grow. To help inform decisions on how to respond to such representations, this study predicted gastrointestinal disease burdens from recreation on a currently protected drinking water supply reservoir in Australia. This study considered a range of scenarios, described in terms understandable to the community, and predicted health implications using screening-level quantitative microbial risk assessment (QMRA). The assessment was limited to microbial risks to drinking water quality – risks from chemical or physical hazards were not considered, nor was ecosystem protection. The QMRA predicted that six reasonably foreseeable scenarios could result in microbial risks that exceeded the health-based target benchmark given in the Australian Drinking Water Guidelines (ADWG). Therefore, additional water treatment would be required to reduce those risks to acceptable levels. However, even with the introduction of additional treatment, permitting increases in the levels of recreational activity in the source water was found to be inconsistent with many of the Guiding Principles of the ADWG and with a landmark state Supreme Court planning decision that had interpreted how those principles should be applied in drinking water catchments. Therefore, the results did not support permitting recreational access to the reservoir, and the importance of source protection was reinforced.