Microbial exposure assessment of an urban recreational lake: a case study of the application of new risk-based guidelines.

New WHO and Australian guidelines promote a risk-management approach for minimising exposure to pathogens in recreational waters. Between 2003 and 2005, they were applied to Lake Parramatta (10 ha, 450 ML), a potential recreation site in Sydney, Australia. A three stage approach was developed involving (1) initial suitability assessment using historic data, (2) revised suitability assessment based on new data and (3) characterisation of hazardous (especially wet weather) events. Contrary to the stage 1 suitability classification, stage 2 baseline data indicated that during dry weather the lake had water quality sufficient for primary contact recreation (95th percentiles for enterococci = 19 MPN/100, n = 50) and the major pathogen source was wildfowl. Guideline principles provided a rationale for collecting microbiological and geographic data needed to understand local cycles of lake contamination/recovery. The concept of hazardous events was particularly useful. Studies of stormwater events led us to identify a transition point (> 10 mm rainfall in 24 h) where human-faecal pathogen risks increased and access needed to be controlled. Together baseline and event data yielded operational tools (i.e. event detection methods, action triggers, auditing criteria, remediation priorities) for minimising bather exposure.