Estimating health risks to water users: Marrying hydrodynamic models and risk models

Epidemiological studies show that discharges of treated human and animal wastes to receiving waters are associated with elevated health risks both to recreational water users and to consumers of raw shellfish. Those studies have relied almost exclusively on faecal indicator bacteria, indicating the potential degree of pathogenicity of the affected waters. However, many receiving waters are substantially different in character from those covered by epidemiological studies. To account for that, QMRA (Quantitative Microbial Risk Assessment) techniques, based on particular pathogens and their dose-response characteristics, are increasingly being used in New Zealand to directly assess the health impacts of wastewater schemes. They have been used to study not only existing infrastructure but also a number of possible alternatives and so are now well-regarded by commissioners hearing permit applications. For coastal discharges QMRA models must generally be married with hydrodynamic models. We present case studies from several outfalls to demonstrate the uses of QMRA, how these can inform understanding of potential health risk to water users, and discuss information/research gaps. Given that coastal QMRA exercises focus on the sources of contamination (and use hydrodynamic models to convey the contaminants to points of exposure), the need for source load minimisation is a natural outcome of the assessment.