Modelling microbial source pathways at the catchment scale

Globally there is strong advocacy and action towards achieving swimmable waterways; loosely labelled as the 'take the plunge' challenge. The benefits are far-ranging in terms of social well-being, healthy ecosystems and economic benefits to the local community. Consequently, a risk-based assessment framework is adopted in developing waterway management strategies that meet waterway health outcomes reflecting the community's environmental values and uses of waterways, whilst minimising risks to human health from pathogenic sources. As part of a risk-based assessment framework, integrated catchment-river modelling can help guide microbial water quality evaluation and target setting in order to meet regulatory recreational water quality objectives and to inform management strategies and policy development. A key challenge of these models is quantifying microbial source pathways, particularly from diffuse sources, and at a catchment-scale providing realistic concentrations, fluxes and loads, that are suitable as inputs for fine spatial-temporal scale hydrodynamic-biophysical receiving water models. This paper discusses the development and performance of a catchment-scale microbial fate and transport model, with particular focus on the representation of diffuse source inputs. The model has been applied to the Parramatta River Catchment, a heavily urbanised catchment that discharges to a coastal estuary in Sydney, Australia. The modelling framework combines wastewater and stormwater systems, cathcment-scale fluvial and esturaine process to simulate microbial fate and transport at a sub-daily temporal scale. The catchment model gives an indication of the degree of risk of microbial contamination from different sources, and has contributed to the evidence-base for the development of the Parramatta River Masterplan that aims to make the river swimmable by 2025.