Swimming in urban estuaries: Understanding stormwater contamination events and recovery from historical data

Cities globally are responding to public demand for water-based recreation by transforming urban rivers and estuaries into swimming destinations. Historical water quality compliance records may help diagnose contamination and recovery dynamics and serve as a valuable reference point for assessing the suitability of proposed urban swimming sites. This study analysed historical bathing water compliance data and concurrent hydrometeorological records to characterise long and short-term contamination and recovery patterns at current and proposed bathing sites in Sydney's Parramatta River estuary. Generalised Linear Mixed Modelling (GLMM) was used to estimate site-specific recovery parameters and explore environmental drivers of variability in the faecal indicator bacteria (FIB) enterococci. Recovery times to health-relevant thresholds ranged from 3 to 125 hours, with background levels reached between 10 to 142 hours depending on initial contamination. Marine-dominated bathing sites were temporarily diluted to near-freshwater conditions following rainfall, as indicated by reductions of up to 97% in baseline salinity. A 10 mm rainfall threshold was identified as a practical trigger for event-based sampling. Salinity and solar exposure were the most influential environmental factors affecting recovery rates. This study demonstrates how routine, compliance datasets can be repurposed to derive short-term, site-specific recovery metrics, quantify stormwater intrusion using a practical salinity proxy (RSR%), and establish actionable rainfall triggers for event-based monitoring, providing an evidence-based blueprint to inform risk management for emerging urban bathing sites when high-frequency data are unavailable.