Study finds rainfall “a blunt proxy” for E.coli risk in coastal waters

(by Karma Loveday)

Rain alone is a poor predictor of when and where bacterial contamination will hit coastal waters. 

That’s according to research by Plymouth Marine Laboratory, published in the Marine Pollution Bulletin. The findings challenge assumptions built into existing water quality forecasting, and suggest there is untracked risk to public health from swimming/recreation and shellfish production. 

The work combined water sampling with advanced computer modelling to trace the journey of E. coli from rivers and sewage overflows to the beaches and shellfish farms of Devon’s Plymouth Sound and Lyme Bay – two areas that support a combined tourism, recreation and shellfish economy worth millions of pounds a year.

The study team sampled water monthly from the River Tamar, River Plym and Plymouth Sound throughout 2024, then used a hydrodynamic ocean model paired with a particle-tracking model to simulate exactly where bacteria released into rivers would travel once they reached the sea. This factored in tides, wind and, critically, the direction and speed of surface currents.

The modelling in the study revealed that coastal currents concentrated E coli at particular locations. Lead author Dr Gavin Tilstone commented: “Rainfall and river volume alone are not sufficient to forecast where E. coli contamination will end up, and coastal current direction is just as necessary. We found instances of poor water quality outside the official bathing season and even during dry weather, at a time when people are still swimming in these waters.”

He added: “The current monitoring approach, based on occasional spot sampling and rainfall alone, is not capturing the full picture.”

Plymouth Marine Laboratory recommended that hydrodynamic and particle-tracking models should be integrated into a new generation of early-warning systems, combining real-time sensors, satellite data and current/tide forecasting. It said: “This would give the public and shellfish producers a genuine, location-specific picture of contamination risk, rather than relying on rainfall as a blunt proxy.”