Bacteria from sewage overflows are spiking on Devon’s beaches and shellfish beds on both wet and dry days alike, according to new research that undermines the long-held assumption that rainfall is the main driver of coastal water quality contamination.

Scientists from Plymouth Marine Laboratory (PML) tracked E. coli – a bacteria whose harmful strains are commonly found in untreated sewage – from source to sea across Plymouth Sound and Lyme Bay, and found that it is coastal currents – and not rain – that determine which beaches and shellfish beds bear the brunt of contamination.

Until now, rainfall and river volume following periods of heavy rain had been the key metrics used to forecast where E. coli contamination might end up. These findings now indicate that coastal current direction is just as necessary to calculate the true risk.

These are findings that land amid intensifying scrutiny of sewage discharges into UK rivers and coastal waters, as ageing infrastructure buckles under increasingly erratic rainfall driven by climate change. 

Water companies in England and Wales routinely release untreated or partially treated sewage from storm overflows. The Environment Agency recorded 291,492 spills totalling 1.87 million hours across England in 2025, with 46,164 spills totalling 407,006 hours in the south west region alone.

Water quality checks currently rely on periodic sampling rather than continuous monitoring, a system that can easily miss short-lived contamination spikes – only around 5% of exceedance events are thought to be captured by routine testing schedules.

Public warning, meanwhile, lean heavily on rainfall forecasts.

The study – now published in Marine Pollution Bulletin – combined in-situ water sampling with advanced computer modelling to trace E. coli’s journey from rivers and sewage overflows to the beaches and shellfish farms of Plymouth Sound and Lyme Bay – two areas that support a combined tourism, recreation and shellfish economy worth millions of pounds each year.

The results challenge the assumptions that are already baked into existing forecasting.

On July 8, 2024 – a dry day before any rainfall – E. coli counts at beaches around Plymouth were already elevated, well above the threshold for ‘sufficient’ bathing water quality. Across 13 years of data from three Devon river catchments, the team found no significant relationship between rainfall and bacteria levels at all: contamination spiked during both wet and dry periods alike.

Currents, instead, appeared to concentrate the bacteria at specific locations. When they flowed one way, E. coli from the River Plym was funnelled east onto popular tourist beaches. On other days, the same source reached the Breakwater and beaches to the west instead.

In Lyme Bay, the researchers traced two contamination events at an offshore mussel farm, in May and October 2019, back to probable sewage discharges via combined sewer overflows on the rivers Exe and Dart – more than 10 kilometres away. Both events were serious enough to trigger a downgrading of the site’s shellfish safety classification, costing the business thousands in lost revenue and reputational damage.

Lead author, Dr Gavin Tilstone, a Bio-optical Oceanographer at Plymouth Marine Laboratory, said: “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.”

Even beaches rated ‘excellent’ carry a measurable health risk – previous studies suggest 2.5 – 4% of bathers can still develop gastrointestinal illness at that standard, and coastal swimmers face higher rates of gut, ear, eye, and respiratory infections than non-swimmers generally.

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

The researchers are now calling for hydrodynamic and particle-tracking models to be built into a new generation of early-warning systems – one that would combine real-time sensors, satellite data and current and tide forecasting to give the public and shellfish producers a genuine, location-specific picture of contamination risk.

They caution, however, that further work is needed to isolate other possible contributors to elevated bacteria levels, including agricultural run-off and seabird guano.

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