New Plymouth Marine Laboratory research suggests Devon sewage pollution monitoring may miss short-lived E. coli contamination events, with coastal currents proving as important as rainfall in determining which beaches and shellfish farms are affected.
The study, published in Marine Pollution Bulletin, combined in-situ water sampling with hydrodynamic and particle-tracking models to trace potential pathways of Escherichia coli from rivers and sewage overflows into Plymouth Sound and Lyme Bay.
Researchers found elevated bacteria levels on both wet and dry days, while analysis of 13 years of monitoring data showed no significant relationship between rainfall and E. coli concentrations across the three Devon catchments studied.
The findings suggest that systems relying heavily on rainfall forecasts and periodic sampling may fail to identify some short-lived pollution events.
Devon sewage pollution monitoring may miss contamination spikes
Plymouth Marine Laboratory said routine monitoring schedules captured only around 5% of exceedance events identified within the datasets analysed by the research team.
Periodic spot sampling can miss short-lived increases in bacterial contamination, particularly where conditions change rapidly between scheduled tests.
The study adds to a wider shift towards continuous water quality monitoring in rivers and coastal waters. H2O Global News has previously reported on continuous monitoring systems being deployed on UK rivers to provide utilities with more frequent environmental data.
According to the Environment Agency, England recorded 291,492 monitored storm overflow spill events in 2025, with a total duration of around 1.87 million hours.
Rainfall alone failed to explain E. coli levels
The research team sampled the River Tamar, River Plym and Plymouth Sound monthly throughout 2024 and combined the results with longer-term monitoring data.
A hydrodynamic ocean model, FVCOM, was paired with the PyLag particle-tracking model to simulate how material released into rivers could move once it reached coastal waters.
The modelling accounted for tides, wind and the direction and speed of surface currents.
On 8 July 2024, a dry day before rainfall, E. coli concentrations at beaches around Plymouth were already elevated and above the level considered sufficient bathing-water quality.
Analysis of 13 years of data from the Plym, Dart and Exe catchments also found no significant relationship between mean rainfall and recorded E. coli levels, with elevated concentrations occurring during both wet and dry periods.
Modelled E. coli concentration pathways show how coastal currents can affect where contamination reaches beaches and shellfish farms. Figure: Plymouth Marine Laboratory
Coastal currents changed which beaches were affected
The particle-tracking modelling indicated that surface-current direction could strongly influence where contamination accumulated after entering the sea.
Under some current conditions, material originating from the River Plym was directed east towards beaches including Jenny Cliff, Bovisand and Wembury.
Under different conditions, the same source was modelled towards Plymouth Breakwater and beaches further west.
Lead author and Bio-optical Oceanographer at Plymouth Marine Laboratory, 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.”
Researchers found no significant relationship between mean rainfall and E. coli concentrations across the Plym, Dart and Exe datasets. Figure: Plymouth Marine Laboratory
Modelling links contamination events to offshore shellfish farm
In Lyme Bay, researchers used backward particle tracking to investigate two contamination events affecting an offshore mussel farm in May and October 2019.
The modelling identified probable pathways from combined sewer overflow discharges associated with the Exe and Dart catchments more than 10km away.
The events were linked with a downgrade in the shellfish site’s safety classification, affecting the business commercially.
However, the authors stress that the modelling provides a probabilistic indication of contamination pathways rather than direct real-time source identification.
Other potential contributors to elevated E. coli levels, including agricultural run-off and seabird faeces, require further investigation.
Researchers call for more integrated early-warning systems
The study recommends combining hydrodynamic and particle-tracking models with real-time sensors, satellite observations and current and tide forecasts to provide more location-specific pollution warnings.
Dr Tilstone added:
“The current monitoring approach, based on occasional spot sampling and rainfall alone, is not capturing the full picture.”
The researchers say such systems could provide more timely information for bathers, regulators and shellfish producers than rainfall-based warnings alone.
The full study, Particle trajectory modelling of Escherichia coli at beaches and shellfish beds along the south-west UK coast, is published in Marine Pollution Bulletin.
Further information is available from Plymouth Marine Laboratory.
For more research and monitoring developments, visit H2O Global News’ Water Quality coverage.
Source: Plymouth Marine Laboratory
