Particle trajectory modelling of Escherichia coli at beaches and shellfish beds along the south-west UK coast

Exposure to bacteria and viruses in fresh and coastal waters, whether through recreation or food, represents a potentially significant source of infectious diseases in humans. The reliance on in situ 'spot' sampling conducted at regular intervals at discharge locations and in bathing waters may not be sufficient to capture fecal contamination events. Hydrographic modelling of contamination risk from bacteria can provide high temporal and spatial simulation of particle trajectories. In this study we use in situ measurements with the Finite Volume Community Ocean Model (FVCOM) and a Lagrangian particle tracking model (PyLag) of Escherichia coli (E. coli) from point source discharge to bathing sites and shellfish farms along the south-west coast of the UK. In situ sampling transects were conducted from the Tamar and Plym Rivers and off-shore to Plymouth Sound from April to October 2024 for the analysis of E. coli cell counts. A forward Lagrangian trajectory model was used to track where the E. coli occurred from the rivers to the coast. During heavy rainfall on 11 April and 16 October 2024, and also during a dry period on 08 July 2024, significant numbers of E. coli were traced to local tourist beaches to both the east and west of Plymouth Sound. In addition, a large mussel farm in Lyme Bay, Devon was affected by elevated counts of E. coli in 2019 resulting in the downgrading of the classification of its offshore shellfish production site. We also used particle back-trajectory modelling, combined sewage overflow and river flow data to give insight into possible sources of the elevated E. coli counts at the mussel farm in May and October 2019. The backward trajectory Lagrangian particle tracking indicated that the potential pathway of E. coli contamination was from sewage discharge in local rivers. The results illustrated that rainfall and river volume alone are not sufficient to forecast trajectories of E. coli, but that surface current and direction are also necessary. The hydrographic modelling method to track fecal contamination is applicable to coastal waters and could enhance marine monitoring of these contaminants both in the UK, EU and globally.