Stream to Science
Wolvercote Mill Stream is at the north end of Port Meadow, a large area of common land which borders the River Thames in the city of Oxford. It’s popular with swimmers, kayakers, canoeists and rowers all year round, but especially in the summer months and school holidays as the shallow sloping banks and barbecue areas are popular with families.
The site was only the second designated bathing water river site in the UK, and the first on the Thames. It was designated in April 2022 after an energetic community campaign including floating protests and citizen science testing for bacteria. In the three bathing water seasons since designation, the results have been mixed. In 2022, both E Coli and enterococci were poor, with high levels seen during the dry and hot months of July and August. In 2023, the higher readings were seen in September, which had some rainfall but not heavier than average. 2024 has not seen high temperatures, and all the sampling was completed before the exceptionally heavy rainfall at the end of September.
Although there are only three seasons’ data to go on, this indicates that both heavy rainfall and high temperatures can increase the levels of bacteria at the site. The heavier rainfall may increase the inputs of bacteria (from agriculture and sewage), whereas warmer temperatures help the bacteria to survive for longer.
Thames Water has plans to install UV treatment on three sewage works upstream of the bathing water site, but not until 2025 and 2026 (and these improvement deadlines have already been pushed back).
The Environment Agency and Thames Water investigated the reasons for high bacteria at the bathing water site in 2022 and 2023. They both found that water quality on the main Thames upstream and downstream of the site was “Good” for both bacteria. This suggests it’s local sources causing bathing water failures. A small brook which goes through farmland is contributing to the high enterococci readings.
