Water quality at Wolvercote Mill Stream
We've been investigating the river water quality at Wolvercote Mill Stream, in Oxford. This is to understand our impact on the condition of the bathing water.
Wolvercote Mill Stream was the second inland river in the UK to get designated bathing water status. This means people can check the water quality data every week in the bathing season (May to September). The Environment Agency (EA) shares this information with the public.
At the end of the season, the water is graded based on how clean it's been over the past four years. Bathing waters can be classified as excellent, good, sufficient, or poor.
Background
Since designation, the bathing water at Wolvercote Mill Stream has been classified as poor. We've been working with the EA to better understand the causes of the elevated faecal bacteria – E. coli and Intestinal enterococci.
We've been assessing the impact of our assets on the water quality. Plus how future investment may support improving its classification.
Our investigation
We've investigated the water at Wolvercote Mill Stream and the catchment upstream of it. Our main initiatives included:
Water quality testing
Our sampling for faecal indicator organisms has been extensive. We’ve taken hundreds of samples on the main river and on smaller streams, with a focus on Dukes Cut and Kingsbridge Brook. We tested some points further up the Thames catchment, too. Indications are that the greatest impacts on the bathing water are from within the Wolvercote Mill Stream.
Microbial source tracking
Some of the samples have also been used for microbial source tracking (MST). This technique uses microbial DNA to identify the origin of the faecal bacteria. This could come from human or wildlife species.
Bacteria modelling
We've been working with scientists from the UK Centre for Ecology and Hydrology. This is helping us understand how bacteria survive in the river under different conditions. And also to learn the likely rate of die-off once it’s left our treatment works. This could be in the treated final effluent or as storm overflow. Together we've replicated a river environment under lab conditions. We've also conducted experiments in the River Thames. This helped the development of a bacterial sub-model. It's based on the mathematical Questor river network water quality model.
Site walkovers
Site walkovers helped assess other local factors that may impact the water quality. These could be:
- Misconnected or blocked sewers or surface water pipes
- Agricultural or industrial inputs
- Private sewage systems not connected to the wastewater network
We also used drones to reach some more inaccessible areas.
What we learned
Our evidence indicates that there is potential for Cassington STW to impact the quality of the bathing water. However, wider catchment sources of bacteria in addition to Thames Water assets will also need to be identified and tackled.
Next steps
In 2026, we are currently installing and testing ultra violet (UV) treatment at Cassington STW. This new technology will reduce the bacterial load in the treated effluent before it joins the River Thames.
We're also planning to meet new government targets for storm overflows within 5km of bathing waters. This includes improvements at the Cassington sewage treatment works.
View the latest bathing water quality profile for Wolvercote Mill Stream, identified by the Environment Agency.