Efforts intensify to improve water quality at Wolvercote Mill Stream bathing spot

• Thames Water has invested more than £4 million in new Ultraviolet Light (UV)  treatment technology at Cassington sewage treatment works.
• Extensive investigations indicate no single source is solely responsible for Wolvercote Mill Stream's poor bathing water classification.
• Thames Water, the Environment Agency and the local catchment partnership continue work collaboratively to better understand and improve water quality at the site.

Thames Water has invested more than £4 million in new ultraviolet (UV) treatment technology at Cassington sewage treatment works, as efforts continue to improve water quality at the Wolvercote Mill Stream bathing spot.

The investment, alongside several years of detailed investigations by Thames Water, the Environment Agency (EA) and the local catchment partnership, forms part of ongoing collaborative efforts to better understand and address the factors affecting water quality at the Oxfordshire site.

Wolvercote Mill Stream was designated as a bathing water in 2022 following an application from Oxford City Council, Thames21, the Rivers Trust and Thames Water. Since then, the bathing water has been classified as poor year on year, despite monitoring showing water quality can vary significantly throughout the season.

To support potential improvements, Thames Water is commissioning a new UV disinfection system at Cassington sewage treatment works, a Thames Water facility that is approximately 3.5km upstream of the bathing water. Operational since May 2026, the technology neutralises bacteria and other pathogens before treated wastewater is returned to the environment.

Investigations carried out since 2023 have included extensive catchment sampling, scientific studies and computer modelling. Findings indicate that Cassington sewage treatment works has the potential to impact the bathing water but is not the only contributing factor. 

Instead, results suggest other pressures closer to the Wolvercote Mill Stream are having a more significant impact to these intermittent inputs, such as agriculture or urban run-off, which can be difficult to identify and capture through routine monitoring.

Paul Hampton, Healthy River Community Manager at Thames Water, said: 

"We've spent several years investigating the factors affecting water quality at Wolvercote Mill Stream and have undertaken extensive sampling, modelling and monitoring across the catchment.

"While these investigations have not identified a single cause for the bathing water's classification, they have improved our understanding of the range of pressures affecting the site. That's why we've also invested more than £4 million in UV treatment at Cassington sewage treatment works to help move the bathing water towards an improved classification.

"We will continue working with the Environment Agency, Oxford City Council and local stakeholders to better understand the remaining sources of bacteria and identify where further improvements can be made."

Professor Jeremy Biggs, Chief Executive Officer at Freshwater Habitats Trust said:

"As catchment host for the Ock Catchment Partnership, Freshwater Habitats Trust welcomes Thames Water's investment in upgrading its works to improve water quality at the Wolvercote Mill Stream bathing water site. We look forward to continued collaborative work with Thames Water and other partners through a catchment-based approach to address the complex and interconnected causes of water pollution, both here and across the wider Ock catchment. The Ock Catchment Partnership is committed to bringing partners together to deliver practical, evidence-based action that benefits both people and the freshwater environment." 

Thames Water, the Environment Agency, and the local catchment partnership will continue to review monitoring results throughout the 2026 bathing season and work together to better understand the factors influencing water quality at Wolvercote Mill Stream.