The only official swimming spot on the Thames has failed bacteria safety tests, prompting calls for Thames Water to upgrade treatment systems.
Testing at Wolvercote Mill Stream by the Environment Agency indicated both E Coli and Intestinal Enterococci are present at levels higher than it is safe to swim in. These bacteria typically get into water from sewage and animal manure.
The environmental charity Thames21 is calling for Thames Water to upgrade its Cassington sewage treatment works to 'stop this from happening again'.
Claire Robertson, Oxford Rivers Project Officer at Thames21, said: "We are disappointed, but not surprised, by these results at Wolvercote, given that Thames Water released untreated sewage into the rivers around Oxford for 5,600 hours in 2021 and given our results from citizen science testing last year.
"However, we are surprised that Thames Water has presented no firm plans to upgrade sewage treatment which affects swimmers on this stretch of river. There are only four more years to achieve a "sufficient" or higher designation at Wolvercote, otherwise the site may be de-designated.
"However, Thames Water states that it plans to halve raw sewage spills by 2030. This is not nearly fast enough for the swimmers and paddlers at Wolvercote, many of them families with young children, nor for the wildlife and plants that call this part of the Thames their home. We need action now."
Water samples were taken every week between May and September and can be found here.
In April, the Oxford-based river became the second inland river in the UK to achieve Designated Bathing Water Status.
According to the Environment Agency's Bathing Water Profile, Wolvercote Mill Stream has not yet been given a water quality classification.
Richard Aylard, Sustainability Director at Thames Water, said: "Taking action to improve the health of rivers is a key focus for us.
"All sewage discharged from Cassington STW since April 2021 has been fully treated and while the sewage treatment process is not designed to remove bacteria, we recognise the concerns raised regarding the presence of bacteria in the river which can come from multiple sources.
"As a member of the Oxford Rivers Project, who made the application for designated bathing water status at Wolvercote Mill Stream, we'll continue to work with our partners to understand what more needs to be done to ensure consistently good water quality in the Thames.
"We have committed to reducing the annual duration of sewage discharges into rivers by 50% across the Thames Valley by 2030 and have planned substantial investment in our local sewer network to reduce the need for untreated discharges, including a major expansion at Witney sewage treatment works in Oxfordshire.
"After a successful trial in the Oxford area, we're finalising the provision of live alerts from all 468 permitted locations across our region, by the end of 2022."