E.coli levels at an Oxford river bathing site more than quadrupled in a month after sewage releases.
Data collected by the Environment Agency show climbing bacteria levels at Wolvercote Mill Stream, part of the Thames, following months of dry weather.
Levels of the harmful bacteria climbed from 300 colonies per 100ml in June to 1,800 in late July, higher than the recommended safe level for swimming, as water levels in the river dropped.
Testing is ongoing at the site which was designated as an inland bathing water spot earlier this year.
Low river flows can exacerbate any bacterial pollution in rivers, as well as reduce dissolved oxygen levels, making it more difficult for wildlife to survive.
Alarming algal blooms on Lake Windermere have worsened in recent months in hot weather.
Other potential sources of sewage
Claire Robertson, Thames21 Oxford rivers project officer, said: “We don’t really know where they’ve come from.
“There are other potential sources of sewage which aren’t just water company outflows - like people who have septic tanks who aren’t connected in properly to the sewage system.
“The Environment Agency is also doing a DNA analysis on the bacteria, so when we get the results of that we should know how much of it is a human source or an animal source.”
Thames21 thinks some of the pollution is coming from treated sewage releases from two Thames Water plants upstream, Cassington and Church Hanborough.
The group’s previous testing at another area just downstream from the Church Hanborough works had found E.coli at 144 times and intestinal enterococci at 24 times the safe level for bathing.
“In drought conditions there’s barely any river for it to get diluted into. Treated sewage is not treated for the bacteria - they have to comply on phosphates and nitrates and things like that.
“We’ve seen examples of where this effluent coming out looks like it’s not even being treated because it’s got such high levels of bacteria, even though officially it’s being treated,” she added.
Bathing waters around the coast and in rivers are tested for E.coli and another type of bacteria, intestinal enterococci, and given a status based on the cleanliness of the water.
The Environment Agency has been testing water at the site weekly between May and September, and it will be given a classification next year.
Local campaigners said they were unsure of the exact source of the bacteria, which could be coming from agriculture or sewage.
The latest readings, from August 9, suggest the concentrations of both bacteria are now below recommended levels, which are set at 1,000 for E.coli and 400 for enterococci.
Wolvercote Mill Stream is the second inland site to be designated as a bathing water.
The River Wharfe in Ilkley, Yorkshire, was the first to be designated in December 2020. Testing throughout 2021 revealed high levels of bacteria, prompting the Environment Agency to advise against bathing.
This week sewage has been released at coastal sites around the country after heavy rainfall overwhelmed sewer systems.
Real-time data on sewage dumps is more readily available for beaches than rivers because they are more likely to be designated bathing sites.
Thames Water has been contacted for comment.