The Environment Agency has confirmed it is investigating a “potential pollution incident” in Church Hanborough near Witney and has asked anyone with any information to get in touch.
A spokesperson said: “We received a number of reports of a potential pollution incident near the Church Hanborough area, and our officers attended the site.
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“We cannot comment further as this is an ongoing investigation. We will not hesitate to take enforcement action if appropriate.
“Anyone who witnesses pollution or an environmental incident is encouraged to report it to our 24-hour incident hotline on 0800 80 70 60.”
The waterway – which has a public footpath going across it – was first reported to be polluted in early April when the Evenlode Catchment Partnership attended the site.
The area is next to the Church Hanborough Sewage Treatment Works, which has discharged untreated sewage for over 2,100 hours across the past six months, though not in April.
The Partnership reported high levels of coliform bacteria, at least 17 times above the quantity deemed acceptable, which they said indicated a pollution incident.
The campaigners’ project manager Ann Berkeley said: “We should not have to accept this level of dangerous faecal contamination of our rivers from Thames Water.
“And this right by a heavily used footpath.
“It is time for the real Chris Weston [chief executive of Thames Water] to stand up and take control of this public health emergency.”
Following this, parts of the Hanborough Stream – which feeds into the Evenlode – began to turn brown, after which Thames Water sent a team.
A spokesperson said: “We are aware of an incident on the watercourse near Church Hanborough and on 10 April reported this to the Environment Agency as we take these matters extremely seriously.
“Our teams deployed containment booms along the watercourse to limit the impact to the watercourse.”
A member of the ECP who was at the stream, described a “disgusting pile of dirty foam, a filthy river and even more foam”.
He added: “Thames Water who say all sewage spills are unacceptable to us is expecting me to walk my little dog across that filthy river. I don’t think so.”
The group was keen to stress that this comes from treated not untreated sewage, and suggested that nocardia bacteria gatherings had formed.
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Nocardia thrives in environments rich in organic matter and it can be dangerous to human health, causing a serious, slow-developing infection known as nocardiosis.
Thames Water did not confirm whether they felt the path by the stream remained safe.
Meanwhile the Partnership urged for a move to public ownership of water and asked members of the public to sign an online petition on this subject, launched by fellow campaigners Windrush Against Sewage Pollution.
This comes after a month ago the Evenlode group handed back money given to them by Thames Water citing a “betrayal of trust”.
