The market town of Fairford, nestling in the Cotswold hills, is perhaps best known for its church, which has the only complete set of mediaeval stained glass windows in England.

But thanks to a more modern phenomenon, an interactive digital map produced by Thames Water, the Gloucestershire town, with its traditional honey coloured limestone houses, is becoming better known for its continuous, gushing, raw sewage overflow.

As of Monday morning the storm overflow from the town’s sewage works had been discharging diluted untreated effluent into the burbling and beautiful limestone River Coln for 745 hours in a stream of effluent that first began on 23 December last year.

For Andrew Doherty, a local resident and Liberal Democrat district councillor, the digital map of sewage discharges tells him nothing new. Standing on the banks of the Coln, opposite the overflow pipe which is pummelling diluted untreated effluent into the river for the 625th hour, Doherty said the problem was a long running one and was down to a lack of investment by Thames Water over many years.

“There is a serious backlog of work to do,” he said. “These are the headwaters of the Thames. This is literally where it all starts, here in the Cotswolds and yet this is what is happening. This is not something that should be happening in the third decade of the 21st century.”

The Coln, a limestone river once famous for its gin-clear appearance and abundance of trout, is not the only Cotswold river receiving continuous raw sewage discharges over hundreds of hours from Thames Water treatment works.

Andrew Doherty, district councillor, besides the River Coln
Andrew Doherty, a district councillor, pictured while diluted untreated effluent was being poured into the River Coln for the 625th hour, in a discharge that began 23 December 2022. Photograph: Graeme Robertson/Guardian

A few miles away, the Ampney St Peter treatment works has been pouring untreated effluent into Ampney brook for 656 hours since 20 December, according to the Thames map. It stopped briefly early last week, but started discharging again on Wednesday.

“Ten years ago this was a beautiful example of a wild trout stream,” said David Reinger, vice president of the Cotswold flyfishers. “But the double whammy of over abstraction by Thames Water in the summer, and winter storm discharges from Ampney works, probably the worst spilling works in this area, mean it is no longer.”

Public consciousness about the scale of Thames Water’s raw sewage discharges via storm overflows has risen sharply since the company this month launched its digital map identifying discharges as they take place.

Thames Water’s map was a response to pressure from campaigners such as Ash Smith, a water activist from the group Windrush Against Sewage Pollution, a few miles away in west Oxfordshire, and latterly to the Environment Act, which has imposed a new duty on water companies to publish near real-time information on the operation of storm overflows.

Thames Water’s real-time Sewage Release Map, on 11 January 2023
Thames Water’s real-time Sewage Release Map, on 11 January 2023, showing logged sewage flows into waterways in the region controlled by the company. Photograph: Leon Neal/Getty

Overflows in the sewage system exist to stop effluent backing up into properties in exceptional circumstances, such as during extreme rainfall. But evidence from activists like Smith, and the academic Peter Hammond, have shown water companies are routinely using storm overflow discharges in their water management.

Data from the water companies showed that in 2021 they discharged raw sewage via storm overflows into rivers 372,533 times, over 2.75m hours.

The overflows into the Coln and other Cotswold rivers continued into last week, days after the end of a period of heavy rain. This is because, Doherty explained, the groundwater ingresses into the sewage pipes as the water table rises and the overflows continue operating for days after the rain ends.

“There are long running sewerage pipes everywhere and it’s through those that the water is coming in,” he said. “You can line pipes and seal pipes but it costs money to do that and we don’t tend to get a lot of investment in an area like this because we are a very rural, relatively small, population.”

Dry spills, which take place when excessive rain has stopped, are not permitted under the Environment Agency rules. A common cause of dry spills is groundwater infiltration through cracks and joints in the pipes transporting wastewater, said Hammond.

His evidence that water companies in England are responsible for at least 10 times more illegal sewage spills than the agency was aware of sparked an investigation by Ofwat and the EA into potentially illegal dumping of raw sewage by six water companies, including Thames.

For Smith, an angler and retired police detective, the new Thames Water map provides welcome transparency but changes nothing. “It has created a lot of interest in what is happening, it’s been remarkable. It should put pressure on the more opaque water companies to do the same. But nothing has changed, the rivers are still being used as a toilet.”

At Witney, a town on the River Windrush, the hundreds of hours of continuous raw sewage spills since the end of last year have been starkly exposed by the Thames Water map. As of Wednesday morning the treatment plant in Witney had dumped diluted raw effluent into Colwell brook, which feeds into the Windrush, for 607 hours and 20 minutes, in a spill which began on 28 December.

Dipping an underwater camera into the stream, Smith points out the sewage fungus covering plant roots, or breaking away and flowing down the cloudy brook. Under a footbridge near the entrance to Witney Lake and Country Park, the smell of sewage is distinct.

“This has been a healthy stretch of water,” said Smith. “It had a fish kill in 2016 when 1,700 fish were killed in here, so this has been a healthy piece of water. Today there aren’t 1,700 fish here to kill any more, so the pollution happening now is not classified as a serious pollution event.”

Thames Water said: “Our current investment programme, which is fully funded, will deliver major increases in treatment capacity at many of our sites. This includes an £8.5m upgrade of Witney sewage works, which has started and will increase the site’s treatment capacity by 60%, with completion expected to be in early 2024. Fairford and Ampney St Peter sewage works will also receive significant increases to their treatment capacities.

“We are absolutely committed to protecting and enhancing our rivers and the communities who love them, and we want to make these discharges of diluted sewage unnecessary as quickly as possible.

“We are the first company to provide these alerts for inland waters … We want to lead the way with this transparent approach to data, and the alerts will be available through an open data platform for third parties, such as swimming and environmental groups, to use.”