THE River Foss 'absolutely reeked of sewage' at the time fish started dying last week, a top York scientist says.

Prof Alistair Boxall and a team from the University of York's environment department were out taking water samples in The Foss on Tuesday June 20 as part of an ongoing, unrelated research project just before the first fish deaths were reported.

They took samples at several points along the river - including at Sheriff Hutton, Strensall and New Earswick, as well as nearer to the city centre.

They reached the New Earswick sampling point at about 4pm. "And it absolutely reeked of sewage," Prof Boxall said.

As reported in The Press, dead fish were seen floating on the surface of the river at various locations through the city.

York Press: Dead fish floating in the River Foss last weekDead fish floating in the River Foss last week (Image: Howard Jones)

Yorkshire Water said they had visited the Foss and found no sewage in the water.

The Environment Agency, meanwhile, told The Press that similar fish deaths had been observed in rivers across the country - and that they were caused by a combination of high temperatures and thunderstorms creating a lack of oxygen in the water.

But Jenny Kent and Kate Ravilious, City of York Council's joint executive members for environment and climate emergency, say that having seen the initial analysis by Prof Boxall and his team, they DO now believe the fish deaths could have been caused by pollution.

They say the water at New Earswick water contained nitrate levels 80 times higher than further upstream at Sheriff Hutton - and that levels of nitrite and ammonia were also elevated.

Prof Boxall confirmed that they had found high nitrate and ammonia levels in the water at New Earswick.

"There was definitely sewage in the river," he said.

He said the final clincher would come when the result of tests for levels of paracetamol in the water came through.

Paracetamol is easily removed from water in the sewage treatment process, he said - so high levels indicate that sewage has been discharged directly into the water, by-passing the normal treatment process.

He says he believes that sewage systems may simply have been overwhelmed by sudden heavy downpours of rain - so that sewage was diverted directly into the Foss.

That would be a sign that the system was actually working, he conceded: the UK sewer system includes 'combined sewer overflows' - in effect safety valves that operate as overflow outlets to reduce the risk of sewage backing up during heavy rain. But it does mean there would have been raw sewage in the Foss.

Cllr Kent and Cllr Ravilious say they are now awaiting the results of the paracetamol tests - and that if these confirm raw sewage was in the water, they will be seeking urgent meetings with both Yorkshire Water and the Environment Agency.

Cllr Kent said: “This is obviously a greatly concerning incident.

"We want to identify the cause and see what can be done to prevent a re-occurrence."

Cllr Ravilious added: “This indicates that the discharge of untreated sewage cannot be ruled out as a cause for the fish deaths.

"And of course this incident won’t just have impacted fish; it will have damaged the entire river ecosystem."

An Environment Agency spokesperson said the organisation was continuing to investigate 'a number of fish deaths and fish in distress incidents across the country, including in Yorkshire'.

The spokesperson insisted that the agency still believed the 'recent event' on the Foss was the result of low dissolved oxygen 'due to a combination of the high temperatures ...and the low atmospheric pressure caused by thunderstorms'.

But they added: "Our officers are investigating at multiple locations to rule out any other causes such as pollution."

A spokesperson for Yorkshire Water said: "We’ve supported the Environment Agency in their investigations into fish deaths. We are not being investigated in relation to these incidents.”