The group asked Welsh Water for pump performance data from the nearby pumping station.
They are, according to Mr Cooper, supposed to pump at 120 litres a second but had only been operating at “95 or 96”.
“That means 20% of what should have gone for treatment was going straight into the river,” he said.
“We calculated over 12 months, and it averages out as 17 tankers a day emptying into this river.
“You can imagine the outcry if on 17 occasions, a tanker pulled up and let it all go.”
Welsh Water data showed the overflow pipe at Picton in 2023 discharged 133 times for almost 647 hours.
The Cleddau Project said it found high levels of ammonia in the water and that tests showed sewage was being discharged at levels toxic to fish.
The group said it was reported to Natural Resources Wales (NRW) several times, which investigated in June 2023, almost a year later.
