Almost 56m litres of sewage was dumped successful a stream successful 2023, according to campaigners.
The Cleddau Project said Welsh Water information showed pumps astatine Picton successful Haverfordwest, Pembrokeshire, were moving astatine 80% capableness from July 2022 to February 2024.
That meant 55,857,000 litres of discarded - equivalent to 17 tankers a time - which should person been going to a sewage attraction plant, was alternatively going into peculiar conservation country the Western Cleddau.
Welsh Water apologised and said its Picton pumping presumption had been upgraded and spills cut.
But it admitted determination could beryllium problems astatine different sites crossed Wales.
The Cleddau Project's Ric Cooper said campaigners investigated due to the fact that an overflow tube astatine Picton playing fields appeared to beryllium spilling much earthy sewage than usual.
The radical asked Welsh Water for pump show information from the adjacent pumping station.
They are, according to Mr Cooper, expected to pump astatine 120 litres a 2nd but had lone been operating astatine "95 oregon 96".
"That means 20% of what should person gone for attraction was going consecutive into the river," helium said.
"We calculated implicit 12 months, and it averages retired arsenic 17 tankers a time emptying into this river.
"You tin ideate the outcry if connected 17 occasions, a tanker pulled up and fto it each go."
Welsh Water information showed the overflow tube astatine Picton successful 2023 discharged 133 times for astir 647 hours.
The Cleddau Project said it recovered precocious levels of ammonia successful the h2o and that tests showed sewage was being discharged astatine levels toxic to fish.
The radical said it was reported to Natural Resources Wales (NRW) respective times, which investigated successful June 2023, astir a twelvemonth later.
The Cleddau Project said it took different 9 months earlier the pumps were up to speed.
Another campaigner, Simon Walters, said the spills would impact sewin and salmon movements.
He added that E. coli levels were "way" supra that permitted for harmless bathing.
He said: "We utilized to aquatics successful present arsenic kids. I wouldn't fto my canine aquatics successful present now."
NRW and Welsh Water bosses, helium believed, needed to resoluteness the matter.
Mid and South Pembrokeshire MP Henry Tufnell, who met the Cleddau Project members, Welsh Water and NRW past week, called the contamination "not acceptable".
NRW needed much resources, helium believed, arsenic it was investigating lone 20% of alleged breaches reported.
He said the UK authorities was taking "definitive action" connected tackling pollution with a measure which would springiness accrued powers to regulators to tackle polluting companies.
In July, Ofwat confirmed it had served ceremonial notices and opened an enforcement lawsuit against Welsh Water and 3 different companies into however they managed discarded h2o attraction works and networks.
Welsh Water "apologised" for immoderate harm to the Western Cleddau.
Waste h2o main Steve Wilson said the instrumentality show travel connected the Picton overflow tube had been fitted earlier the deadline of regulator NRW.
"To upgrade that pumping station, we had to instal caller pumps and a caller panel, which really took 26 weeks to beryllium built," helium added.
It was present operating correctly, helium said.
Mr Wilson said the institution was upgrading sites successful Wales arsenic accelerated arsenic imaginable to amended h2o quality.
The company, helium said, planned to walk £4bn betwixt 2025 and 2030, with £100m connected betterment and attraction successful Pembrokeshire, taxable to Ofwat approval.
NRW confirmed it was made alert of a occupation successful June 2023, and was told by Welsh Water caller pumps would beryllium installed by 31 October 2023.
It said the deadline was missed and Welsh Water were fixed a ceremonial warning.
The enactment to repair the pumps was completed successful February 2024, it said.
NRW said the pumping presumption was "subject to a ceremonial ineligible probe for different matter".