Admissions that Dŵr Cymru has pumped untreated sewage into the river Erch has prompted concern from local politicians.
A study by Afonydd Cymru shows Dŵr Cymru illegally released wastewater at Abererch on 374 days over the last six years.
Concern has also been raised over spillages at Caernarfon/Seiont which saw 14 breaches that led to 165 million litres of untreated sewage pour into the river.
Dŵr Cymru has not disputed the findings of the report, but says it is aiming to improve.
Reacting to the report, Dwyfor Meirionnydd MS Mabon ap Gwynfor and MP Liz Saville Roberts said: “This is an alarming admission from Dŵr Cymru, which raises very serious questions, both for them and for Natural Resources Wales – not only due to the sheer number of wastewater treatment plants involved in the release of untreated sewage, but also because both organisations seem to have been complicit in failing to redress the problem for nearly a decade.
“A total of 374 breaches were identified at Abererch treatment works in our constituency, with a further 14 breaches recorded at the Seiont treatment works in nearby Caernarfon.
“This is significantly concerning for our natural environment. Left untreated, sewage can lead to a myriad of problems for our rivers, seas and the natural ecosystems they support.
“We need full transparency from Dŵr Cymru and immediate answers are required as to why these failures were allowed to go unchecked for nearly a decade with no action being taken.
“Welsh Government must use all available powers at its disposal to toughen environmental governance over water. Plaid Cymru has secured legislative commitment in this regard, through our Co-operation Agreement with Welsh Government.’”
Jane Dodds, MS for Mid and West Wales, was also critical, saying: “We have the second highest bills across England and Wales, bosses have been awarded thousands in bonuses and at the same time have been illegally dumping sewage into our rivers.
“I have repeatedly led calls in the Senedd for tougher action on sewage dumping and for a full review of Dŵr Cymru and it’s operation. The Welsh Labour Government must take those calls seriously or risk the future of our rivers.”
Dŵr Cymru said in response: “Each site presents its own and sometimes very complex challenges, with NRW reviewing the information we provide, agreeing appropriate timescales for resolving the issues, and taking enforcement action where appropriate.
“We have over 5000 environmental permits as Wales has some of the largest numbers of treatment works, pumping stations and storm overflows in the UK. We are continually monitoring and when we find issues, we share this data with regulators, investigate and deliver improvements. We are investing around £1 million a day in improving our services.”
Ann Weedy, NRW operations manager said: “Both regulators, Ofwat and NRW, have been clear that the current use of storm overflow discharges is unacceptable and needs to change.
“We understand the concern of many across Wales that overflows are operating too frequently, and we are taking steps to ensure our regulation of overflows responds to the needs of the environment and public.”