Pa asesiad y mae'r Gweinidog wedi'i gynnal o effeithiau gollyngiadau carthion ar ansawdd dŵr. Adroddodd Top of the Poops y bu 3,375 o ollyngiadau carthion sy'n cyfateb i 34,818 o oriau o hyd, sef 9.2 gollyngiad carthion bob dydd ar gyfartaledd ar 44 o safleoedd gollwng yn afonydd Llynfi, Garw, Ogwr ac Ewenni yn 2022.
Natural Resources Wales is responsible for assessing water quality in accordance with legislative drivers such as the Water Environment (Water Framework) (England and Wales) Regulations 2017. The latest classification for Wales’ water bodies can be found here Water Watch Wales (naturalresourceswales.gov.uk).
Of the four rivers you mention, sewage is not identified as a reason for failing to meet good ecological status.
Our Programme for Government commits us to strengthen water quality monitoring. The action plan issued in July 2022 sets out how we will improve the regulation and performance of wastewater assets, in particular storm overflows and will increase our understanding of the impact of storm overflows.
https://www.gov.wales/environmental-regulation-overflows-action-plan-html#100072
I am aware of the “Top of the Poops” (TOTP) website which is run in collaboration with the pressure group Surfers Against Sewage (SAS). It is a matter for Natural Resources Wales (NRW) to provide an assessment as to the reliability of the figures reported for each of the rivers you have listed and to report any impacts upon the water quality.