Environment Agency Wales say that tests are being carried out to assess any damage that an orange coloured discharge found in the River Rhymney could have on water quality and wildlife.
They said samples will be taken and "if they confirm the pollution is caused by minewater, the Coal Authority will be asked to work with the Agency to try to improve the situation."
"Discharges of this type can occur many miles from the mine, as drainage tunnels were dug over long distances."
Environment Agency Wales say they have ruled out 'active industry' as the cause of the discolouring of a stretch of the River Rhymney near Caerphilly.
The Environment Agency Wales are investigating the source of an incident that has 'discoloured' a 6.5 mile stretch of the River Rhymney.
The river from Caerphilly to Machen has turned orange, possibly due to a discharge from an old mine.
The pollution is thought to have entered the river from a brook near Bedwas House Industrial Estate, and officers have been at the site since early this morning.
They say the exact source will be difficult to find due to the complexities of the underground mine workings.
A large amount of slurry has polluted a tributary of the River Taf in Carmarthenshire.
Environment Agency Wales were called to the Nant Cwmfelinboeth, north of Whitland, late last night after a call from member of the public.
Biologists and fisheries officers are currently assessing the full impace on the environment and local wildlife.
Any one who spots any further signs of pollution in the river should report it via the agency's incident hotline on 0800 807060.
Environment Agency Wales is investigating how a fire broke out at a waste plant in Nantyglo, near Brynmawr, on Saturday. It will look into whether it was caused by any environmental offences at the site.
The fire continued burning throughout the weekend. There's no sign of any damage to the local area.
With yet more heavy rain forecast this weekend, people driving home after Christmas are advised to plan their journeys carefully and allow extra time.
There is currently one flood warning in place for the River Ritec at Tenby and seven flood alerts in place across Wales.
Parts of Wales are at risk of localised flooding as forecasts indicate yet more bad weather is in store.
Bands of heavy rain are expected to affect south-west, mid and north-west areas of Wales over the weekend.
Experts say the incessant wet weather over the Christmas period, combined with last month’s heavy rainfall, means the ground remains saturated and at risk of flooding.
Environment Agency Wales advises there is currently one flood warning in place for the River Ritec at Tenby and seven flood alerts in place across Wales. They say it is likely that more flood alerts and warnings will be issued across the country.
These new flood defences in Riverside, Newport, are successfully protecting 420 homes and businesses from flooding.
The problem of flooding has been an historical problem along the eastern bank of the River Usk.
The new defences are part of a seven million pound scheme to protect homes in Newport from flood damage.
The project was built with money from the Welsh Government and the European Regional Development Fund.
Environment Agency warn if river levels continue to drop there could be big impact on fish and wildlife across Wales.