Serious flooding which forced villagers from their homes was potentially caused by a mine shaft “blow out” during Storm Christoph, authorities have said.
About 80 people were evacuated as water rushed through Skewen, Neath Port Talbot, on Thursday.
Residents have been told they will not be able to return home this weekend or “possibly longer”.
The Coal Authority said initial checks suggested water had built up in the shaft and flooded the village.
Carl Banton, from the Coal Authority, said there had been a “tremendous amount” of rain recently and potentially a blockage in the drainage system could have caused the mine shaft to “blow out”.
Mr Banton reassured people that officers had visually checked other mine shafts in the area and were “not concerned” any would collapse.
“The mine shaft in question is the one that was on actually on the water level, it has found its point of weakness,” he said.
A major incident was declared as water rushed into the village on Thursday, leaving eight streets underwater as Storm Christoph caused widespread flooding across Wales.
On Friday, as firefighters continued to pump water out of the village, Natural Resources Wales (NRW) confirmed the Tennant Canal had been polluted “from mine water”.
Late on Friday evening, Neath Port Talbot council said, for safety reasons, people forced to leave their homes would “not be able to return home this weekend, and the wait could possibly longer”.
A support centre will open at Abbey Primary School from Saturday, with council officers on site to help people access emergency support.
The Coal Authority, which manages the effects of historical coal mining, are investigating the cause of the flooding.
Mr Banton said initial findings showed there may have been a build-up of water on the hillside which had “found its way out” through the mine shaft, flooding the village.
“The flow appears to be subsiding… but what we are unsure of is if there is a feed of additional water into the mine workings, from the extensive mine workings on the hillside,” he added.
Mr Banton said officers would drill down into the shaft and investigate on Saturday, in the hope that people could soon be allowed back into their homes.
“A lot of the mining in this area is very old… some of it dates back to the early 1800s… we have no details of how the shaft in question here was originally filled or capped,” he said.
“We will ensure the mine shaft is properly capped and sorted out.”
Martyn Evans, of NRW, said officers were looking at how to minimise the risk of pollution to nearby rivers, and investigating any impacts on the River Neath.
“We have also carried out tests on other watercourses in the vicinity of the incident. Results indicate there has been no significant impact on those at present,” he said.
On Thursday night a further 20 homes were evacuated by emergency services as the water continued to rush through the village.
First Minister Mark Drakeford confirmed on Friday financial support would be made available to people affected by the recent floods, up to £1,000 per household.
“This is the same level of support available a year ago when storms Ciara and Dennis hit Wales, just before the pandemic,” he said.
Skewen resident John Thomas said he returned home from a funeral with wife Lynne on Thursday to find their house had turned into “a lake”, he told BBC Radio Wales Breakfast.
He said: “The water was around the level of the bottom of the doors so we couldn’t go in, so we just had to stand there and watch this orange-coloured water just piling up and up and up.”
Mr Thomas said that with water up to his waist, he was unable to get in to rescue possessions.
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He added: “We’re in a bit of a dip on the road, so you could see it gradually coming up, they were worried it might have been a sinkhole because of the coal mines.
“It’s definitely mine workings, just by looking at the colour of the water, it’s an orange colour.
“Other people who were evacuated had the chance to move things upstairs, I didn’t have a chance to do that because I couldn’t get in to it.”
The couple are now staying with their daughter, with everyone else who was evacuated from their homes finding accommodation and told to avoid the area.
More than 30 residents of Cwrt-Clwydi-Gwyn care home were among those moved as a precaution.
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‘Terrifying’
Teresa Dalling, who lives in Dynevor Road, said she had spent the night fearing for her safety.
“I haven’t slept. I was up the back door every two hours checking the water level,” she said.
“I didn’t know we lived near old mines and if there’s been a collapse, my fear is more could follow and that’s terrifying.”
Up to 45 firefighters were involved at the scene at the height of the flooding.
In a joint statement, the police, fire service and Neath Port Talbot Council urged people not to return to their homes until it was safe.
Ch Supt Trudi Meyrick said: “We appreciate people are eager to get back to their homes and we are working with partners to allow this to happen as soon as it is safe to do so.
“In the meantime we ask people to please be patient as their safety is our top priority.”
Deputy Chief Fire Officer Roger Thomas, of Mid and West Wales Fire and Rescue Service, said firefighters remained in the village, pumping out water.
He said: “We will continue to monitor the situation and support our partner agencies and those affected over the next few days.”
Neath Port Talbot council said a local rest centre was available, and measures had been put in place to protect against Covid-19.
Chief executive Karen Jones said they would continue to support residents who had to leave their homes and they would ensure others had a safe place to go if further evacuations were necessary.
Network Rail said engineers had checked for any potential damage to the railway line, but had found no “cause for concern”.
A severe flood warning remains in force for the Lower Dee Valley, from Llangollen to Trevalyn Meadows.
Three flood warnings are in place for the River Wye at Monmouth, River Ritec at Tenby, and Bangor-on-Dee, where people were forced to leave their homes on Thursday as flooding saw a major incident declared. Eleven flood alerts are also in place.
Snow and ice could also exacerbate issues for emergency services and those forced to leave their homes, with temperatures forecast to plummet in coming days.
