WHEN miners extinguished their lamps across Wales and the pumps were shut down, water began to fill the underground seams.
Naturally-occurring metals like iron and manganese dissolved into this water, which then rose to the surface.
Once back out in the open the reverse process happened and these polluting materials settled onto riverbeds, affecting fish and plants.
Metal mines were more of a threat as heavy metals like zinc, copper, lead, cadmium and tin emerged with the water into daylight.
Fifteen river systems in Wales are still affected, or potentially affected, by the metal mines of old.
Every year the Coal Authority treats more than 13 billion litres of Welsh mine water.
“Historically mines were dug in hillsides, but as they got deeper and deeper they needed to be pumped to keep dry,” said Carl Banton, the Coal Authority’s operations director.
“When the mines closed the pumps were switched off. They gradually began to flood.”
It could take two months, he said, for rainwater to trickle to the bottom of a 600m-deep mine.
The Coal Authority treats water at 15 sites in Wales – some where gravity does the hard graft and water is already discharging, and others where the water is actually pumped.
The largest two sites in terms of treatment capacity are Morlais, north of Llangennech, Carmarthenshire, and Taff Merthyr, Merthyr Tydfil.
The amount of water pumped varies from month to month and year to year, depending on rainfall, the mines’ depth and the rock strata.
“Sometimes we have to pump more at different times of the year,” said Mr Banton.
Coal mine water is treated by sending it running down a concrete cascade to oxygenate it. Iron and manganese settles in special lagoon areas, while the water moves on to reed beds where it is filtered.
Metal mine water is more of a challenge to treat, with bacteria or chemicals used.
Consent for this process is needed from Wales’s environment body Natural Resources Wales (NRW).
It goes on year after year, decade after decade.
The Coal Authority has a rolling plan for the next 100 years, but commercial opportunities are now part of its thinking.
Ideas include supplying water warmed naturally from the Earth’s crust to homes and businesses, using mines for water storage during heavy rainfall events, and using mine water to store data servers.
“I think it’s quite exciting,” said Mr Banton. “The mines are there – can we think about them as an asset rather than a liability?”
Surveying work for such a district heating scheme is due to get under way soon at a village in County Durham, England, while the Coal Authority is advising on a similar proposal near Maesteg.
“We’re right at the start of the journey in Wales for this,” said Mr Banton.
Meanwhile, the Welsh Government is handing NRW £4.5 million to help clean up metal mines, which it said are one of the principal causes of failures of water standards in Wales.
The Welsh Government said there are 1,300 abandoned metal mines in the country, which have been estimated to impact nearly 400 miles of river catchment areas.
The worst affected rivers, said NRW, include the Afon Goch, Anglesey, the River Conwy, North Wales, and the Afon Rheidol and Upper Wye in West Wales.
The Coal Authority also reacts to incidents, advises on planning applications, and owns and manages 26 mining-related tips.
It is currently undertaking a review of all tips for the Welsh Government following the collapse of 60,000 tonnes of spoil at Tylorstown, Rhondda Cynon Taff, in February.
It poured down the hillside as Storm Dennis dumped vast amounts of rain in the area.
“The big issue with tips is water,” said Mr Banton. “If you make sure the water management is done, they will remain stable.”
Mr Banton said there were two underground and three surface mines which still operated in Wales, but he said some were in the process of being restored.
Carmarthenshire Council is considering an application to remove 110,000 tonnes of coal from the Glan Lash site, north-west of Ammanford.
Applicant Bryn Bach Coal Ltd has said that three-quarters of it would be used for water filtration and brick manufacture.
The Ammanford area is no stranger to mining.
“The coal in West Wales is anthracite (cleaner-burning than bituminous coal) and is very close to the surface,” said Terry Norman, secretary of the Ammanford and District Archaeology and History Society.
The development of more powerful earth-moving equipment, he said, drove open-cast operations.
Mr Norman said the defunct Tirydail colliery left an unsettling legacy.
“A lagoon filled up from below,” he said. “It was black.”
Another watercourse nearby, he said, “ran orange like Tango for several years”.
Both have since been cleaned up.
Mr Norman added: “Most of the coal tips have been removed or landscaped.”
But there is another hidden problem – subsidence.
“There’s a warren of mine shafts. There used to be pit props, but they were allowed to collapse.
“That can cause subsidence to houses above, but you can get grants for it.”
Mines are just a memory – if that – for most people in Wales.
The country has nearly 31,000 recorded “coal mine entries”, according to the Coal Authority.
Some historians think Wales would have remained a large rural society without mining.
Between 1801 and 1901 Wales’s population more than trebled, from 587,000 to more than two million.
The Rhondda had a population of around 900 people in both valleys in 1830, soaring to more than 100,000 by 1900.
Speaking to BBC Wales in January this year, Ceri Thompson, of National Museum Wales, said by 1921 there were 271,000 people in Wales directly involved in the coal industry.
“The south Wales valleys would have had a scattered hill farming population (without coal),” said Mr Thompson.
“Cardiff, Swansea and Newport would have been small villages and wouldn’t have developed into docks.”
The Coal Authority’s Mr Banton reckoned there was still a lot more coal left in the ground than has been mined over the years in Wales.
And he expected that it would remain there, although he said an argument could be advanced for indigenous coal for Tata’s steelworks in Port Talbot, rather than coal imported from afar.
“But it’s extremely expensive to open a new mine,” said Mr Banton, a former mine surveyor.
There is also the all-pervading question of the environment.
A Welsh Government spokesman said: “It is our policy objective to avoid the continued extraction and consumption of fossil fuels, in line with the objectives set out in our declaration of a climate emergency.”
Proposals for opencast mines, deep-mine development or colliery spoil disposal should not, he said, be permitted.
The spokesman added: “Any exceptional proposals or applications, which must contribute to decarbonisation, must be considered from the perspective of overall global impacts as well as Welsh prosperity and emissions targets.”
Decarbonising the electricity supply has been a relative success story for the UK, although gas accounts for around a third of our electricity generation.
In the first quarter of 2020, coal – once the mainstay of our energy needs – generated less than 4% of the UK’s electricity.
“If you think where we are going with the environment, renewable sources of energy are more favourable,” said Mr Banton.
Conservatives’ Lack of Action on Obscene Energy Profits “Indefensible” says Welsh Lib Dems
New Audit Office Report on Poverty in Wales supports Plaid Cymru’s calls
Successful Operation targeting anti-social driving across Newport and Monmouthshire