You can’t just close the door, or in the case of a mine, cap the shaft, and walk away.
In extracting the coal from the mines of Northumberland other materials were also disturbed and these are coming into contact with water as the mine refloods.
One of those materials is the mineral pyrite. You might get quite excited if you found some, as it looks just like gold. But for good reason it is known as “Fool’s Gold” as it is actually made of iron and sulfur.
When pyrite comes into contact with oxygen (from the air) and water, chemical reactions begin. The pyrite dissolves, turning the water acidic (sulfuric acid) and carrying the iron away in the water too.
If the water eventually flows out of the mine, you can clearly see by the colour that it is not just water. Compare the clean water of the River Tweed below with the pictures on this link.
If left alone, as the mines reflood, the polluted mine water will mix with and contaminate other water, both in surface rivers and underground. Thus it is a threat to both river life and water supply.
The Bates Minewater Treatment Scheme, in Northumberland, is one of more than 70 across the UK that together prevent an estimated 4000 tonnes of iron entering our waterways each year.
But the iron is dissolved, so how do they get it out of the water? We will find out next week.