Science: A pit's poisoned legacy

An old mining village has made good its polluted past.

THE FORMER pit village of Quaking Houses in County Durham displays its heritage proudly. On the village green, visitors are greeted by two large black iron tubs, once used for bringing the coal to the surface of the mine but now bursting with flowers. The far end of the green is dominated by a vast mine wheel set into the ground.

But the mining industry has left more sinister legacies in its wake. At the bottom of the hill flows a small stream, the Stanley Burn, enjoyed by the locals walking their dogs, and by children "plodging" barefoot in the water during the summer.

About nine years ago the clear waters of the burn changed alarmingly. A Quaking Houses resident, Chas Brooks, explains. "When I first came here there was plenty of water life - particularly a lot of water voles. Then they put this new pipe in and the water went every colour of the rainbow. The voles disappeared; all the plants started dying off. There wasn't even a pond skater left. It was completely dead."

For several years villagers lobbied the water authorities to investigate the problem, but then Kim Bradley, a postgraduate student in the civil engineering department at the University of Newcastle, came across the pollution and reported it to his supervisor, Dr Paul Younger.

"The water was strongly acidic with very high iron, aluminium and sulphate concentrations," says Dr Younger. "When we inspected the stream we found that the bed was coated extensively with orange-red ochre - ferric hydroxide. In the upper reaches there were floating masses of white froth with the consistency of candy floss. This was aluminium hydroxide spume. These are classic - if extreme - symptoms of acidic mine-water pollution."

The source of the pollution was identified as the nearby spoil heap from the now defunct Morrison Busty Colliery. Water percolating through the heap is collected in a network of drains and discharged into Stanley Burn. Soon afterwards Dr Younger was invited to a public meeting organised by Durham County Council on the problems caused by mine-water pollution. There he met Diane Richardson, of a local action group, the Quaking Houses Environmental Trust.

That serendipitous meeting culminated in a remarkable collaboration between scientists and engineers from the university, villagers and a local arts charity, to tackle the pollution by creating a new wildlife habitat. Earlier this year the scheme won the pounds 5000 first prize in the "conservation engineering" category of the Henry Ford European Conservation Awards.

The spoil heap consists mainly of broken shale, sandstone, clay and fragments of coal. Much of the shale and coal contains iron disulphide, commonly known as iron pyrites.

"When these minerals are brought out of the ground and exposed to air, you get oxidation reactions," says Dr Younger. "The pyrites oxidises to sulphuric acid and soluble iron salts. This resulting increase in acidity then drives reactions in the clay minerals, releasing aluminium salts." Iron, aluminium and acidity are all poisonous to water life.

Dr Younger was keen to attempt to treat the water "passively", that is, without adding more chemicals. The idea is to use limestone to help neutralise the acidity and bacteria to attack the chemicals in the water, producing harmless by-products.

In 1996, after a pilot study, the Northumbrian Water Kick Start Fund gave the Quaking Houses Pollution Remediation Consortium - consisting of Dr Younger's team, the local environmental trust and a local arts charity - a grant of pounds 54 000 to construct a full-scale wetland.

Using pulverised fuel ash, a waste product from a nearby power station, the retaining walls for a 900-square-metre basin were created. The base was spread with limestone and above that a foot-thick layer of compost and manure.

"The process relies on sulphate-reducing bacteria, which are found naturally in manure and compost," says Dr Younger. "The bugs strip the oxygen out of the sulphate to create sulphide. This reacts with the soluble iron salts in the water to precipitate insoluble iron sulphide.

"As you take out the sulphate the acidity is lowered and this has the effect of making the aluminium insoluble. On some days the whole thing looks white as the aluminium drops out, and on some days it is green with the iron."

The incoming polluted water takes about 36 hours to pass through the wetland, which treats the water at a rate of between 600 and 700 litres each minute. The water emerging from the wetland is considerably less acidic than when it entered it.

The wetland has been planted with a variety of water plants, and a year after its completion the "clean" end of the pond is becoming colonised with invertebrate life.