Reporter 423, 21 September 1998

When the mines close down and the rivers run red

Pollution from abandoned coal-mines poses a serious threat to the environment. David Adam, right, the University’s new Publicity Officer, has spent the last four years at Leeds researching a method of treating it using mushroom compost. So, he asks, can constructed wetlands prevent the revenge of Old King Coal?

The decline of the UK’s coal mining industry has left a devastating environmental legacy. Drainage from abandoned mines carries an acidic cocktail of pollutants into surrounding rivers and can quickly choke them of life.

A new and environmentally friendly method of treating this pollution is to divert the contaminated water through constructed wetlands – swimming pool-sized concrete basins filled with plants and compost. Chemicals and bacteria within the wetland help to neutralise the acidity and remove dissolved metals, significantly improving the quality of the water.

These wetlands are viewed by many scientists as a long-term solution to the problem of acidic mine drainage, but just how realistic is that hope?

Professor Rob Raiswell in the Environment Centre is trying to find out. By analysing a recently-established wetland system at Tonmawr, South Wales, Professor Raiswell and a number of postgraduate students are trying to unravel the different processes that occur. “The wetland is supposed to be self-sustaining,” he explains, “assuming that there is sufficient food for the right bacteria to grow.”

The desired microbes are sulphate-reducing bacteria, which thrive on the high levels of sulphate present in mine drainage.

In the Tonmawr wetland, microbial food is supplied as mushroom compost, which the bacteria use to produce distinctive-smelling hydrogen sulphide. Dissolved pollutants (most commonly iron) react with the hydrogen sulphide to form insoluble compounds, which sink and are trapped in the wetland.

Mushroom compost may seem an unlikely food but appears to go down well with the bacteria, and importantly, large quantities are available free of charge. The previous owners of abandoned coal-mines are excused legal liability for any subsequent pollution, so any treatment must be paid for from public funds. Low cost is, therefore, an important prerequisite of any applicable treatment method.

But a food source that produced faster bacterial activity would speed up the pollution treatment, or allow the wetland to treat more heavily polluted water.

In an attempt to find such a material, the bacteria have also been grown on a number of different waste products in laboratory experiments. Newspaper, sewage sludge and the discharge from a local brewery all promoted promising growth, though the bugs clearly found the taste of used tea bags and potato peelings less appealing.

Other experiments showed how the bacteria reacted when the temperature was varied, and when different microbes which compete for the same food were present, helping to simulate the wetland conditions more accurately. Though the bacteria had the ability to rapidly adapt to changing conditions, the results suggested that they are not responsible for the bulk of the pollution treatment. So what is?

Back in the Tonmawr wetland, Professor Raiswell has been studying the chemical reactions which could contribute to the overall clean-up effort. The relative importance of each of these is unclear, but they could have significant implications for the wetland’s long-term chances of success.

Large amounts of acid-neutralising limestone were added at the scheme’s inception, and similar chemicals within the mushroom compost also dissolve over time. If these chemicals are removing the pollution as they reverse the acidity, then effective water treatment will stop when these supplies are exhausted.

Professor Raiswell hopes that analysing the different forms of carbon found in the wetland will offer clues about this. The sulphate reducing bacteria feed exclusively on a lighter form of carbon, so the production of more compounds containing the heavier form is evidence of chemical, not biological, activity.

The debate over the long-term prognosis for constructed wetlands has not lessened their appeal and Professor Raiswell will soon be applying his expertise to two new wetland projects. One of these will be based on a bright orange stretch of the River Don in South Yorkshire, attempting to reverse nearly a century of man-made pollution.

'The previous owners of abandoned coal mines are excused legal liability for any subsequent pollution so any treatment must be paid for from public funds.'

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