In the UK, about 200 km of river lengths are polluted by mine water and a further 400 km by discharges from abandoned metal mines. The problem is likely to get worse because the closure of many deep-shaft coal mines in the early 1990s will result in further flooding and discharges of polluted mine water. In a survey of pollution caused by mine-water drainage in Scotland, it was found that there were 59 discharges of mine water: 25 from abandoned mines, 17 indirectly associated with mining, 8 from active mines and the remainder from coal waste tips and opencast mines.1
When a coal mine is in use, the tunnels and shafts are cleared by pumping out the water which seeps into them. When the coal has all been extracted, the mine is closed and the pumping equipment is removed. Gradually the mine workings flood with ground water until eventually it reaches the surface and flows into a stream. This process can sometimes take many years after closure depending on the depth of the mine shafts, the rate of flooding and the extent of the workings.
In some areas the ground water that emerges from abandoned coal mines is of good quality and can be used for public supply, but these instances are rare. Usually, the mine water is polluting because of the high concentration of various ions, particularly sulphate and iron (water containing high concentrations of the latter is called ferruginous). This is because, adjacent to coal seams, the ground contains pyrites, FeS2. With the opening of the mine and the passage of air through the tunnels, the pyrites becomes oxidized to ferric sulphate Fe2(SO4)3. The closure and subsequent flooding of the mine cuts off the oxygen and the ferric sulphate is reduced to ferrous sulphate, FeSO4. This reduced form of iron is water soluble so the flooding mine water becomes contaminated with iron. The ground water also becomes acidified because of the presence of sulphur bacteria in the walls of tunnels. These bacteria utilize the oxygen present in iron sulphates as an energy source because of the lack of oxygen in the air and, as a result, sulphuric acid is formed. These chemical reactions can be summarized as follows:
2FeS2 + 7O2 + 2H2O ^ 2(FeSO4) + 2H2SO4 2FeSO4 + H2SO4 + V2O2 ^ Fe2(SO4)3 + H2O Fe2(SO4)3 + 6H2O ^ 2Fe(OH)3 + 3H2SO4
The result of these reactions is that the mine water, as it emerges from the ground some time after abandonment of the mine, has the following characteristics:
• it has high levels of sulphate ion
• it contains soluble iron in its reduced, ferrous, form
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