3.3.2 Water pollution from coal mines
Most underground and some surface mines lie well below the water table. Both therefore have the potential to pollute any groundwater that flows through them. The root cause of the problem is the action of aerobic bacteria on pyrite (FeS2) within the coal sequence. This process releases metal and sulphate ions into solution, which in turn causes the acidity of the mine water to increase:
Before mining, groundwater flow through coal sequences is usually sluggish and in general chemically slightly reducing. During mining, most of the original water is extracted by pumping to keep the mine dry. Pumping exposes the coalbearing rocks to moist air, leading to oxidation of pyrite within the sequence, helped by the catalytic action of bacteria. When mines close and pumping stops, the water table can rise again. Soluble products of pyrite oxidation will pollute groundwater with sulphuric acid, iron and manganese cations, sulphate anions, and sometimes highly dangerous arsenic ions. The roadways of abandoned coal mines form a very effective artificial network of underground watercourses, which can channel polluted water to the surface. Such acid mine drainage ( AMD; see Smith, 2005) is highly damaging to most forms of aquatic life and to humans who come into contact with it. Where this water emerges, surface streams show the telltale sign ofAMD: ochreous slimes (Figure 32).
Surface mines represent only a transient problem. Water is pumped out of them during working into settling ponds, which ensure that only clean water leaves the site. The mined area is backfilled after use, and the water table generally equilibrates to that within the surrounding area. Preventive measures usually centre around excluding oxygen, water or bacteria from underground exposures to prevent pyrite oxidation. In old mines, products of pyrite oxidation have already accumulated over centuries of exposure, and sealing old mine workings to reduce water flow is impractical.
A solution to AMD is not easy to find. Continued pumping after a pit has closed down is expensive, and routing polluted mine waters into special treatment areas is impractical because it is almost impossible to predict the movement of groundwater in anything other than general terms. Environmental scientists have looked at other ways of improving the quality of mine waters, such as the use of wetlands in which some plants can permanently absorb pollutants, and neutralizing mine waters by adding alkaline calciumcarbonate or sodium hydroxide. Neutralization also decreases the solubility of dissolved metal ions, so that they precipitate and settle out of solution.