Although underground coal mines operations are not as visible as surface mining, their overall environmental impact can be greater than that of the typical surface mine. A key environmental problem is subsidence. Underground mines are large cavities in the rock, and depending on the strength of the intervening strata, the depth of the mine, and the type of mining and roof support, the rock walls can fail, causing cracks and land collapse at the surface. Typically, coal seams at depths greater than about 200 feet are extracted by underground mining methods rather than by surface mining, with the exact depth principally based on the relative amount of coal and overburden. However, before improved technology made surface mining so affordable, the trade-off occurred at much shallower depths; some abandoned underground mines are only 35 feet below the land surface. (more…)

Before the passage of regulations dictating mined land reclamation and mine water discharge standards, streams and rivers down-gradient of mine sites were often contaminated with high levels of suspended and dissolved solids. In the eastern United States, Acid Mine Drainage was also a major problem. Nowadays, streams and rivers near active mine sites have much less of an impact. Sediment ponds are constructed to collect suspended solids and if the mine water does not meet regulations, chemicals [typically lime, Ca(OH) 3 ] are added to neutralize acidity and precipitate dissolved metals. (more…)