ENVIRONMENTAL GEOLOGY 27 landslip activity In deep mines, the emission of methane and other gases is a problem, while wastewaters may be charged with iron and may be acidified by the breakdown of pyrite and other sulphides Acid mine drainage is a growing problem in many disused metal mines and has adverse affects on groundwater supplies and water bodies, such as estuaries, rivers, and lakes Mining subsidence is a result of the removal of geological materials underground, creating a void space, which subsequently collapses (Figure 2) Similar situations occur when voids are created for purposes other than mineral extraction Longwall mining is associated with deep coal extraction and generally involves the planned subsidence of a relatively large area Pillar-and-stall mining, which dates back centuries, can lead to the differential collapse of the pillars, giving rise to graben-like failure structures In other cases, removal of deep salt deposits through the process known as brining – the pumping out of brine-charged waters – can lead to severe subsidence Large-scale subsidence is often difficult to manage, and the possibility of constructing buildings over an unknown area of mining activity is a very live issue, even where the mining is known to date back to antiquity, as in Rome Raft foundations, grouting, deep piling, and excavation and backfill are all possibilities, depending on the depth and extent of the problem Mine wastes are produced at two stages during the mineral-extraction process: during mining, when waste rock or spoil is produced; and during further Figure Subsidence associated with longwall coal mining processing of the materials extracted, which creates a further set of mineral wastes, generally much finer than simple spoil and referred to as tailings Spoil is of variable grain size and generally accumulates in open tips close to the mine workings Tailings produced by mineral processing tend to be much richer in minerals and more uniform and of a finer grain size owing to milling In some cases these materials have a high toxicity, as hazardous chemicals are used in the separation of mineral particles Tailings are also more likely to be distributed by weathering and erosion, raising the consequent possibility of hazardous air-borne particulate matter The long-term stability of mine wastes is also of concern, as with the South Welsh Aberfan disaster in 1966, where 111 000 m3 of debris moved down a 13 slope, enveloping a school with tragic loss of life Dealing with waste tips is a matter of concern for environmental geologists, and the solution depends very much on the nature of the material In the UK, tax on aggregate extraction led to the reuse of some tax-exempt spoil, particularly slate and shale, as poorquality aggregate Other mineral spoil may be reused for rock fill or inert landfill or even in dealing with mine subsidence and quarry-restoration projects Tailings, on the other hand, given the potentially high level of toxicity, have to be treated with caution Wet tailings have to be ponded, treated, and removed, while dry tailings have to be carefully monitored so that water passing through the wastes does not lead to