The most obvious environmental effect of surface mining is the disruption of the land surface; surface mining is generally more visually obtrusive compared to underground mining. In most cases, coal occurs at depth and is covered by soil and rock that must be removed to allow access to the coal, leaving large holes and piles of removed material. Even when the coal outcrops on the land surface, removing the coal leaves a hole in the ground. At prelegislation mines, the land was sometimes left in a disrupted state, but mining companies are now required to separate topsoil from the overburden material, to fill in the excavation as mining proceeds, to cover the regraded land with topsoil, and then to revegetate the land surface. Moreover, in most countries, the regraded and reclaimed land has to be left in a final condition, the approximate original contour (AOC), that approximates the topography that existed before mining. Also, in the United States, bond money must be posted before mining is permitted; before the bonds can be released, the land must pass certain sustainable revegetation requirements, based on the premining soil conditions. For example, if the land was originally prime farmland, the reclaimed land must be productive enough to qualify as prime farmland (Fig. 2). Finally, the reclaimed land surface must be stable; measures must be taken to avoid erosion.
Given the fact that coal has been extracted, it might seem problematic to refill the excavated pit completely. In practice, the problem is typically the reverse; once the consolidated overburden rock is disrupted and broken, it cannot be placed in the same volume of space that it occupied before mining. Excess material (often called ‘‘spoil’’ or ‘‘fill’’) must be placed so as to be stable, either on level ground created during the mining process or in nearby valleys. At some sites, a pit is intentionally left and allowed to flood, providing a pond or small lake.
This avoids the expense of hauling back spoil material already placed elsewhere, but the issue of water quality in such ponds, and indeed at all mine sites, is a key concern.
At many coal mines, iron sulfide (pyrite) associated with the coal and the overburden strata oxidizes on exposure; this produces water with elevated concentrations of sulfate, acidity, and dissolved metals. If there is sufficient alkalinity present (typically in the form of limestone), the alkalinity will neutralize the acidity. In such cases, the environmental impact on water quality may be relatively minor. However, if there is insufficient alkalinity, the water becomes acidic. This type of water is known both as acid mine drainage (AMD), in most coal-mining regions and especially in the eastern United States, or as acid rock drainage (ARD), in most metal ore-mining regions. The term ARD is also preferred by those who like to point out that the same water quality results from such rock exposure in road cuts and construction projects (e. g., the Halifax Airport). ARD can typically be recognized, wherever it occurs, by the color of the water, i. e., red or yellow-orange, which is caused by dissolved and suspended iron (Fig. 3).
The prediction of postmining water quality is a key component in obtaining a mining permit. This is typically determined by analyzing rock cores for pyrite and alkalinity-producing rock strata. Then, using one of several procedures, a prediction is made about whether the water will be acidic. If it appears that it will probably generate AMD, or have other adverse hydrologic consequences, the regulatory agency may deny permission to mine or require the mining company to undertake special measures to decrease the likelihood of AMD. For example, the mining company may be required to handle pyritic
strata selectively, and place it in such a manner that it will be less exposed to air and/or water, or to mix it with the more alkaline rock strata, or to import additional alkalinity to the site.
AMD can contaminate many miles of stream down-gradient from a mine, sometimes rendering it toxic to aquatic life. By regulation, any site that generates water not meeting regulatory standards (typically pH 6-9, no net acidity, iron less than 3mg/liter, and manganese less than 2mg/liter) must treat the water before it can be discharged. The water treatment must continue for as long as the untreated water fails to meet these discharge criteria. However, it should be noted that at surface mines, after the pyritic rock is buried, AMD typically moderates. Acid salts formed during exposure to the atmosphere continue to dissolve, but once these are dissipated, acid generation begins to decrease. It may take decades, but water quality, at even the worst of these sites, does improve.
In the United States, AMD is a major problem in Pennsylvania, northern West Virginia, western Maryland, and eastern Ohio, but also occurs at many mine
sites in other states. Elsewhere in the world, acid drainage is often associated with coal mining, though the extent of the problem varies with local geology, site conditions, mining methods, etc. In the western United States, AMD is less likely to be a problem, due in part to the lower concentrations of pyrite associated with the strata there and in part due to the fact that the climate is drier. In fact, the lack of adequate precipitation can make reclamation difficult, and also introduces the problem of sodic spoils. Such materials are exposed during mining and contain elevated concentrations of sodium salts, which dissolve and add to the salinity of the soil and the downstream waterways.
Other environmental problems associated with surface mines include fugitive dust (airborne particles that blow away in the wind), ground vibrations and noise associated with blasting, loss or conversion of wildlife habitat, and loss of aesthetics (visual resources). The first two are temporary problems, but the latter two can be temporary or permanent, depending on the eventual land use. However, if an area is viewed as important to the life cycle of an endangered species, permit denial is almost certain. In contrast, mining companies sometimes take advantage of land disturbance and reclamation to enhance wildlife activity; for example, in the United States, an exception was granted to the AOC requirement to allow a mining company to establish an appropriate habitat for a bird that was native to the area but declining in population.
The lack of aesthetics generally attributed to mined and reclaimed land, though it varies with the individual site, generally refers to the fact that mined land is typically reclaimed in a bland and uniform manner. The lack of visual contrast, rather than the actual disturbance, is what is noticed. This is one, of many, objections that citizens often make about mountaintop removal, which is currently the most controversial form of permitted surface mining, and deserves to be specifically mentioned here. Moun – taintop removal, or mountaintop mining, is used in mountainous areas such as southern West Virginia to extract multiple seams of coal over large areas. The excess overburden is placed in what were previously valleys, with French drains constructed to handle the intermittent stream flow that might have been there previously, though major drainage patterns are preserved. The original rugged ridge and valley appearance of the land is converted to a more sedate topography, creating usable flat land where before there was none, and typically reducing the hazard of storm-related flooding; local inhabitants, although they may appreciate the jobs brought to the area, find their neighborhood forever changed (some would say ruined). Local biota is of course affected as well. There have been numerous court fights attempting to end the practice of mountaintop removal, and it is not yet clear how the issue will finally be resolved, but an interesting and so far unexplained finding that elevated levels of selenium have been found downstream of West Virginia mountaintop removal operations may turn out to be significant.