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, AMD 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. Consequently, the remaining sources of contaminated water discharging to streams and rivers in mined areas are generally from abandoned or orphan mines. However, before moving on to water remediation at such sites, the techniques used to prevent, ameliorate, or remediate mine water problems at active operations need to be addressed. As alluded to previously, a powerful tool in preventing AMD is the permitting process. Variation exists in how much certainty of AMD generation a regulatory agency requires before a permit is denied. In the United States, Pennsylvania is the most conservative, and can legitimately boast that it has improved the accuracy of its permitting decisions from about 50% in the 1980s to 98%; however, it should be noted that this definition of success is based on AMD generation at permitted operations (following reclamation) and does not include mine sites that had permits denied and might not have produced AMD if allowed to operate.
In contrast, adjacent states (and many areas outside of the United States) permit a higher percentage of mines to operate, but generally require that measures be taken to reduce the risk of acid generation. In addition to the selective handling of overburden and the importation of alkalinity from off-site mentioned earlier, minimizing exposure of the pyrite to either oxygen or water can decrease the amount of acidity generated. Restricting exposure to oxygen generally means that the pyritic material must be placed beneath the eventual water table, which is difficult to do at many surface mines. Alternatively, the operator can attempt to place the pyritic material in an environment where it is dry most of the time, well above the potential water table and capped by material of low permeability. Compaction can also be used to reduce permeability, though not on the final soil surface, where compaction makes it difficult to establish a vegetative cover. Water can be diverted around the mine site by intercepting overland flow with ditches, and by constructing drainage ways along the final highwall to intercept groundwater and prevent it from flowing through the overburden material.
At many sites, mining companies have been given special permission to remine old abandoned mines to improve water quality. The mining companies harvest coal left behind by the old operations. For example, old room-and-pillar mines sometimes left as much as 50% of the coal behind to support the roof rock. Some of these old mines are relatively shallow and can be inexpensively surface mined using modern machinery. Because the coal pillars are sources of acid generation, removing them generally improves water quality. Similarly, old surface mines could not economically remove as much overburden as is now possible. The exposed highwall can now be economically mined with modern machinery. The mining companies are required to reclaim the land to current standards, but their water discharge requirements typically only require them to at least meet the water quality that existed before the remining operation. In most cases, the water quality improves and the land is reclaimed, at no cost to the public.
A more exotic approach of controlling AMD involves the inhibition of iron-oxidizing bacteria. These ubiquitous bacteria normally catalyze pyrite oxidation; inhibiting them reduces acid generation significantly. In practice, however, it is difficult to do this. The only cost-effective approach that has been developed involves the use of anionic surfactants (the cleansing agents in most laundry
FIGURE 7 Mine water can be directed through a specially constructed wetland to passively improve the water quality.
detergents, shampoos, and toothpaste); their use selectively inhibits the iron-oxidizing bacteria by allowing the acidity that they generate to penetrate through their cell walls. This approach has been used effectively to treat pyritic coal refuse, reducing acid generation 50-95%. Slow-release formulations have been developed for application to the top of the pile before topsoil is replaced and the site is revegetated.
Discharge criteria must be met, and if the at – source control measures are not completely effective, some form of water treatment is required. Sometimes water treatment is required only during the mining and reclamation operation, and water quality improves soon after reclamation is completed. Sometimes the water quality remains poor. Chemical treatment is simple and straightforward, but is expensive to maintain for long periods.
Passive and semipassive water treatment technologies, though by no means universally applicable, are an option at many sites. These techniques developed as a result of observations at natural and volunteer wetlands, which were observed to improve the quality of mine drainage. The simplest of these techniques, appropriate for near-neutral mine water that is contaminated with iron, involves the construction of shallow ponds, planted with plants that will tolerate the water, such as Typha (commonly called cattails in North America) (Fig. 7). The iron in the water oxidizes and precipitates in this constructed wetland instead of in the stream, allowing natural stream biota to repopulate. These constructed wetlands cannot be viewed as equivalent to natural wetlands; they have been designed to optimize water treatment, and any associated ecological benefits, though they may be significant, have to be viewed as secondary.
Water that is acidic can be neutralized by the inexpensive addition of alkalinity. Several passive methods have been developed, generally using limestone and/or sulfate-reducing bacteria. Alkaline waste products (e. g., steel slag) have also been used. Water quality, site considerations, and flow dictate which approach is most cost-effective at a given location.
The realization that passive techniques can be used to treat low to moderate flows of mine water has made it possible for state reclamation agencies and local watershed associations to remediate mine water at abandoned and orphan mines without a long-term financial commitment for chemicals. However, highly contaminated AMD or high flows still cannot be cost-effectively treated passively. But the technology is continuing to evolve. Semipassive techniques, such as windmills and various water – driven devices, are being used to add air or chemical agents to mine water, extending and supplementing the capabilities of the passive treatment technologies.
Contaminated water in abandoned or orphan underground mines represents the ultimate challenge because of the large volume of water that must be treated. One option currently being explored in the United States is whether or such mine pools can be used as cooling water for a power plant. Locating new power plants is becoming more difficult because of the water needed for cooling purposes. A large mine pool could be a good resource for such an operation. The power plant would have to treat the mine water chemically, but this cost could be partially subsidized by the government, which would otherwise have to treat the water chemically or allow the mine discharge to flow untreated into the local streams and rivers.