The acid drainage problem has been recognized for at least 50 years and has been the subject of intensive research since the 1960's. Research efforts have proceeded on two fronts:

• Prediction of acid generation;
• Preventive/mitigative methods; and
• Acid Drainage Technology Initiative.

Predictive Methods

Prediction of acid generation based on geochemical analysis has been practiced for about 20 years. The most widely used method, Acid/Base Accounting (Sobek and others, 1978), quantitatively balances pyrite against carbonates and other alkaline materials. Its original use was to identify topsoil substitutes and root zone media, not a quantitative predictor of drainage quality. As a water quality predictor, it has been accurate in some instances and misleading in others (Erickson and Heiden, 1988). Research has therefore, continued on improving predictive methods.

A variety of simulated weathering tests have been developed and studied as drainage quality predictors (Caruccio, 1967; Sturey, 1982; Renton and others, 1988) Test details differ, but all methods attempt to mimic cyclic wetting/drying and flushing of spoil piles. Currently, there is no consensus on which method most accurately reflects field conditions. Questions have also arisen regarding length of laboratory test time and extrapolation to field weathering time.

It has been observed that pyrites of different origins can exhibit varying levels of reactivity. Laboratory studies have been conducted (Hammack and others, 1988) to determine why certain pyrites are more chemically reactive. "Evolved gas analysis," which involves the thermal decomposition of sulfur compounds under controlled conditions, has been used to characterize pyrite reactivity. The basic premise is that lower temperature decomposition reflects unstable and more reactive pyrite. Current research efforts (Sheetz, 1990) are focused X-ray diffraction studies of subtle differences in crystal structure and possible trace inclusions in the crystal lattice, in combination with evolved gas analysis. The goal of these research efforts is to identify the controlling factors and develop a reproducible test that discriminates reactive and nonreactive pyrites.

Computer models are another approach to prediction of acid generation. Most of these models incorporate a number of chemical and physical parameters to describe the chemical reactions of acid generation, microbial catalysis and leaching (transport) of the weathering products (Jaynes, 1991; Scharer and others, 1991). Many of these parameters are difficult to measure or must be estimated and verification is generally lacking. One model uses a "lumped variable" approach, rather than a large number of individual parameters (Rymer and others, 1990; Hart and others, 1991). One combined variable estimates acid generation and a second variable accounts for leaching of weathering products. This model is still undergoing testing and verification.

Prevention/Mitigation

Research on acid prevention and mitigation has focused on three main areas: chemical inhibition of the acid generating reactions; inhibition of the microbes responsible for catalyzing the acid generating reactions; and physical or geotechnical treatments to minimize water contact and leaching.

The benefits of lime and other alkaline agents have long been recognized in mitigating acid drainage. However, the complex chemistry of spoil materials have given varying levels of effectiveness in alkaline addition studies. Direct mixing and contact with pyritic materials appears most effective but an optimum lime to pyrite ratio remains unknown. Indirect treatments such as alkaline recharge (Caruccio and Geidel, 1989) and borehole injection (Aljoe and Hawkins, 1991; Ladwig and others, 1985) have also yielded mixed results. Field studies of alkaline addition (Brady and others, 1990) have been conducted but it has been difficult to identify definitive cause and effect relationships. Further research is continuing in this area.

Phosphate

The application of rock phosphate is another technique under study as a pyrite oxidation inhibitor (Renton and others, 1988; Evangelou and others, 1991). Pyrite weathering ultimate produces free ferric iron which acts to oxidize additional pyrite, thus establishing a cyclic and self-propagating series of reactions. Dissolution of rock phosphate in acid media releases highly reactive phosphate ions, which will combine with iron to form insoluble iron phosphate compounds. The formation of insoluble iron phosphates would halt or inhibit the cyclic reaction of iron and pyrite. Phosphate treatment has effectively reduced acid generation in laboratory studies; one field study showed a reduction of about seventy percent in acid load compared to a control (Meek, 1991). For reasons not yet completely understood, an application rate of about two to three percent rock phosphate provides the most effective control. Thorough mixing of phosphate and pyritic material also appears necessary for effective treatment. Further research is continuing in this area.

Coatings and Sealants

Other ongoing research activities are focusing on the surface chemistry of pyrite and development of various types of sealers, coatings and inhibitors to halt acid production.

Bactericides

The catalytic role of bacteria in pyrite oxidation has been well documented (Kleinmann and others, 1981). Many compounds have been screened as selective bactericides and the anionic surfactants sodium lauryl sulfate and alkyl benzene sulfonate are considered to be the most reliable inhibitors. Application of bactericides has reduced acid loading in field experiments. Bactericides are generally water soluble and will leach from the spoil. Currently, the time required for leaching of bactericides is uncertain. It is also unclear whether the sulfur and iron oxidizing bacteria will repopulate the spoil and catalyze the acid-producing reactions when the bactericide is depleted.

Encapsulation/Physical Barriers

The third major prevention/mitigation research area involves physical or geotechnical techniques to isolate or encapsulate pyritic material. Fly-ash, cements, bentonite, and other clays are a few of the materials studied as sealants and flow barriers (Skousen and others, 1987; Bowders and Chiado, 1990). Successful application of these methods in the field is heavily dependent on good engineering and construction practices and site conditions. Other investigations have attempted borehole injection to isolate buried pyritic material. Research is ongoing in this area and may escalate as solid waste disposal rules become more stringent.

Mine-spoil Hydrology

Although sometimes not considered an AMD research topic, mine spoil hydrology plays a crucial role in determining drainage quality. Relatively few studies of hydrogeologic processes have been conducted in the context of controlling mine drainage quality, and it is a subject in need of further investigation.

Much useful research has been conducted in predictive and preventative acid drainage techniques. No universally effective technologies have yet been developed, however.

In a partnership-based joint venture called the Acid Drainage Technology Initiative (ADTI), OSM has joined with industry, the States, academia, and other government agencies and groups to identify science-based solutions to AMD problems. The ADTI Operations Committee has set up two working groups: one on AMD prevention/remediation and one on AMD prediction. The AMD remediation effort is expected to result in a multi-part agreed upon work product in the form of a user manual/notebook on AMD remediation methods, including historic case studies. This will be a compilation of all previously conducted AMD remediation technology experiments, data and information. The AMD prediction effort is expected to result in multi-party agreed upon recommendations for best science-based testing standards for AMD prediction and convert them into a best technology user manual for those working on AMD problems in the eastern United States.