THE ENVIRONMENTAL-IMPACT OF MINE WASTES - ROLES OF MICROORGANISMS ANDTHEIR SIGNIFICANCE IN TREATMENT OF MINE WASTES

Mine wastes have been generated for several centuries, and mining acti vity has accelerated significantly during the 20th century. The mine w astes constitute a potential source of contamination to the environmen t, as heavy metals and acid are released in large amounts. A great var iety of microorganisms has been found in mine wastes and microbiologic al processes are usually responsible for the environmental hazard crea ted by mine wastes. However, microorganisms can also be used to retard the adverse impact of mine wastes on the environment. Conventionally, the mine drainage as well as the waste itself can be treated with alk ali to increase pH and precipitate metals. The main drawback of this m ethod is that it has to be continuously repeated to be fully effective . There may also be negative effects on beneficial microorganisms. Sev eral other treatment methods have been developed to stop weathering pr ocesses thereby reducing the environmental impact of mine wastes. One approach has been to influence the waste deposit itself by reducing th e transfer of oxygen and water to the waste. This can be achieved by c overing the waste or by placing it under water. Vegetating the cover w ill probably also decrease the transfer of oxygen and water, and will give the deposit area a more aesthetical appearance. The other main ap proach to reduce the environmental impact of mine wastes is to treat t he drainage water. Various methods aim at using microorganisms for thi s in natural or engineered systems. Sulfate-reducing bacteria, metal-t ransforming bacteria and metal accumulating microorganisms are some ex amples. Often, some kind of reactor design is needed to effectively co ntrol these processes. Recently, much interest has been focused on the use of natural or artificial wetlands for treatment since this genera lly is a low-cost and low-maintenance method. Bacterial sulfate-reduct ion and microbial metal accumulation are processes wanted in such syst ems. Few studies have dealt with long-term effects of wetland systems, but there are some indications that the wetland material has to be re placed for effective treatment. Furthermore, bacterial iron reduction may take place instead of sulfate-reduction in some wetlands. In gener al, the activity of microorganisms is neglected in the design of mine waste treatment systems, and the treatments are created merely from a technical point of view. This can result in situations where unexpecte d microbial processes take over, and, in the worst scenario, the overa ll effect is opposite to the desired.