EBA Engineering Consultants Ltd. has prepared this report, based on literature review, to provide information to assist the Yukon mining industry in remediating environmental problems caused by acid rock drainage (ARD) with its associated heavy metals contamination. There are three main sections to this review. First, the history of ARD is presented. Second, the chemistry and microbiology of ARD and the treatments that are used worldwide, with emphasis on cold climate treatments, are reviewed. Finally, we present the most promising technologies developed in temperate regions, that could be tested under typical Yukon conditions. This is the main challenge given the short growing season and cold temperatures.
Acid mine drainage is caused by oxidization of metal ores, containing sulphur and metal sulfides found in coal. There are three steps in this process. First, oxygenated water, from rain, for example, oxidizes metal sulfides producing acidic water and ferrous iron. When the water becomes moderately acidic, a number of bacteria can assist in further oxidization and increase the acidification of the drainage water. Finally, when the water reaches a pH of 3.5, an iron bacterium, Thiobacillus ferroxidans, can further dissolve metal sulfides, such as pyrite, producing ferric hydroxide, which can smother vegetation. Also, the sulphuric acid is acutely toxic. Heavy metals that are toxic, are also present in the ARD. Some common treatments for ARD identified by EBA including neutralizing the acidity of water using limestones, minimizing water contact with metal sulfides, or using organic amendments to bind with heavy metal contaminated waters. Important developments in using natural wetlands for ARD, have taken place in the late 1990s, which have identified that anaerobic (oxygen free conditions) are important in treating ARD. High sediment loads in streams have been found to limit neutralization by coating carbonates and decreasing microbial reduction of metal, as well as preventing metal uptake by vegetation.
