Acid Mine Drainage and Metal Leaching Potential at Makum Coalfield, Northeastern India

Acid mine drainage (AMD) is one of the primary environmental concerns in the coal mining industries. It is relatively more common in the coalfield deposited influenced by marine environment. The Makum coalfield, the largest Tertiary coalfields in India, is located in Assam and well known for generating AMD. The assessment of potential AMD has been done using the acid base accounting (ABA) study which serves as the principal tool for predicting the post mining water quality. In this paper a detailed mineralogical study of overburden, the static ABA test for both coal and overburden samples along with their leaching studies have been conducted. The coal seams and laminated carbonaceous shale contain significant amount of pyrite. Mineralogical study of the overburden reveals that dolomite is the dominant controlling mineral with trace amounts of calcite for neutralizing the acid generated by the oxidation of pyrite. The relatively higher amount of dolomite is found to be associated with siltstone and non-laminated (massive) carbonaceous shale. The ABA results indicated that all the coal seams, and few overburden materials have acid generating potential. Concentration of Cr, Cu, Mn, Ni, Pb and Zn in overburden is above their respective crustal abundances. However, Pb is above its crustal abundance in coal. Leaching experiment infers that overburden containing high pyrite has more potential for releasing metals relative to those having less pyrite. Concentrations of Mn, Ni and Pb in the leachate are found to be much higher than their respective water quality guideline. Abnormally high concentration of Mn is found to be associated with the dissolution of siderite. Coal releases significantly high concentration of Ni. The results of ABA and leachate study are nearly consistent with the direct mine discharge quality.