REE GEOCHEMISTRY OF AN ACID MINE DRAINAGE SYSTEM IN WESTERN INDIANA

Rare earth element (REE) geochemistry was investigated at the Green Valley coal mine site in western Indiana. Acid mine drainage (AMD) discharges from several seeps at the site and flows into a nearby natural stream. Five locations (sites 1-5) in a constructed AMD channel were sampled to determine REE concentrations. Sites 1-4 are located in the upper section of the AMD channel, with site 5 located in a mixing zone near the confluence of the AMD channel with the natural stream. Unmixed AMD from sites 1-4 ranged in pH from 2.7 to 5.7 with an average pH of 3.7. The pH at site 5 was higher, ranging from pH 4.1 to 6.6. Concentrations of REE in unmixed effluent ranged from two to four orders of magnitude higher than concentrations reported for Ohio River water. Overall, REE data showed a distinctive trend in which REE concentrations increased with decreasing pH. The concentrations of total REE to Al and Fe also reflect changes in pH. Where pH increased due to mixing at site 5, heavy REE were significantly diminished compared to light REE. REE concentrations normalized against CI chondrites for sites 1-4 show a localized elevated concentration in the intermediate REE.