GEOCHEMISTRY OF THE OBEY RIVER BASIN, NORTH-CENTRAL TENNESSEE - A CASE OF ACID-MINE WATER IN A KARST DRAINAGE SYSTEM

The East Fork of the Obey River, a tributary of the Cumberland River, drains a 523 km2 basin along the western margin of the Cumberland Plat eau. The upper basin lies on clastic rocks that include coals that wer e mined earlier in this century. The lower basin is drained through a deep gorge where the East Fork has cut downward into the Mississippian carbonate rocks. The lower basin is karstic, with a 9 km length of th e river and most of its tributaries underdrained by conduit systems. M any of the upper basin tributaries of the East Fork are contaminated b y acid mine drainage and have high levels of acidity, sulfate, iron, a nd aluminum. As the acid mine waters sink in the limestone portions of the basin, they are buffered but acidity is not rapidly reduced and a cid waters appear at a large spring deep in the karst having survived 5 km of transport. Sulfate waters gradually convert to bicarbonate wat ers as more tributaries from carbonate rocks enter the system. Aluminu m in solution decreases with increasing pH, and precipitates as colloi dal sized particles. Although alkalinity increases in the lower reache s of the basin, most of the waters remain highly undersaturated with r espect to calcite.