Processes Governing Flow and Chemical Characteristics of Discharges from Free-Draining, Underground Coal Mines

In the Uniontown Syncline of Southwestern Pennsylvania, discharges from unflooded, free-draining coal mines are acidic with high sulfate concentrations. Flow and water quality data obtained in 1998-2000 for an unflooded mine discharge in the Uniontown Syncline were evaluated using a tank reactor fill-and-draw model to describe seasonal variations in outflows over time observed for the mine as well as to simulate discharge water quality. The hydraulic model was coupled to a chemical mass balance using estimates of recharge water quality and in-mine chemical production/loss. Field data indicated that the concentrations of sulfate, iron, and acidity were fairly constant even when flow varied greatly. Flow-related mass production functions for these constituents were obtained by fitting the field data. The hydraulic-chemical model was used to simulate sulfate and acidity production from pyrite dissolution and total carbonate loss in the mine. Model simulations indicated that in-mine acid production correlated with recharge rate, due to the sustained presence of oxygen which drives pyrite dissolution, and that recharge water chemistry had a significant influence on discharge characteristics. For the mine studied, alkaline recharge water mitigates the acidity of the discharges.