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

Abstract

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.