Hydrologic and Geochemical Factors Governing Chemical Evolution of Discharges from an Abandoned, Flooded, Underground Coal Mine Network

Discharges from some underground flooded coal mines have exhibited increases in pH and reductions in contaminant loadings with time. Data from a study of mine water quality evolution in interconnected, flooded mines of the Uniontown syncline, Southwestern Pennsylvania were evaluated with the aid of modeling to elucidate the hydrologic and geochemical factors responsible for such changes. Coal barriers left in place from mining operations define three hydraulically distinct but interconnected zones: the southern, central, and northern pools. Assuming each mine pool to behave as a completely mixed tank reactor, a steady-state, tanks-in-series model was developed to describe system hydraulics. Chemical modeling components were coupled with the tank reactor hydraulic model to simulate inputs to the mine voids, acid generation from pyrite dissolution, and discharge water quality. Empirical in-mine chemical production terms were estimated for each of the mine pools based on discharge data from 1974 to 1975 and 1998 to 2000. The production terms were then used to simulate discharge water quality for each of the mine pools over a 50  year period. Simulated water quality in the northern and central mine pools reached steady-state conditions approximately 25-30  years after the mine pools flooded, evolving over time to reflect the recharge water quality. The simulation results indicate that the evolution of mine water quality in the flooded mine voids has been governed by alkaline recharge water slowly displacing acidic "first flush" water.