Long-term changes in quality of discharge water from abandoned underground coal mines in Uniontown Syncline, Fayette County, PA, USA.

Changes in water quality over 25 years have been documented for discharges from an extensive network of abandoned underground coal mines in the Uniontown Syncline, Fayette County, PA, USA. A baseline study of 136 mine discharges in the syncline was conducted in 1974-1975. In 1998-2000, follow-up water flow and quality monitoring was conducted at 21 selected discharges for 2 years to assess the degree of mine water-quality improvement since 1974-1975. The data from the two periods of time were compared, with consideration of differences in measurement methods. The degree and rate of water-quality improvement was found to be highly dependent on the amount and duration of flooding in the mine voids. Water quality of discharges from the substantially flooded mine voids improved significantly, going from acidic water with high sulfate and iron concentrations in 1974-1975 to alkaline water with substantially lower sulfate and iron concentrations in 1998-2000. In contrast, the water quality in the unflooded mines showed less improvement over the 25 years between studies. The water discharging from the unflooded mines in 1974-1975 was acidic with high sulfate concentrations and in 1998-2000 was still acidic but showed somewhat lower sulfate and iron concentrations, reflecting depletion of readily available pyrite. The data obtained provide insight into the potential and rate of natural amelioration of mine water quality in different abandoned underground coal mine systems.