Long-Term Water Quality Trends in a Partly Flooded Underground Coal Mine

Abstract

Water quality trends for an 8 year period were analyzed for two acidic springs draining from a partially flooded underground coal mine, and the composite mine-pool outflow of 10 discharges. Time series analysis was used to separate long-term data trends from short-term noise. Short-term variation usually constituted less than 30 percent of the trend concentration. Exponential functions were fitted to the trend data, and time estimates (t 50C) for concentration to decline 50% were generated for Total Acidity, Fe, Al, SO 4 , Co, Ni, and Zn concentrations. Iron decline is similar at two springs with an estimated t 50C of 60 months. Sulfate t 50C is about 60 to 70 months at one spring and for the aggregate mine-pool. Cobalt, Ni and Zn declines are more rapid, with estimated t 50C of about 30 to 50 months. Aluminum decline is 2 to 3 times slower than rates for other parameters, and mine-waters are near apparent equilibrium for the mineral jurbanite, Al(SO 4)OH*5H 2 O. Constituent fluxes are controlled mostly by flow, and decline with time. Estimated time (t 50F) for flux to decline 50% for the composite mine-pool outflow is about 85 months for Fe, 80 months for SO 4 and 105 months for acid flux. Fluxes are 1.5 to 3 times greater in spring than fall, and reflect seasonal distribution of precipitation and recharge to the mine. Most of the improvement in mine-pool discharge results from declining pollutant concentrations. These trends suggest a slow decline in pyrite oxidation, with significant water quality improvement occurring on the order of years to decades.

Water Quality Evolution in Flooded and Unflooded Coal Mine-Pools

“…Under current conditions, Al concentration shows the slowest rate of improvement of the mine drainage parameters examined. Similar trends were found in another acidic unflooded mine (Perry and Rauch 2004) where a half life for Al in mine drainage was 2 to 3 times longer than other metals. So long as mine-pool pH remains below about 4.0, dissolution of alumino-silicate minerals and Al hydroxides could occur.…”

“…Mineral saturation and precipitation should be favored during dry periods, and dissolution promoted during high flow conditions. We observed similar trends in another unflooded mine-pool (Perry and Rauch, 2004). The jurbanite saturation index is more scattered beginning in 1999, and corresponds to limestone injection.…”

“…Inhibition of pyrite oxidation as a result of the limestone injection is not apparent in any of the compiled water quality data. A comparable effect was observed with grout injection in another mostly unflooded underground coal mine in a similar hydrogeologic setting (Perry and Rauch, 2004). Aggressive reclamation techniques like injection into underground mine-works may cause short-term deterioration in water quality, before beneficial effects of the treatment are seen.…”