LONG-TERM WATER QUALITY TRENDS IN A PARTLY FLOODED UNDERGROUND COAL MINE

摘要

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 percent 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_2O. Constituent fluxes are controlled mostly by flow, and decline with time. Estimated time (t_(50F)) for flux to decline 50 percent 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.