Long-term water quality trends in a partly flooded underground coal mine

Abstract

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. Additional