GEOCHEMICAL VARIATIONS IN ABOVE- AND BELOW-DRAINAGE COAL MINE DISCHARGES (1975 TO 2012)

Water samples from 24 Coal Mine Discharges (CMDs) in the anthracite region of Pennsylvania were collected in 2012 to evaluate long-term (37 year) changes in water quality that may be related to geochemistry, hydrology, and natural attenuation processes. It is hypothesized that decreasing rates of acid production will be observed over time because of diminishing quantities of unweathered pyrite, decreased access of O₂ to the subsurface after mine closure, and relatively constant influx of alkalinity from groundwater. Below-drainage systems receive groundwater inputs with low dissolved O₂, resulting in a decrease of pyrite oxidation and the gradual improvement of CMD water quality. Above-drainage systems with exposed rock surfaces facilitate O₂ transport and continual pyrite dissolution, alkalinity consumption, and less change in water quality over time. Compared to measurements in 1975, 1991, and 1999, Fe and SO₄ concentrations generally decreased while pH increased. Matched-pair tests between each year sampled indicate differences in pH, Fe, and SO₄ were significant (p<0.05) for below-drainage mines. Above-drainage discharges did not see any significant changes. The fluxes of these constituents tend to track with increases in discharge between 1991 and 2012, suggesting that dilution has an influence on CMD geochemistry over time. Geochemical equilibrium models indicate that Fe(II) is the dominant Fe species, and transformation to Fe(III) may be limited by O₂ transport. Saturation of Fe(III) precipitates varies with pH and Fe and SO₄ concentrations: increasing pH and decreasing concentrations of Fe and SO₄ limit the precipitation of K-jarosite and schwertmannite and favor precipitation of Fe(III) oxides.

Observations are consistent with a general hydrogeochemical model for the evolution of CMDs. Initial pyrite dissolution occurs in O₂-rich systems, mainly during active mining, which results in the accumulation of relatively soluble secondary sulfate minerals in mined rock. After mine closure, 1) groundwater entering the mines continues to dissolve the sulfate minerals and export Fe and SO₄; 2) recharge dilutes Fe and SO₄ concentrations, while pH increases, and 3) steady-state conditions eventually approach natural long-term weathering rates.