Geochemistry and stable isotope investigation of acid mine drainage associated with abandoned coal mines in central Montana, USA

The Great Falls-Lewistown Coal Field (GFLCF) in central Montana contains over 400 abandoned underground coal mines, many of which are discharging acidic water with serious environmental consequences. Areas of the mines that are completely submerged by groundwater have circum-neutral pH and relatively low concentrations of metals, whereas areas that are only partially flooded or freely draining have acidic pH (<3) and high concentrations of metals. The pH of the mine drains either decreases or increases after discharging to the surface, depending on the initial ratio of acidity (mainly Al and Fe ²⁺) to alkalinity (mainly HCO ₃ ^-). In acidic, Fe-rich waters, oxidation of Fe ²⁺ after exposure to air is microbially catalyzed and follows zero-order kinetics, with computed rate constants falling in the range of 0.97 to 1.25mmol L ^-1 h ^-1. In contrast, Fe ²⁺ oxidation in near-neutral pH waters appears to be first-order with respect to Fe ²⁺ concentration, although insufficient data were collected to constrain the rate law expression. Rates of Fe ²⁺ oxidation in the field are dependent on temperature such that lower Fe ²⁺ concentrations were measured in down-gradient waters during the day, and higher concentrations at night. Diel cycles in dissolved concentrations of Zn and other trace metals (Mn, Ni) were also noted for down-gradient waters that were net alkaline, but not in the acidic drains.The coal seams of the GFLCF and overlying Cretaceous sandstones form a perched aquifer that lies ~50m above the regional water table situated in the underlying Madison Limestone. The δD and δ ¹⁸O values of flooded mine waters suggest local derivation from meteoric water that has been partially evaporated in agricultural soils overlying the coal mines. The S and O isotopic composition of dissolved sulfate in the low pH mine drains is consistent with oxidation of biogenic pyrite in coal under aerated conditions. A clear distinction exists between the isotopic composition of sulfate in the acid mine waters and sulfate in the adjacent sedimentary aquifers, making it theoretically possible to determine if acid drainage from the coal mines has leaked into the underlying Madison aquifer.