Chemical evolution of coal mine drainage in a non-acid producing environment, Wasatch Plateau, Utah, USA

The causes and problems of coal mine drainage, particularly acid mine drain age, in the Eastern and Interior Coal Provinces of the United States are we ll documented. West of the Mississippi River, where coal mines account for about 45% of total US coal production and where acid mine drainage is rare, the chemical evolution of coal mine drainage is less well documented and u nderstood Zn this investigation, we have used solute and isotopic compositi ons of non-evolved inflow groundwater and evolved mine discharge water to q uantify the chemical evolution of mine discharge water in a western undergr ound coal mine. Water enters the mine from fractures and roof bolt holes, which intercept g roundwater in the overlying rock. Carbon-14, and H-3 data indicate that the se waters recharged between 12,000 and 19,500 years ago. The TDS and solute compositions of roof drip waters are spatially zoned and TDS concentration s range from about 300 to 550 mg l(-1). After the water encounters minerals and other substances in the mine, the chemical differences between various mine regions become more pronounced and the TDS of mine drainage water inc reases to about 850 mg l(-1). The TDS of mine drainage is related to water- rock ratios. Mine drainage issuing from the older mined areas, where water- rock ratios are low, has the greatest TDS. Geochemical and isotopic mass balance calculations were performed to quanti fy chemical reactions in the mine, and to identify sources contributing to the TDS of mine drainage. Chemical reaction pathways evaluated include pyri te oxidation, dissolution of native and rock dust gypsum, dissolution of ca lcite and dolomite, precipitation of calcite, ion exchange, precipitation o f iron hydroxide, and organic decomposition of mining machine emulsion flui d. Solute and isotopic mass transfer reaction calculations demonstrate that the oxidation of pyrite triggers a series of cascading in-mine chemical re actions that are the primacy cause of the elevated TDS of mine drainage rel ative to the TDS of roof-drip water. Pyrite oxidation does not result in ac id drainage because of the buffering effect of abundant carbonate minerals. Dissolution of gypsum, both native and gypsum dust previously used as rock dust, is also a significant contributor of SO42+. Ion exchange of Ca2+ on the sodium zeolite analcime, which occurs in the coal, accounts for an incr ease in Na+ concentrations. Oxidation of fugitive longwall emulsion fluid p roduces abundant CO2(g) some of which indirectly affect the TDS of mine dra inage. (C) 2000 Published by Elsevier Science B.V.