Oxygen transport and pyrite oxidation in unsaturated coal-mine spoil

An understanding of the mechanisms of oxygen (0₂) transport in unsaturated mine spoil is necessary to design and implement effective measures to exclude 0₂ from pyritic materials and to control the formation of acidic mine drainage. Partial pressure of oxygen (Po₂) in pore gas, chemistry of pore water, and temperature were measured at different depths in unsaturated spoil at two reclaimed surface coal mines in Pennsylvania. At mine 1, where spoil was loose, blocky sandstone, Po₂ changed little with depth, decreasing from 21 volume percent (vol%) at the ground surface to a minimum of about 18 vol% at 10 m depth. At mine 2, where spoil was compacted, friable shale, Po₂ decreased to less than 2 vol% at depth of about 10 m. Although pore-water chemistry and temperature data indicate that acid-forming reactions were active at both mines, the pore-gas data indicate that mechanisms for 0 2 transport were different at each mine. A numerical model was developed to simulate 0₂ transport and pyrite oxidation in unsaturated mine spoil. The results of the numerical simulations indicate that differences in 0₂ transport at the two mines can be explained by differences in the air permeability of spoil. Po₂ changes little with depth if advective transport of 0₂ dominates as at mine 1, but decreases greatly with depth if diffusive transport of 0₂ dominates, as in mine 2. Model results also indicate that advective transport becomes significant if the air permeability of spoil is greater than 10^-9 m², which is expected for blocky sandstone spoil. In the advective-dominant system, thermally-induced convective air flow, as a consequence of the exothermic oxidation of pyrite, supplies the 0₂ to maintain high Po₂ within the deep unsaturated zone.