Oxygen transport and pyrite oxidation in unsaturated coal-mine spoil

Abstract

An understanding of the mechanisms of oxygen (0 2 ) transport in unsaturated mine spoil is necessary to design and implement effective measures to exclude 0 2 from pyritic materials and to control the formation of acidic mine drainage. Partial pressure of oxygen (Po 2 ) 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, Po 2 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, Po 2 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 0 2 transport and pyrite oxidation in unsaturated mine spoil. The results of the numerical simulations indicate that differences in 0 2 transport at the two mines can be explained by differences in the air permeability of spoil. Po 2 changes little with depth if advective transport of 0 2 dominates as at mine 1, but decreases greatly with depth if diffusive transport of 0 2 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 m 2 , 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 0 2 to maintain high Po 2 within the deep unsaturated zone.