Simulated Aerobic Pedogenesis in Pyritic Overburden with a Positive Acid—Base Account

Abstract

AbstractReclamation of surface‐mined land is often hindered by the excess salts and acidity produced by the weathering of pyritic overburden. This study was conducted to document the initial transformations that occur when pyritic overburden containing excess acid neutralizing potential is used as parent material in minesoil construction. An overburden containing 0.8% FeS2 (pyrite) and 1.6% inorganic carbonate (predominantly dolomite) was collected from the highwall of an active lignite surface mine in Panola County, Texas. The overburden was lightly crushed through a 13‐mm sieve and packed into three replicate lysimeters (0.75 by 0.75 by 1.2 m). The lysimeters were leached monthly with 63.5 mm of deionized water for 24 mo. The initial material had a pH of 8.3 and an excess acid neutralizing potential. Progressive FeS2 oxidation released H2SO4 and the pH decreased to 6.8. The dolomite dissolved, neutralizing the acidity, with subsequent release of Ca and Mg ions into solution. Leachate Ca2+ and SO2−4 concentrations exceeded the ion activity product of gypsum in the lower 60 cm of the lysimeters. Thin‐section analysis revealed that gypsum crystals precipitated along margins of residual pyrite particles and in conductive vughs and channels. The continued accumulation of gypsum in minesoil development could eventually lead to the formation of a gypsic or a petrogypsic horizon. A restrictive layer such as this would decrease vertical movement of water and O2, which would reduce vegetative growth, increase runoff and erosion, and thus increase the probability of reclamation failure.