Modelling the effect of chemical heterogeneity on acidification and solute leaching in overburden mine spoils

Abstract

The generation of acid mine drainage from overburden spoil piles at open-pit lignite mines is impacting the quality of groundwater and surface water bodies in large parts of the Lusatian mining area in Germany. Values of pH as low as 1 have been observed in the groundwater. After decommissioning, mine pits are generally converted to lakes which may also be acidic owing to the acidic groundwater discharge. The acidic effluent is generated by sulphide oxidation in the unsaturated zone of the spoil pile which generally extends to large depths as a result of dewatering. The long-term evolution of the acidification is still largely unknown. Our research focuses on the effects of physical and chemical heterogeneity caused by mixing of soil materials that may have already been oxidized to different degrees during the deposition of the spoil pile. Processes considered include variably saturated groundwater flow, oxygen diffusion in the soil gas, kinetic pyrite oxidation and acidic effluent generation, advective–dispersive transport of the aqueous components, equilibrium geochemical reactions between the chemical components and the soil minerals, and possible buffering and acid neutralization. Several existing numerical codes were coupled to represent the complete set of processes. Simulations were carried out in one- and two dimensions using representative characteristics of mine spoil piles, with the two-dimensional representation being based on spatially heterogeneous random fields of hydraulic conductivity and sulphide mineral fractions. Results show the long-term evolution of the oxidation front, the mass flux of oxidation products and the effects of system heterogeneity. Under conditions of constant flow, the system is found to return to neutral conditions over a time period on the order of several decades. Further work, including sensitivity analyses with respect to the controlling parameters and model calibration using site-specific field data, will be necessary to obtain a full understanding of the complex interacting mechanisms. The simulation approach shows promise as a potentially useful tool for predicting the effects of sulphide oxidation on the water quality of pit lakes.