Modelling the impact of physical and chemical heterogeneity on solute leaching in pyritic overburden mine spoils

Spatial variability of physical and chemical properties may affect acidification and acid mine drainage in sulfide-bearing overburden mine spoils. This study compares heterogeneous with vertically layered and horizontally averaged spatial distributions of chemical components in a generic 2D-vertical spoil cross-section with heterogeneous distributions of water and air contents and of water flux densities. The initial spatial distributions of the fraction of sulfur and the pre-oxidized zones follow autocorrelated random functions. The initial background chemistry distribution is correlated to the degree of pre-oxidation using five classes of differing initial chemical systems. Processes considered include variably saturated water flow, oxygen diffusion, shrinking-core kinetics of pyrite oxidation, multicomponent reactive solute transport, and geochemical equilibrium reactions between aqueous and mineral components. Spatial distributions were generated using the SGSIM-GSLIB geostatistical simulation algorithm and estimated parameters. Results show differences in vertical concentration profiles of chemical components and in integrated breakthrough curves at 30 m depth between the heterogeneous and the layered case for both—high and low—sulfur content scenarios. Differences are relatively large in the beginning and with respect to the distribution of solid phase components. A heterogeneous sulfide mineral distribution results in vertical spreading of the oxidation front while precipitation and dissolution of the secondary minerals affects the acid mine drainage, by locally retarding and releasing the solutes.