Combined effects of dissolution kinetics, secondary mineral precipitation, and preferential flow on copper leaching from mining waste rock

A probabilistic Lagrangian approach to reactive subsurface transport is used to investigate possible processes that may explain reported discrepancies between modelled and observed copper concentrations at the Aitik waste rock heaps in northern Sweden and to quantify the implications of these processes for the long-term dynamics of copper leaching. The presented Lagrangian transport system couples two different types of primary dissolution kinetics with flow heterogeneity and pH-dependent equilibrium precipitation/dissolution of secondary copper-bearing minerals. Of the investigated processes, which we chose to study based on previous geochemical studies of the site, only flow heterogeneity in the form of preferential flow paths can provide a possible explanation for the low drainage water concentrations of copper currently measured in the field. The investigated pi-I-dependent precipitation-dissolution of secondary copper bearing minerals provides an explanation for even lower copper concentrations observed in laboratory column experiments in fresh waste rock; these experiments correspond to the initial part of the weathering phase in the field. If both preferential flow and secondary copper precipitation/dissolution occur at the Aitik site, the assumed prevailing type of primary dissolution kinetics does not influence the modelled expected long-term copper leaching nearly as much as it does under homogeneous flow conditions without formation of secondary minerals. The present sensitivity analysis does not provide a comprehensive prediction model for the Aitik site, since not all possible hydrological and chemical processes have been included in the investigation. This study rather exemplifies the application of the probabilistic Lagrangian approach as an investigation methodology for field-scale geochemical problems by using and extending results of detailed geochemical modelling.