Reactive transport modeling of unsaturated hydrology and geochemistry of neutral and acid rock drainage in highly heterogeneous mine waste rock at the Antamina Mine, Peru

Drainage quality from variably-saturated mine waste-rock dumps is controlled by multiple processes that are effective at different scales. The objective of this research is to improve the conceptual understanding of coupled hydrological and geochemical processes in mine waste rock using reactive transport modeling. Multicomponent reactive-transport was modeled using the code MIN3P to investigate sulfide oxidation and acid buffering reactions constrained by two field-scale studies of fine-grained reactive intrusive material at the Antamina mine, Peru: 1) 1 m-high field barrel and 2) 10 m-high experimental pile. At the field-barrel scale, the uniform flow and solute transport model was able to capture long term concentration trends in the discharge. Sulfide mineral oxidation along with pH-buffering reactions, and Cu and Zn secondary mineral precipitation/dissolution were considered the main processes controlling metal concentrations. Results indicate seasonal fluctuations in dissolved concentrations controlled by precipitation/dissolution of secondary minerals in wet and dry cycles and a long-term trend towards more acidic drainage. At the pile scale, the uniform-flow and solute transport model was successful in matching the field-observed basal discharge and cumulative outflow. The results demonstrate the importance of preferential flow not only in rock-like, but also in soil-like waste-rock piles and indicate that calibrating an unsaturated flow model to observed outflow alone is insufficient to evaluate flow patterns and residence times in waste rock. Therefore, mobile-immobile, dual-porosity and dual-permeability approaches were implemented into the MIN3P code and the enhanced code was used to improve the simulation of tracer breakthrough at the pile scale, relative to the uniform flow and solute transport model. Although substantial improvements could be obtained by using the dual domain approaches, observed tracer peak concentration and tailing were not well captured, suggesting the presence of a third immobile region with a very slow release rate. Based on the geochemical system developed for the field barrel scale and the dual-domain model developed for the pile scale, the applicability of the enhanced code for modeling of multicomponent reactive transport in waste rock at the pile scale was demonstrated, and in the process, the distribution of reactivity in preferential flow and matrix regions was evaluated.Science, Faculty ofEarth, Ocean and Atmospheric Sciences, Department ofGraduat