Controls of uncertainty in acid rock drainage predictions from waste rock piles examined through Monte-Carlo multicomponent reactive transport

Heterogeneity in waste rock piles (WRPs) determines uncertainty in acid mine drainage (ARD) predictions from these deposits. Numerical modeling based on a novel and efficient stochastic framework to evaluate influential heterogeneity-linked factors controlling such uncertainty. The analysis considers a representative WRP with a mean neutralization potential ratio NPR over bar =2. The heterogeneity-linked factors are: (1) Scale-dependent mineralogical variability. At the local scale, the variability within individual rock blocks in the waste rocks (10 s of cm) is measured through the correlation coefficient (rho) between acid producing and acid consuming minerals, here considered a geogenic property of the site. For the analyzed conditions, as rho -> 0 WRPs tend generate a higher risk of ARD and higher variability among results, which can be explained by the increasing mineralogical mixing (blending) as rho grows. At the field scale, the coefficient of variation (CV) is measured as the mineralogical variability of all rock blocks within the WRP. Since CV is an engineering design parameter of a WRP, the results suggest that building WRPs with lower CVs results in less uncertain predictions of long-term neutralization capacity of the piles. (2) Flow heterogeneity. The variance of solute travel times through a pile, here measured by sigma w2, can be used to characterize flow heterogeneity, where high variance means stronger preferential flow in the WRP. Simulated ARD mass loadings with strong flow heterogeneity (sigma w2 >= 1) leads to significant differences to the homogeneous case, increasing the uncertainty in the estimation of the ARD risk. (3) Pore gas concentration. In well-ventilated WRPs the effect of mineralogical heterogeneity is enhanced (WRPs generate much higher risk than WRPs with diffusion-limited gas transport modalities. Gas diffusion limits the amount of acidity produced in sulfidic-rich zones, thus attenuating the effect of mineralogical variability at the scale of the WRPs compared to well-ventilated piles.