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

Abstract

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 $\overline{NPR} = 2$. 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 $\rho \to 0$ 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 $\sigma_{w}^{2}$, 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 ($\sigma_{w}^{2} \ge 1$) 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.