Stochastic multicomponent reactive transport analysis of low quality drainage release from waste rock piles: Controls of the spatial distribution of acid generating and neutralizing minerals.

In mining environmental applications, it is important to assess water quality from waste rock piles (WRPs) and estimate the likelihood of acid rock drainage (ARD) over time. The mineralogical heterogeneity of WRPs is a source of uncertainty in this assessment, undermining the reliability of traditional bulk indicators used in the industry. We focused in this work on the bulk neutralizing potential ratio (NPR), which is defined as the ratio of the content of non-acid-generating minerals (typically reactive carbonates such as calcite) to the content of potentially acid-generating minerals (typically sulfides such as pyrite). We used a streamtube-based Monte-Carlo method to show why and to what extent bulk NPR can be a poor indicator of ARD occurrence. We simulated ensembles of WRPs identical in their geometry and bulk NPR, which only differed in their initial distribution of the acid generating and acid neutralizing minerals that control NPR. All models simulated the same principal acid-producing, acid-neutralizing and secondary mineral forming processes. We show that small differences in the distribution of local NPR values or the number of flow paths that generate acidity strongly influence drainage pH. The results indicate that the likelihood of ARD (epitomized by the probability of occurrence of pH<4 in a mixing boundary) within the first 100years can be as high as 75% for a NPR=2 and 40% for NPR=4. The latter is traditionally considered as a "universally safe" threshold to ensure non-acidic waters in practical applications. Our results suggest that new methods that explicitly account for mineralogical heterogeneity must be sought when computing effective (upscaled) NPR values at the scale of the piles.

Slurry wall containment performance: monitoring and modeling of unsaturated and saturated flow.

A specific 2-year program to monitor and test both the vadose zone and the saturated zone, coupled with a numerical analysis, was performed to evaluate the overall performance of slurry wall systems for containment of contaminated areas. Despite local physical confinement (slurry walls keyed into an average 2-m-thick aquitard), for at least two decades, high concentrations of chlorinated solvents (up to 110 mg l(-1)) have been observed in aquifers that supply drinking water close to the city of Milan (Italy). Results of monitoring and in situ tests have been used to perform an unsaturated-saturated numerical model. These results yielded the necessary quantitative information to be used both for the determination of the hydraulic properties of the different media in the area and for the calibration and validation of the numerical model. Backfill material in the shallower part of the investigated aquifer dramatically affects the natural recharge of the encapsulated area. A transient simulation from wet to drought periods highlights a change in the ratio between leakages from lateral barriers that support a specific scenario of water loss through the containment system. The combination of monitoring and modelling allows a reliable estimate of the overall performance of the physical confinement to be made without using any invasive techniques on slurry wall.