Numerical model to assess the effect of hydrogeological characteristics of mine waste piles on capping efficiency in unsaturated conditions

Oxygen capping is widely adopted to control pyrite oxidation and ion transport in waste piles. Hydrogeology of waste piles significantly affect capping performance. However, the research studies to quantify the impact of hydrogeological conditions on capping efficacy are limited. This study aims to examine the impact of hydrogeological factors on capping efficiency and pyrite ion transport in unsaturated waste piles. A numerical model is formulated by integrating 1-D vertical flow and 2-D reactive transport models. The impact of hydrogeological factors such as porosity, water content and soil texture on the oxygen infiltration, pyrite ion (ferrous, sulphate, ferric) transport and pH distribution is investigated. Sensitvity index based on the zeroth spatial moment (ZSM) of ferrous, sulphate, and ferric ion is estimated to quantify the impact of these factors on pyrite ion transport in mine waste piles. Outcome of the analysis shows that increment in the porosity of waste pile reduces pyrite oxidation. As the porosity increases from 0.2 to 0.6, the peak levels of ferrous, sulphate and ferric ions are reduced by around 55%, 68%, and 85% respectively. Moreover, increment in initial water content is found to enhance peak ionic levels in waste piles. Oxygen capping is found to be more effective to control ferric and sulphate ionic levels in mine waste at a water content of 0.9. pH levels in mine waste gets increased with the increment in the water content. However, soil texture is found to not have considerable impact on the ionic concentration levels and capping efficiency.