Impact of capping on pyrite oxidation ion transport in unsaturated mine waste pile: A hydrogeochemical modeling study

The present study employs a hydrogeochemical model to investigate the effect of regulating oxygen ingression and infiltration on the transport of pyrite oxidation ions in unsaturated waste piles. The model aims to couple flow, pyrite oxidation and multi-component reactive ion transport in unsaturated systems. The coupled hydrogeochemical model is solved using fully-implicit finite difference numerical technique. Hydrogeochemical transport is simulated in a two-dimensional domain, whereas the infiltration is simulated in vertically downward direction. The coupled model is solved to obtain `oxygen, ferrous, ferric, sulphate, and pH distributions within the waste pile. Flow and transport simulations are carried out for various oxygen capping and infiltration scenarios. The model findings suggest that the oxygen penetration depth within the waste pile is limited to 4.5 m for the non-capping scenario. An increase in the capping level from 25 to 75% reduces the peak concentrations of ferrous, ferric, and sulphate by 41.7%, 60%, and 37.5%, respectively. However, the pH values in the waste pile are found to be identical across all capping rates investigated. The efficiency of oxygen capping on the mass of sulphate and ferrous ions varies with the infiltration rate and oxygen diffusion coefficient. As the oxygen diffusion coefficient increases, the ionic ferrous and sulphate mass in the waste pile becomes more sensitive to the capping rate. The findings of the current model may be utilised to assess the efficiency of different capping scenarios and to adopt the optimal conditions for oxygen capping and infiltration control in mine waste piles.