Hydrogeochemical modelling of pyrite oxidation ion mobility in unsaturated mine waste rock piles

The present study is aimed to formulate a numerical model to simulate hydrogeochemical reactions controlling pyrite oxidation in unsaturated aquifers under non-uniform flow conditions. A coupled flow and multi-component transport model is solved using a finite difference scheme to simulate the concentration distribution of oxygen (O₂), ferrous (Fe²⁺), ferric (Fe³⁺), sulphate (SO₄²⁻) and hydrogen (H⁺) ions. An attempt is made to quantify the transport characteristics of pyrite oxidation ions by analysing the spatial moments of concentration profiles. The simulation results suggest that the variable flow rate in the vertical direction significantly enhances the pyrite oxidation near the ground surface. Peak concentration of Fe²⁺, Fe³⁺, and SO₄²⁻ near the ground surface is found to be approximately 2–3 times higher in the case of variable flow. The pH level near the ground surface drops considerably when variable flow conditions exist in the unsaturated zone. The variation in flow parameters such as porosity, and water saturation and reaction parameters such as oxygen diffusion rate and the oxygen consumption rate are found to affect the mass and mobility of Fe²⁺, Fe³⁺, and SO₄²⁻ considerably. Although there has been substantial research on hydrogeochemical events associated with pyrite oxidation in the past, there has been very less focus on the transport qualities of the ions produced in groundwater. The mobility assessment of pyrite oxidation ions in aquifer presented in the study is beneficial to understand the groundwater quality risk associated with mining and to plan and implement effective remediation strategies for the restoration of affected aquifers.