Hydrogeochemical Forensics of Pyrite Oxidation in Unsaturated Mine Overburden: A Numerical Simulation Framework for Groundwater Contaminant Migration.

Groundwater contamination arising from mining activities represents a persistent and complex environmental challenge, particularly in coal-bearing regions where sulfide-rich mine overburden is extensively exposed to atmospheric conditions. Upon interaction with oxygen and moisture, pyrite oxidation generates acidic by-products and mobilizes dissolved constituents, such as ferrous, ferric iron, sulfate, and hydrogen ions. Understanding and predicting the spatiotemporal evolution of contaminant plumes in such systems remains challenging due to the coupled nature of variably saturated flow, multicomponent geochemical reactions, and microbially mediated processes. This study develops a comprehensive numerical modeling framework for simulating contaminant transport and remediation processes associated with oxidation reactions in unsaturated mine overburden systems. Variably saturated flow is represented through discretization of the governing flow equation in the vertical domain using hydraulic head-based parameters, and the resulting tridiagonal system of linear equations is efficiently solved using the Thomas algorithm. The model couples variably saturated groundwater flow, represented by Richards’ equation, with multicomponent reactive transport equations describing the generation and migration of key oxidation products (Fe²⁺, Fe³⁺, SO₄²⁻, and H⁺). In addition, sulfate reduction mediated by sulfate-reducing bacteria (SRB) is incorporated to capture biologically driven attenuation mechanisms relevant to natural and engineered remediation scenarios. Simulations are performed for a total of 22 years (8030 days). A time step of 0.1 day and a grid size of 0.2 m are identified as the optimal choices for the simulations. The simulation results indicate that the concentrations of oxidation-derived species decrease significantly from 200 to 40 mol/m³ in clay, 300 to 95 mol/m³ in loam, and 1 to 0.2 mol/m³ in sand. Sensitivity analysis shows that peak sulphate sensitivity in clay with a sensitivity index (SI) of 0.65 and in loam with an SI of 0.5 under high saturation condition (water content, w_c = 0.9), while ferrous ions exhibit maximum sensitivity in loam under low saturation condition (w_c = 0.2) with an SI of 750. The findings support the development of predictive frameworks that can inform sustainable groundwater management, optimize remediation strategies, and address key challenges in the practical application of contaminant transport models.