Assessing the impact of acid mine drainage on groundwater resources using 3D solute transport modelling in fractured rock settings: The Dareh-Zar copper mine, Iran

Acid mine drainage (AMD) poses a major risk to groundwater quality in mining regions due to the oxidation of sulphide minerals and the subsequent release of sulphate and heavy metals. This study aimed to assess the transport behaviour of sulphate and copper within the fractured aquifer system of the Dareh-Zar copper mine in south-central Iran, where mining activities have significantly altered local hydrogeological conditions. A three-dimensional numerical groundwater flow and solute transport model was developed to simulate contaminant migration under current site conditions. Sulphate was treated as a conservative tracer, while copper was modelled as a reactive solute influenced by geochemical attenuation mechanisms such as adsorption and precipitation. The model incorporated geological structures, hydrochemical gradients, and depth-dependent hydraulic properties to evaluate contaminant transport behaviour. Results showed that sulphate formed a widespread plume aligned with regional groundwater flow directions and fault zones, while copper remained localised near the source. Transport in upper fractured volcanic units was enhanced due to their higher permeability, whereas plume development was limited in deeper layers with lower hydraulic conductivity. A remediation scenario demonstrated a 27% reduction in annual sulphate loading, decreasing from 2190 to 1597 tonnes per year. These findings highlight the influence of geological structure on solute transport and demonstrate the potential effectiveness of natural attenuation and source control strategies in managing AMD impacts on groundwater systems.