Evaluating contaminant pathways in an altered vadose zone: a multidisciplinary approach in open-pit quarry environments

Abstract The closure of mining operations presents significant environmental challenges for groundwater protection and sustainable closure planning. Fractured and altered aquifers, which supply drinking water to nearly half the world’s population, are vulnerable to disruptions caused by mining. This study investigates groundwater flow and contaminant dynamics in an altered vadose zone and fractured rock environment at a quarry situated 20 km east of Pretoria, South Africa. The primary objective is to develop effective monitoring strategies for groundwater protection post-mine closure. The heterogeneous geological structures, including a network of fractures and a diabase sill, generate distinct water types and flow dynamics, with active groundwater circulation despite seasonal fluctuations. Field investigations and tracer analyses revealed seasonal variations in transport parameters. Blasting activities modified fracture characteristics, creating new pathways and increasing connectivity between fracture networks, complicating contaminant migration. Tracer tests analysed with the MDMi and MDP-2RNE analytical models identified variations in Péclet numbers and mean transit times, with higher transport velocities and lower retardation factors during the wet season. The MDMi model was more sensitive to fracture connectivity changes, while the MDP-2RNE model highlighted seasonal differences in flow velocities and dispersion. These findings highlight the challenges of parameterisation and the necessity for seasonal calibration in modelling contaminant transport. A conceptual site model (CSM) illustrates how mining has transformed groundwater flow patterns and contaminant transport mechanisms, providing critical insights into sustainable groundwater management and mine closure planning. This research highlights the importance of targeted monitoring strategies to protect groundwater in altered vadose zone environments post-mine closure.