Combined use of environmental and artificial tracers to characterise the anthropogenically altered vadose zone and groundwater system

Abstract

Understanding groundwater flow and contaminant transport in fractured rock environments is critical for sustainable water resource management, particularly in regions impacted by mining activities. Open-pit quarries significantly alter the natural hydrogeological regime by modifying recharge pathways, introducing contaminants, and enhancing preferential flow through fractures. This research systematically investigates the transport mechanisms and pathways of environmental and artificial tracers within the anthropogenically altered fractured vadose zone of an open-pit quartzite and sandstone quarry located approximately 20 km east of Pretoria, South Africa. The study employs a combination of environmental tracers (δ¹⁸O, δ²H, ³H) and artificial tracers (uranine, rhodamine WT) to characterise both the vadose and saturated zones, quantify fluid dynamics, and explain important tracer transport processes. A hydrocensus assessment and hydrochemical analyses were conducted and revealed two distinct water types: shallow groundwater samples displayed a calcium-magnesium bicarbonate facies, indicating relatively fresh shallow groundwater with little chemical alteration from recharge to discharge, while pit water samples exhibited a calcium-magnesium-sulphate facies, reflecting sulphide mineral dissolution and contamination from legacy waste disposal. Stable isotope analyses (δ¹⁸O:-5.40‰ to-1.20‰, δ²H:-32.0‰ to-5.80‰) highlighted diverse recharge sources and evaporation effects, with D-excess values (4.04‰ to 14.94‰) further differentiating groundwater and surface water interactions. Tritium concentrations ranged between 0.0 and 2.3 Tritium units(TU), with a mean of 0.9 TU. Groundwater samples exhibited tritium concentrations between 0.4 and 1.0 TU, suggesting relatively older water or a mix of old and young recharge. Quarry pit water samples displayed tritium values of 1.0 to 1.1 TU, indicating a more recent recharge isotopic signature, suggesting that the pit water is primarily influenced by recent precipitation, runoff, and groundwater seepage. These findings provide insights into the groundwater recharge dynamics and the influence of quarrying activities on flow systems. Tracer experiments revealed significant seasonal variations in transport dynamics. During the wet season, mean residence times ranged from 0.689 to 2.043 hours, with an average of 1.1–1.3 hours, reflecting slower transport under higher soil saturation. In contrast, the dry season showed faster tracer movement, with residence times between 0.106 and 0.184 hours (average 0.13–0.15 hours), aligning with the observed higher flow velocities (0.009 m/s) despite lower saturation. This suggests that increased flushing volume temporarily enhanced hydraulic gradients by increasing the volume and pressure of water moving through the system. The resulting higher flow velocities likely reflect a more pronounced hydraulic gradient, even under lower soil saturation. If there has been any anthropogenic disturbance of the aquifer, such as changes in permeability due to mining or excavation activities, this could also contribute to localised increases in hydraulic conductivity, further accelerating transport. This would also be consistent with the lower recovery rate of 10% due to greater flow divergence from the injection point. The distance from the tracer injection to the breakthrough point was 6.5 meters, further emphasising the observed variations in transport dynamics. The application of analytical models (MDMi and MDP-2RNE) effectively captured seasonal variations in flow velocities, dispersion, and fracture connectivity. Péclet numbers and mean transit times varied between seasons, with higher velocities and lower retardation factors during the wet season, reflecting enhanced connectivity under saturated conditions. A steady-state water balance analysis was conducted to quantify inflows and outflows within the quarry pit lake system. The results indicated a water balance deficit of -30.96 ML, suggesting that estimated inflows (74.88 ML/a) were significantly lower than outflows (105.84 ML/a). This discrepancy was attributed to unaccounted surface runoff, which was not explicitly incorporated into the initial balance calculations. Given the quarry’s inward-draining nature, with steep gradients directing water towards the pit, additional runoff contributions were estimated based on a mean annual runoff rate of 42.2 mm/a for the quaternary catchment. When incorporating this missing runoff component (MARunoff = 79.3 mm/a), the observed deficit was largely reconciled, reinforcing the importance of considering surface runoff in hydrogeological assessments of open-pit mining environments. The hydrogeologic conceptual site mode (CSM) was developed from the geology, borehole lithology, structural geology, groundwater and surface water features. The CSM was supported by modelling results, which successfully validated the tracer results and represented groundwater flow and contaminant transport pathways. This model highlighted the impact of fractures and human-induced changes on the vadose zone, accurately capturing the key flow and transport processes. Key findings demonstrate the effectiveness of combined tracer methods in characterising the vadose zone and emphasise its critical role in groundwater contamination assessments. This research provides innovative insights into flow and transport dynamics in fractured, unsaturated systems, contributing to mine water management and understanding contaminant behaviour in such environments. Furthermore, these findings have broader implications for contaminant transport in anthropogenically altered vadose zones beyond mining environments. For example, previous research on potential SARS-CoV-2 contamination of groundwater in cemeteries has highlighted the risks associated with sinkhole formation and enhanced infiltration in altered vadose zones, which share similar hydrogeological characteristics with open-pit quarries. This study, therefore, shows the importance of understanding fluid pathways and contaminant mobility in disrupted subsurface environments, informing sustainable water resource management in both mining and burial site contexts. By integrating empirical data with tracer based analytical techniques, this work further highlights the importance of understanding complex interactions within anthropogenically altered vadose zones, ultimately supporting sustainable water resource management in open-pit mining operations.