Investigation of groundwater flow systems in the Eastern Karoo Basin along the Drakensberg Mountains to the Indian Ocean transect

Eastern Karoo Sedimentary Basin, KwaZulu-Natal Province, South Africa. Groundwater flow in mountainous terrains is often assumed to migrate from high-elevation recharge zones toward lowland discharge areas; however, such systems remain poorly understood in geologically complex and data-scarce regions. This study investigates groundwater flow systems along a transect from the Drakensberg Mountains to the Indian Ocean, emphasizing the influence of geological controls on flow dynamics. A multi-method approach was employed, integrating six independent lines of evidence: potentiometric surface mapping, wetlands and drainage patterns, spring distribution, baseflow analysis, isotope and hydrochemical tracers, and subsurface geological structure analysis. Results reveal a dominantly localized and compartmentalized groundwater flow system, in contrast to the expected regional flow from uplands to lowlands. This compartmentalization is largely controlled by extensive dolerite intrusions (dykes and sills) that impede vertical and lateral groundwater movement. Radon-222 concentrations, TDS, and Na⁺/Ca²⁺ ratios show limited spatial coherence along topographic gradients, further confirming a lack of regional connectivity. Springs and groundwater-dependent wetlands are predominantly located in the highlands, indicating local discharge zones. Stream baseflow analysis also suggests larger groundwater contribution in upper catchments. These findings underline the role of geology controls over topography in governing groundwater flow in the Eastern Karoo, even when topography is permissible for regional flows. The conceptual model developed offers a valuable framework for future hydrogeological studies in similar sedimentary basins worldwide. • Understanding groundwater flow is key to effective water resource management. • A multi-method approach was used to study groundwater flow systems. • Results show a mainly localized and compartmentalized groundwater flow system. • Groundwater compartmentalization is controlled by dolerite intrusions. • Geology plays a greater role than topography in controlling groundwater flow.