Investigating groundwater and surface water exchange in the Umgeni Catchment, Kwazulu-Natal, South Africa.

Abstract

The understanding of the role of groundwater in river basin hydrology and its contribution to the water resources base is limited. Past studies have focused on quantifying and modelling surface water. Recent South African policies emphasise the need for conjunctive use of water resources, including adding groundwater into the national water security mix and water balance accounting. Groundwater and stream networks fed by groundwaters are proven to be resilient against global climate change. The current study aims to shed light on groundwater recharge processes and its connection to the stream network in the Umgeni Catchment of KwaZulu-Natal province in South Africa. The groundwater recharge-flow-discharge processes are examined across altitudinal gradients and scales with specific emphasis on the role of wetlands, lithology, geomorphology, dykes, land use and land cover in controlling the connection between groundwater and surface water. Proven isotope tracers (Oxygen-18 (18O) and Deuterium (2H) and Radon (222Rn)), piezometry, electrical conductivity and baseflow separation were employed to investigate groundwater recharge processes and the connection between groundwater and surface waters. About 220 rainwater samples have been collected on a daily to monthly basis between March 2022 and January 2024 at three locations (Durban, Pietermaritzburg and Howick). In addition, we collected 180 samples, including streams (105), springs (18), wetlands (15) and boreholes (43), representing both the wet and dry seasons. The results showed that rainfall predominantly recharges the groundwaters. 222Rn data from the stream network revealed that the headwater streams of the Umgeni catchment are dominated by groundwater inflow, while the stream network downstream shows little sign of groundwater inflow. The findings highlight the critical role of rainfall in recharging groundwater and the strong influence of geology on groundwater contributions to surface waters in the Umgeni Catchment. The identification of groundwater-dominated headwaters and surface water-dependent downstream areas underscores the need for regionspecific water management strategies. Protecting recharge zones, implementing sustainable groundwater abstraction, and integrating groundwater-surface water management will be essential for ensuring long-term water security.