Integrated characterisation of groundwater-ecosystems-surface water interactions in the Tuli-Karoo transboundary aquifer

Abstract

groundwater-supported ecosystems’ incorporation in resource management. This phenomenon is underresearched in data-scarce southern Africa, especially in transboundary aquifer settings. To investigate this interrelationship, this study was initiated in the semi-arid South African portion of the Tuli-Karoo transboundary aquifer, shared with Botswana and Zimbabwe. Water Table Fluctuation (WTF) and Chloride Mass Balance (CMB) methods were employed for recharge; Environmental isotopes (δ 2H, δ18O, and 3H) analysis for recharge mechanism; and baseflow algorithms for groundwater contribution to baseflow. The estimated alluvial aquifer average recharge ranges from 17.3 to 43.9 mm/a (5 – 12.9% of MAP), while for the sandstone aquifer it ranges from 0.4 to 8.2 mm/a (0.12 – 2.5% of MAP). Local precipitation is the source of post-evaporation recharge. Isotope analysis indicated bi-directional groundwater-surface water interactions. Baseflow average is 3.4 m3/s with a low baseflow index of 7.6%. On average, groundwater contributes 0.39 m3/s and 0.22 m3/s during wet and dry periods, respectively. Additionally, groundwater levels were observed to be declining gradually, which could negatively impact groundwater component of baseflow, and riparian and wetland vegetation. It can be concluded that rainfall seasonal fluctuations control groundwater-ecosystems-surface water interactions. These aquifer-specific findings are crucial for comprehensive implementation of integrated water resource management that is inclusive of associated ecosystems and their protection.