Use of environmental isotopes to investigate groundwater and surface water interaction in the South Phuthiatsana Catchment, Lesotho.

Abstract

Lesotho, often referred to as the "Water Tower of Southern Africa," plays a crucial role in regional hydrology due to its high-altitude water sources, which significantly contribute to the Orange-Senqu River system. Despite its importance, comprehensive data on the country's catchment hydrology remains limited. This underscores the need to enhance understanding of Lesotho’s water resources to support sustainable management under the pressures of population growth and climate change. Isotopic techniques provide valuable insights that complement conventional hydrological methods and are critical in identifying groundwater recharge, essential for effective policy development and implementation. This study investigates groundwater and surface water (GW-SW) interactions in the South Phuthiatsana catchment, addressing a significant knowledge gap in Lesotho’s hydrology. A multi-parameter, multiscale approach was applied, employing stable water isotopes (δ¹⁸O and δ²H), the radioactive isotope ²²²Rn, baseflow separation, and in-situ electrical conductivity (EC) measurements to systematically analyse flow dynamics. Over 21 months from April 2022 to January 2024, water samples were collected from 50 springs, five precipitation stations, four rivers, five wetlands, and 11 boreholes. These were supplemented by additional datasets collected between 2018 and 2023, offering a comprehensive hydrological snapshot of the catchment. Stable isotope analysis led to the development of Lesotho’s Local Meteoric Water Lines (OxLMWL, Ms-LMWL, and Abia-LMWL), which revealed key isotopic signatures associated with the amount effect, altitude effect, and seasonal variability in precipitation. These LMWLs served as essential baselines for assessing recharge processes and determining the origins of water sources. The results show that highland wetlands are recharged primarily by precipitation and groundwater (subsurface) inflows, playing a critical role in sustaining upstream rivers with baseflow. Downstream river segments exhibited significant evaporative fractionation, marked by enriched δ¹⁸O and δ²H values, indicating prolonged residence times, warmer temperatures, and wider channel areas. Moreover, river classification using δ¹⁸O, 222Rn and EC revealed a systematic altitudinal trend: isotopic depletion at higher elevations and enrichment downstream, alongside decreasing ²²²Rn concentrations, highlighting reduced groundwater discharge with decreasing elevation. However, elevated ²²²Rn near the confluence of the Liphiring and South Phuthiatsana Rivers suggests localised zones of increased subsurface inflow. Boreholes, plotting above the LMWL, displayed isotope signatures consistent with rapid recharge from meteoric water. In contrast, spring samples exhibited more varied isotope values and plotted below the Abia-LMWL, suggesting evaporation before recharge, possibly due to shallow flow paths or delayed infiltration. While ²²²Rn measurements were intentionally excluded from boreholes and springs to avoid groundwater contamination during isotope sampling, EC was measured across all water sources. Although no consistent spatial pattern emerged in EC values for boreholes and springs, some localised areas exhibited elevated EC levels, warranting further investigation. Importantly, all EC readings remained below 1000 µS/cm, indicating generally low mineralisation and limited salinity issues within the catchment. Overall, the study demonstrates strong interconnectivity between GW-SW in the catchment, with hydrological processes governed by elevation, geology and climate. Key contributions include the establishment of local isotopic baselines, enhanced understanding of wetland recharge mechanisms and identification of spatial variability in GW-SW interactions. The findings provide a robust scientific foundation for integrated water resource management in Lesotho. By illustrating the hydrological connectivity between GW-SW water systems, the research supports a shift from isolated to joint resource management approaches. The insights gained can inform catchment planning, protection of the highlands recharge zone and adaptive water allocation strategies in response to environmental change. This study thus strengthens the evidence base necessary for safeguarding Lesotho’s water security through informed decisionmaking and sustainable resource governance.