Isotopic Characterisation of Groundwater Recharge Sources in the Sokoto-Rima Basin: An Empirical Analysis

Understanding recharge sources and groundwater residence times in the semi-arid Sokoto-Rima Basin is essential for sustainable aquifer management, yet direct empirical evidence from environmental isotopes remains sparse in the published literature for this region. This study undertakes an isotopic characterisation of groundwater recharge sources in the Sokoto-Rima Basin using oxygen-18, deuterium, tritium, and carbon-14 tracers applied to a systematic sampling programme across the basin. Groundwater samples were collected from sixty boreholes screened in the Rima Group and Gwandu Formation aquifers, along with rainfall, surface water, and soil water samples for isotopic reference datasets. Oxygen-18 and deuterium compositions were measured by cavity ring-down spectroscopy and plotted relative to the Global Meteoric Water Line and the local meteoric water line derived from a three-year rainfall isotope monitoring programme. Tritium activities and carbon-14 percentages of modern carbon were determined by liquid scintillation and accelerator mass spectrometry respectively to estimate mean residence times. The study tests the hypothesis that the deep Gwandu Formation aquifer is sustained by modern recharge rather than palaeowater, with implications for safe abstraction yield calculations. Results demonstrate a statistically significant age gradient between shallow and deep aquifer compartments, with evidence for lateral recharge from the Niger River floodplain in the southern basin. Findings provide a quantitative evidence base for the Sokoto State Water Board's aquifer management plan. Keywords: isotopic hydrology, groundwater recharge, Sokoto-Rima Basin, oxygen-18, residence time.