Isotopic Tracing and Geochemical Modelling of Groundwater Recharge Dynamics and Contamination in the Chad Formation Aquifer System, North East Nigeria

The Chad Formation aquifer system, which underlies vast areas of Borno, Yobe, and Adamawa States, is the primary freshwater resource for tens of millions of Nigerians in the North East geopolitical zone, yet the recharge dynamics, residence times, mixing processes, and geogenic contamination origins of this aquifer system remain poorly constrained by modern isotopic and geochemical modelling approaches, creating fundamental uncertainties in sustainable groundwater management. This study applies a multi-tracer isotopic approach, combining stable isotopes of oxygen-18 and deuterium, radiocarbon (carbon-14), tritium, chlorofluorocarbons, and sulphur hexafluoride, to characterise groundwater age, recharge sources, and mixing dynamics in the Chad Formation aquifer across 45 boreholes distributed across three hydrogeological units in Borno, Yobe, and Adamawa States. Major ion hydrochemistry and trace element analysis by inductively coupled plasma mass spectrometry define hydrochemical facies and geogenic contamination sources, particularly fluoride, arsenic, and uranium of natural geochemical origin. Geochemical modelling using PHREEQC inverse and forward modelling deciphers water-rock interaction reactions controlling groundwater chemistry evolution along flow paths. An original groundwater vulnerability and recharge zone delineation framework is developed by integrating isotopic residence time data, unsaturated zone transit time estimates, and a modified DRASTIC index calibrated with local hydrogeological parameters. Geostatistical kriging produces spatially continuous maps of key hydrochemical and isotopic parameters. The study delivers original conceptual and quantitative hydrogeochemical models for the Chad Formation that directly support sustainable groundwater resource management policy in the North East geopolitical zone. Keywords: isotopic tracing, Chad Formation aquifer, groundwater recharge, hydrogeochemistry, North East Nigeria