Isotopic and Geochemical Fingerprinting of Groundwater Pollution Sources in the Crystalline Basement Aquifer of the Middle Belt, Nigeria

This dissertation develops and validates an original isotopic and geochemical fingerprinting methodology for identifying and apportioning pollution contributions from multiple diffuse and point sources in the fractured crystalline basement aquifer system of Nigeria's Middle Belt zone, making a fundamental contribution to groundwater source attribution science in geologically complex terrains. The Middle Belt of Nigeria encompassing Benue, Plateau, Niger, and Kogi States contains crystalline basement aquifers that supply drinking water to millions but are exposed to multiple pollution pressures including agricultural fertiliser application, pit latrine seepage, artisanal mining acid drainage, and urban wastewater, and distinguishing between these sources in complex multi-contributor mixing scenarios requires multi-tracer approaches beyond the capability of conventional water chemistry analysis alone. The dissertation integrates environmental isotope ratios of nitrogen-15 and oxygen-18 in nitrate, strontium-87 to strontium-86 ratios for geogenic source discrimination, stable carbon and sulphur isotopes for organic and mining contribution tracing, and hydrochemical end-member mixing analysis in a Bayesian stable isotope mixing model that propagates source signature variability and measurement uncertainty. Field sampling involved 120 groundwater sites and thirty candidate source water samples across four study catchments spanning geological and land use gradients. Laboratory analysis was performed to international geochemical standards at a university analytical facility. The mixing model is validated against an independent dataset from a fifth catchment. Original contributions include the Bayesian multi-tracer mixing model adapted for crystalline basement aquifer conditions, the source signature database for the Middle Belt geological and land use context, and the validated apportionment results for the four study catchments. Keywords: isotopic fingerprinting, groundwater pollution, Middle Belt, Bayesian mixing model, source apportionment.