Hydrogeochemical Evolution of Groundwater in the Basement-Sedimentary Transition Zone of Niger State

The hydrogeochemical evolution of groundwater across the basement-to-sedimentary cover transition in Niger State, North Central Nigeria, involves a systematic change in water-rock interaction character that has not been empirically quantified, creating gaps in the understanding of aquifer connectivity and contaminant transport in this transitional setting. This study characterises the hydrogeochemical evolution of groundwater along a forty-kilometre transect crossing the basement-sedimentary transition zone in Niger State, using major ion chemistry, trace element data, and isotopic tracers to reconstruct water-rock interaction histories. Forty-five groundwater samples were collected from boreholes screened in basement fractured aquifers and overlying Nupe Sandstone aquifers distributed along the transect. Major and minor element analyses were performed by ion chromatography and inductively coupled plasma optical emission spectrometry. Saturation index calculations using PHREEQC geochemical modelling code identified the dominant mineral equilibria controlling water chemistry evolution along the flow path. Mixing proportions between basement-derived groundwater and meteoric recharge were estimated using conservative tracers. The study tests the hypothesis that ion exchange on smectite clay at the basement-sedimentary interface drives the observed transition from calcium-bicarbonate to sodium-bicarbonate water types. The quantified geochemical evolution framework provides a basis for contamination risk assessment and managed aquifer recharge planning in the transition zone. Keywords: hydrogeochemistry, basement-sedimentary transition, water-rock interaction, Niger State, PHREEQC.