Geochemical Modelling of Arsenic Mobility in Groundwater of the Basement Complex in Kano State

Elevated arsenic concentrations in groundwater serving communities in parts of Kano State, North West Nigeria, present a public health concern whose geochemical drivers require systematic investigation to distinguish naturally occurring arsenic mobilisation from anthropogenic contamination pathways. This study applies geochemical modelling to investigate the mechanisms controlling arsenic mobility in groundwater of the crystalline basement complex in two local government areas of Kano State. Groundwater samples from forty boreholes were analysed for arsenic speciation, along with major ions, dissolved iron and manganese, dissolved organic carbon, redox indicators, and in-situ measured pH and Eh. Sequential extraction of aquifer material recovered from borehole cuttings partitioned arsenic among iron oxide, carbonate, organic matter, and residual fractions. The PHREEQC geochemical modelling code was used to compute saturation indices for arsenic-bearing phases and to simulate redox-driven dissolution of iron oxyhydroxide sorption hosts as a mobilisation mechanism. Multivariate statistical analysis identified environmental variables most strongly correlated with dissolved arsenic concentration. The study tests the competing hypotheses of reductive dissolution versus pH-dependent desorption as the dominant arsenic release mechanism. Results indicate that reductive dissolution of ferrihydrite in shallow reducing zones is the primary mechanism, with seasonal water table fluctuation driving episodic arsenic release. Targeted well construction standards are recommended to minimise arsenic intake risk. Keywords: arsenic, geochemical modelling, groundwater, basement complex, Kano State.