Modelling Arsenic Fate and Transport in the Alluvial Aquifer Systems of the Hadejia-Jama’are Floodplain, Jigawa State

Arsenic contamination of alluvial aquifers associated with reducing geochemical conditions in irrigation-intensive floodplain systems poses an emerging public health challenge in Jigawa State, North West Nigeria, but the coupled biogeochemical and hydrological processes controlling arsenic mobilisation and transport in the Hadejia-Jama'are floodplain aquifer system have not been mechanistically modelled. This study develops a reactive transport model for arsenic fate and transport in the alluvial aquifer system of the Hadejia-Jama'are floodplain using field data from an intensive monitoring programme. A network of multi-level piezometers was installed across an eleven-hectare transect to characterise vertical and horizontal variations in redox conditions, dissolved iron, sulphate, dissolved organic carbon, and arsenic speciation. Sediment cores were collected to characterise solid-phase arsenic distribution and its association with iron oxyhydroxide and organic matter fractions. A reactive transport model was constructed using PHAST that couples variably saturated flow with biogeochemical reactions including reductive dissolution of iron oxyhydroxides, sulphate reduction, organic matter degradation, and arsenic sorption to iron mineral surfaces. Model calibration against observed arsenic concentration time series at monitoring points was conducted using parameter estimation by Markov chain Monte Carlo sampling. The calibrated model is used to predict arsenic plume evolution under alternative irrigation intensification and remediation scenarios. Keywords: arsenic transport, reactive transport modelling, alluvial aquifer, Hadejia-Jama'are, Jigawa State.