Groundwater Flow Modelling Constrained by Geophysical Data in the Imo River Basin, South East Nigeria

Sustainable management of groundwater resources in the Imo River Basin, which straddles Imo and Abia states in South East Nigeria, requires a quantitative understanding of groundwater flow dynamics that integrates subsurface geometry derived from geophysical surveys with hydraulic parameters from field testing. This study develops a numerical groundwater flow model for the Bende-Ameke Formation aquifer system in the Imo River Basin, using subsurface geometry constrained by electrical resistivity tomography and vertical electrical sounding surveys. Seventy-two VES stations and eight ERT profiles provided a three-dimensional framework defining aquifer thickness and lateral extent for model construction. The MODFLOW numerical model was implemented in the USGS ModelMuse interface, discretising the modelled aquifer system into grid cells of 500 by 500 metres over a 1,200-square-kilometre domain. Hydraulic conductivity and specific yield values were calibrated against seasonal groundwater level data from 22 observation boreholes over a two-year simulation period. Steady-state calibration produced a mean absolute head error of 1.8 metres, considered acceptable for the basin scale of the model. Transient simulations reproduce observed seasonal water table fluctuations with a normalised RMS error of 8 percent. Predictive simulations of increased abstraction scenarios indicate that a 50 percent increase in abstraction would lower water tables by 3 to 7 metres in the central basin within five years, approaching the critical threshold for shallow borehole failure. The study provides the first quantitative groundwater resource model for the Imo River Basin. Keywords: groundwater modelling, MODFLOW, Imo River Basin, aquifer, ERT