Integrated geophysical and hydrochemical characterization of geologically and structurally complex groundwater systems in the Mekelle area, northern Ethiopia

Study region Mekelle area, northern Ethiopia. Study focus Groundwater in sedimentary basins is influenced by lithology and structural deformation, yet these controls remain poorly understood in many sedimentary basins. This study investigates subsurface architecture and groundwater characteristics using an integrated geophysical and hydrochemical approach. Vertical electrical soundings (VES, DC resistivity method), supported by borehole lithological logs, were used to delineate subsurface units and structures. Major-ion chemistry of 107 groundwater and surface water samples was analyzed to identify hydrochemical facies, water–rock interactions, and anthropogenic influences. New hydrological insights for the region Integrated geophysical and hydrochemical results indicate a structurally controlled, heterogeneous aquifer system with pronounced hydraulic compartmentalization. Geophysical evidence shows that dolerite intrusions mainly behave as resistive units that locally restrict groundwater flow but also serve as productive aquifers where sufficiently fractured. Fault and fracture networks enhance secondary permeability and form preferential groundwater pathways that control aquifer connectivity. Hydrochemical data reflect this structural control, with shallow groundwater dominated by Ca–HCO₃ and Ca–Mg–HCO₃ facies, indicating active recharge conditions. In contrast, Ca–SO₄ facies in deeper groundwater are linked to interaction with gypsum-bearing formations. Elevated nitrate and chloride concentrations in shallow groundwater further indicate influence from surface-derived contamination. These findings demonstrate the strong influence of lithology and structural features on groundwater systems and support improved groundwater management in complex sedimentary basins.