Characterizing volcanic aquifers in the upper Awash river sub-basin using 3D lithological, hydrostratigraphic, and groundwater flow modeling

Volcanic aquifers in Ethiopia’s Upper Awash River sub-basin serve as essential groundwater sources for domestic, agricultural, and industrial uses. However, their complex geological structures present significant challenges for resource assessments and sustainable management. This study integrates geological, geophysical, and hydrological data to characterize the aquifer system using 3D lithological and hydrostratigraphic models and groundwater flow simulation. A three-dimensional subsurface model was developed using RockWorks based on borehole logs, vertical electrical sounding (VES) data, digital elevation models (DEMs), and geological maps. Hydrostratigraphic units were delineated using kriging and inverse distance weighting (IDW) and five major hydrostratigraphic units were identified, with the lower basalt aquifer holding the highest groundwater volume (515.7 Bm³). A steady-state MODFLOW-2005 model was constructed and calibrated in the Groundwater Modeling System (GMS) using trial-and-error and PEST methods, achieving strong agreement between the observed and simulated heads Normalized Root Mean Square Error (NRMSE < 0.011, R = 0.999). Sensitivity analysis revealed that hydraulic conductivity and recharge parameters strongly influenced groundwater flow, particularly in Zones 6, 4, and 7. Scenario simulations showed a significant drawdown under increased pumping (up to 18.85 m) and complete recharge loss (up to 87.4 m). River leakage was the dominant water budget component, accounting for over 63% of inflows and outflows. These findings highlight the spatial variability and vulnerability of the volcanic aquifers to stress. The integrated modeling approach provides critical insights into sustainable groundwater development, emphasizing the need for zone-specific management strategies, controlled abstraction, and enhanced recharge efforts in high-risk areas.