Physical Characterization of Volcanic Aquifer Systems in the Southwestern Ethiopian Highlands: A Case of the Gilgel Gibe Catchment

Volcanic aquifers are globally important groundwater resources, but their pronounced heterogeneity arising from lava emplacement, volcanic facies, structural features, and differential weathering makes hydrogeological characterization challenging. In many developing regions, limited field data further constrain aquifer characterization, increasing uncertainty in groundwater development and management. This challenge is evident in the Gilgel Gibe Catchment, where increasing groundwater demand and limited hydrogeological information require improved understanding of aquifer properties and productivity. This study characterized the aquifer systems of the Gilgel Gibe catchment primarily through lithological log and pumping test interpretations, supported by spring discharge measurements. A total of 30 deep boreholes were analyzed using pumping test data, including 14 with suitable recovery test data. Lithostratigraphic logs were integrated with diagnostic plots to identify aquifer conditions and select appropriate analytical solutions. The aquifer system is predominantly confined, followed by leaky-confined and locally unconfined conditions, interpreted using the Theis, Hantush-Jacob, and Neuman methods, respectively. Estimated transmissivity ranges from 3.9 × 10−6 to 1.3 × 10−2 m2/s, while spring discharges varied from 0.002 to 5 L/s. The Setema, Upper, Middle, and Lower Basalt, and Trachyte flows form low to moderately productive, fracture-controlled aquifers. The Lower Pyroclastic deposits and stratified tuffs form low-productivity aquifers due to the predominance of tuffs, while rhyolite exhibits low productivity due to the predominance of massive lava layers. Alluvial deposits form shallow aquifers dominated by intergranular porosity, underlain mainly by low-permeability tuffaceous materials. These findings provide a hydrogeological basis for guiding groundwater exploration, supporting well-field development, and informing sustainable groundwater management in volcanic terrains.