Hydrogeochemical Processes and Water Quality Assessment in Volcanic Aquifers of the Gilgel Gibe and Upper Dhidhessa Catchments, Southwestern Ethiopia

Groundwater is a critical resource for domestic, agricultural, and industrial use in the Gilgel Gibe and Dhidhessa catchments of southwestern Ethiopia, where volcanic aquifer systems are the main sources. However, groundwater quality in these catchments has been under pressure from anthropogenic activities such as population growth, land-use changes, and pollution driven by rapid development and poor resource management. This study investigates hydrogeochemical processes and evaluates groundwater quality in volcanic aquifers using hydrochemical analyses and a stable isotope approach applied to 115 water samples. The spatial distribution of various physicochemical and hydrogeochemical parameters shows a distinct contrast between the highland and lowland regions, indicating topography-driven variations in water quality and geochemical processes. In hand-dug wells, springs, and surface waters, the ionic order is Ca2+ > Na+ > Mg2+ > K+ and HCO3− > NO3− > Cl− > SO42−, whereas deep wells show Na+ > Ca2+ > Mg2+ > K+ and HCO3− > Cl− > SO42− > NO3−. The predominant groundwater type is Ca-HCO3, followed by Na-HCO3 and Ca-NO3, with other types including Ca-Mg-HCO3, Ca-Na-HCO3, and Na-Ca-HCO3. Water types of Ca-HCO3 and Ca-Mg-HCO3 dominate the upland areas, indicating relatively young groundwater with moderate total dissolved solids (TDSs) and enrichment in δ18O and δ2H, where highly mineralized Na-HCO3 water types prevail in the deep aquifers of the lowland regions, where δ18O and δ2H are relatively depleted. Principal component analysis, cross-plots of major cations versus HCO3−, and mineral stability diagrams indicate that aluminosilicate weathering and dissolution are the dominant processes controlling groundwater chemistry in the study area. The higher saturation index values observed in the deep wells indicate water closer to mineral equilibrium, suggesting more extended water–rock interaction relative to the shallow wells. The CO2 partial pressures calculated using PHREEQC exceed atmospheric levels (~10−3.5 atm), indicating sources from atmospheric influx, soil, or biogenic activity for most samples, and deeper sources such as mantle degassing may be found in a few deep wells. Scatter plots of Cl− vs. SO42− and Cl− vs. NO3−, associated with Ca(NO3)2, NaNO3, and CaCl2 water types, suggest that anthropogenic inputs are the second major factor influencing the area’s water chemistry. Stable isotope analyses and hydrochemical data indicate that groundwater in the area primarily originates from local precipitation, with isotopic signatures reflecting strong groundwater–surface water interaction. These findings improve understanding of regional hydrogeochemistry and groundwater quality and help identify promising zones for sustainable groundwater development. This study provides valuable insights into groundwater resource management both in the study area and in regions sharing comparable geological contexts.