Hydrogeochemistry of the groundwater system of the transboundary basement and volcanic aquifers of the Bulal catchment, Southern Ethiopia

The evaluation of groundwater geochemistry of transboundary aquifers has an important role in the management and sustainable use of groundwater resources in arid and semi-arid regions. This study presents the results of an integrated hydrogeochemical study of the Bulal transboundary aquifers located in the Borena zone, along the Ethiopia-Kenya border. The main aquifers are basic volcanic rocks, subordinate felsic pyroclastic deposits, and metamorphic basement rocks. These aquifers are overlain by alluvio-lacustrine sediments, eluvial soil and regolith. The groundwaters are dominated by Na⁺ and HCO₃⁻ and mixed water types. Silicate weathering and evaporation are the dominant hydrogeochemical processes controlling the chemistry of the groundwaters in most parts of the catchment. However, dissolution of evaporitic minerals might have also played a significant role in adding major ions into the groundwater system. Groundwater in the volcanic aquifers are Na–HCO₃, Ca–Mg–HCO₃, Na–Ca–HCO₃ and Na–Cl–SO₄ water types while Ca–Mg–HCO₃, Na–Ca–Cl–SO₄ and Ca–Mg–SO₄ water types are dominant in the metamorphic basement aquifers. The groundwater in the catchment mainly evolved from Na–HCO₃ in the west, Ca–Mg–HCO₃ in the north, and Na–Ca–Cl–SO₄ in the central east and southeast to Na–Ca–HCO₃ type in the southern low-lying areas along the groundwater flow direction. The study has significant implications for the management of groundwater quality in the region.