Origin and geochemical evolution of groundwater in the Abaya Chamo basin of the Main Ethiopian Rift: application of multi-tracer approaches

The fractured volcanic aquifer of the Abaya Chamo basin in the southern Ethiopian Rift represents an important source for water supply. This study investigates the geochemical evolution of groundwater and the groundwater flow system in this volcanic aquifer system using hydrochemistry and environmental tracers. Water types of groundwater were found to transform from Ca-Mg-HCO₃ (western part of Lake Abaya area) to Na-HCO₃ (northwestern part), from the highland down to the Rift Valley. Silicate hydrolysis and Ca/Na ion exchange are the major geochemical processes that control groundwater chemistry along the flow path. Groundwaters are of meteoric origin. The δ¹⁸O and δD content of groundwater ranges from −4.9 to −1.1‰ and –27 to 5‰, respectively. The δ¹⁸O and δD values that lie on the summer local meteoric water line indicate that the groundwater was recharged mainly by summer rainfall. δ¹³C_DIC values of cold groundwater range from −12 to −2.7‰, whereas δ¹³C_DIC of thermal groundwater ranges from −8.3 to +1.6‰. The calculated δ¹³C_CO2(g) using δ¹³C_DIC and DIC species indicates the uptake of soil CO₂ for cold groundwater and the influx of magmatic CO₂ through deep-seated faults for thermal groundwater. In the western part of Lake Abaya area, the shallow and deep groundwater are hydraulically connected, and the uniform water type is consistent with a fast flow of large gradient. In contrast, in the northern part of Lake Abaya area, water underwent deep circulation and slow flow, so the water types—e.g. high F⁻ (up to 5.6 mg/L) and Na⁺—varied laterally and vertically.