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 delta O-18 and delta D content of groundwater ranges from -4.9 to -1.1 parts per thousand and -27 to 5 parts per thousand, respectively. The delta O-18 and delta D values that lie on the summer local meteoric water line indicate that the groundwater was recharged mainly by summer rainfall. delta C-13(DIC) values of cold groundwater range from -12 to -2.7 parts per thousand, whereas delta C-13(DIC) of thermal groundwater ranges from -8.3 to +1.6 parts per thousand. The calculated delta C-13(CO2(g)) using delta C-13(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.