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Hydrochemical and isotope data in conjunction with hydraulic head and spring discharge observations were used to characterize the regional groundwater flow dynamics and the role of the tectonic setting in the Gidabo River Basin, Ethiopian Rift. Both groundwater levels and hydrochemical and isotopic data indicate groundwater flow from the major recharge area in the highland and escarpment into deep rift floor aquifers, suggesting a deep regional flow system can be distinguished from the shallow local aquifers. The δ O values, the thermal groundwater is found to be recharged in the highland around 2,600 m a.s.l. and on average mixed with a proportion of 30 % shallow groundwater. While most groundwater samples display diluted solutions, δ 13 C data of dissolved inorganic carbon reveal that locally the thermal groundwater near fault zones is loaded with mantle CO 2 , which enhances silicate weathering and leads to anomalously high total dissolved solids (2,000-2,320 mg/l) and fluoride concentrations (6-15 mg/l) exceeding the recommended guideline value. The faults are generally found to act as complex conduit leaky barrier systems favoring vertical mixing processes. Normal faults dipping to the west appear to facilitate movement of groundwater into deeper aquifers and towards the rift floor, whereas those dipping to the east tend to act as leaky barriers perpendicular to the fault but enable preferential flow parallel to the fault plane.

Being located in the tectonically active Ethiopian Rift, the hydrogeology of the Gidabo River Basin is complex due to the disruption of lithologies by faults and the variability and lateral discontinuity of the aquifers. Hydrogeochemical and isotopic data suggest that the aquifers within the rift floor receive a relevant contribution of groundwater recharged in the highland. However, the incomplete knowledge about the aquifer properties, the hydraulic behavior of the faults, and the boundary conditions cause uncertainties in this conceptual hydrogeological model. To account for these uncertainties fourteen different numerical models with a stepwise increase from 7 to 40 adjustable parameters were developed, calibrated against the same hydraulic head observations, and ranked according to the Akaike information criteria (AIC and AICc) and Bayesian information criterion (BIC). Based on the information criteria five plausible models were identified, all of which were successfully verified against the river baseflow. The highest likelihood is attributed to a model with eleven adjustable parameters that does not explicitly account for the fault zones; other plausible models considering faults as semi-barriers achieve a slight improvement in model fit but have lower likelihood due to the increased number of calibration parameters. Thus, the effect of faults on groundwater flow needs further investigation, particularly at a local scale.On a regional scale, the hydraulic head distributions of the plausible models agree reasonably well with the equipotential map interpolated from well observations. The estimated transmissivity values range between 30 m 2 / day and 1350 m 2 / day and generally increase from the mountains towards the rift floor. The water budget shows that 75 % of the groundwater recharge supplies baseflow to the rivers. The remaining water infiltrates to the deeper aquifers where less than 1 % is abstracted by pumping wells and the rest flows towards Lake Abaya. Within the rift floor, the majority of inflow to the aquifers is from direct recharge; nevertheless, 35 % of the inflow is contributed by mountain block recharge (lateral groundwater flow from the escarpment and highland). The results of this study strongly advocate the idea to incorporate alternative plausible models instead of relying on single models in the practice of groundwater modeling especially in areas of complex hydrogeology and limited data availability.Die Hydrogeologie des im tektonisch aktiven Äthiopischen Graben gelegenen Gidabo-Flusseinzugsgebiets ist aufgrund der störungs bedingten Unterbrechungen der Lithologien sowie der Variabilität und lateralen Diskontinuität der Aquifere komplex. Hydro geochemische und Isotopen-Daten weisen darauf hin, dass in den Aquiferen der Grabensohle ein relevanter Beitrag von Grundwässern vorliegt, die im Hochland neugebildet werden. Die unvollständige Kenntnis der Aquifereigenschaften, der hydraulischen Wirkung der Störungen und der Randbedingungen führen jedoch zu Unsicherheiten in diesem ko...

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