Analysis of Inter-Basin Groundwater Flow System Using Hydrogeochemical and Isotope Hydrology (Δ2h Andδ18o) Between Blue Nile and Awash Basins, the Case of Gerado and Borkena River Catchments

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Addis Ababa University


Hydrochemistry and environmental isotope techniques has been used to analyze inter basin groundwater flow and hydrodynamic system between Gerado and Borkena River catchments located at the southwestern block of the Afar depression where it occupies parts of the northern plateau in the northwestern part, western Afar marginal area and associated Graben. The sratigraphy of the area is characterizes by volcanic episodes were by the extrusion of flood basalts, with interbeds of pyroclastic rocks of rhyolitic or less commonly trachytic compositions, particularly at upper stratigraphic levels and seismically active right stepping N-S trending step system of discontinuous graben that are tens of kilometers long and 5–15 km wide which follow the curving N-S trend of the western Afar margins. Lithostructurally, Ashange basalt sequence is strongly faulted and tilted by 20° to 30° towards east. The weathering and jointing with the inclined contact is a favorable condition for ground-water flow from highly elevated plateau to the marginal graben. Boreholes and spring in Gerado catchment are characterized mostly by low TDS (< 500 mg/l) and Ca-Mg-HCO3 water type. While the Borkena catchment show a complex groundwater type and variation in TDS (500- 938 mg/l) Based on its flow pattern. The water types in Borkena catchment generally dominated by Na and Ca cations. Based on inverse Geochemical Modeling and ionic ratio calculations dominant reaction happened on the flow paths of Groundwater is likely to be cation exchange which Na is Exchanging Ca and dissolution of silicate minerals. Most of the groundwater samples from Gerado and Borkena catchments have relatively depleted isotopic signature of 18O and D from -2.28 to -6.83 and -2.5 to -7.8 respectively. Isotopically depleted springs found in the adjacent areas of Borkena Catchment while most of the springs found in the Gerado catchment are fall in a line of local Meteoric water. Spring patterns suggest there is an inter-basin groundwater flow between the catchment. The springs have preferential alignment in emerging with a considerable discharge which support inter-basin groundwater flow.



Interbasin Groundwater Flow, Blue Nile Basin, Awash Basin, Gerado, Borkena