Groundwater recharge,circulation and geochemical evolution in the source region of the Blue Nile River,Ethiopia |
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| Institution: | 1. Laboratory of Hydrogeology, University of Avignon, 33 Rue Louis Pasteur, 84000 Avignon, France;2. Department of Geology and Geophysics, Addis Ababa University, P.O. Box 1176, Addis Ababa, Ethiopia;1. División de Geociencias Aplicadas, Instituto Potosino de Investigación Científica y, Tecnológica, San Luis Potosí, Mexico;2. Centro del Agua para América Latina y el Caribe, Tecnológico de Monterrey, Monterrey, Mexico;1. Department Earth and Environmental Sciences, University of St. Andrews, KY16 9AL, UK;2. Department Geography and Earth Sciences, Aberystwyth University, Wales, SY23 3DB, UK;3. NERC Isotope Geosciences Laboratory, British Geological Survey, Keyworth, NG12 5GG, UK;4. Isotope Geoscience Unit, Scottish Universities Environmental Research Centre, East Kilbride, G75 0QF, UK;1. Faculty of Water Resources and Irrigation Engineering, Arba Minch Water Technology Institute, P.O. Box 21, Arba Minch University, Arba Minch, Ethiopia;2. Faculty of Hydraulic and Water Resources Engineering, Arba Minch Water Technology Institute, P.O. Box 21, Arba Minch University, Arba Minch, Ethiopia;1. University of Tübingen, Department of Geosciences, Tübingen, Germany;2. Max Planck Institute for Chemistry, Climate Geochemistry Department, Mainz, Germany;3. Leibniz University Hannover, Institute of Mineralogy, Hannover, 30167, Germany;4. UMR 8148 GEOPS, Université Paris-Saclay – CNRS, 91405, Cedex, France;5. Senckenberg Centre for Human Evolution and Paleoenvironment (S-HEP), Tübingen, Germany;6. University of Cologne, Department of Geosciences, Cologne, Germany;7. Department of Mining and Geological Engineering, Botswana International University of Science and Technology, Palapye, Botswana;8. Addis Ababa University, School of Earth Sciences, Addis Ababa, Ethiopia |
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| Abstract: | Geochemical and environmental isotope data were used to gain the first regional picture of groundwater recharge, circulation and its hydrochemical evolution in the upper Blue Nile River basin of Ethiopia. Q-mode statistical cluster analysis (HCA) was used to classify water into objective groups and to conduct inverse geochemical modeling among the groups. Two major structurally deformed regions with distinct groundwater circulation and evolution history were identified. These are the Lake Tana Graben (LTG) and the Yerer Tullu Wellel Volcanic Lineament Zone (YTVL). Silicate hydrolysis accompanied by CO2 influx from deeper sources plays a major role in groundwater chemical evolution of the high TDS Na–HCO3 type thermal groundwaters of these two regions. In the basaltic plateau outside these two zones, groundwater recharge takes place rapidly through fractured basalts, groundwater flow paths are short and they are characterized by low TDS and are Ca–Mg–HCO3 type waters. Despite the high altitude (mean altitude ∼2500 masl) and the relatively low mean annual air temperature (18 °C) of the region compared to Sahelian Africa, there is no commensurate depletion in δ18O compositions of groundwaters of the Ethiopian Plateau. Generally the highland areas north and east of the basin are characterized by relatively depleted δ18O groundwaters. Altitudinal depletion of δ18O is 0.1‰/100 m. The meteoric waters of the Blue Nile River basin have higher d-excess compared to the meteoric waters of the Ethiopian Rift and that of its White Nile sister basin which emerges from the equatorial lakes region. The geochemically evolved groundwaters of the YTVL and LTG are relatively isotopically depleted when compared to the present day meteoric waters reflecting recharge under colder climate and their high altitude. |
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