Sustainability assessment of the groundwater resources in the upper Blue Nile basin (Ethiopia): Isotopic approach.

Over 70 percent of the main Nile flow originates from the Ethiopian plateau via the Blue Nile river which has in turn its source from lake Tana. Due to seasonality in surface water resources, and the frequent drought, groundwater resources play a major role in domestic water supply in rural Ethiopia. Classical isotope hydrology, coupled with hydrogeochemistry have been used to assess the groundwater discharge, recharge, residence time, and then its potentiality for sustaining the surface water in dry periods. Highly variable values in δ¹⁸O contents (-1 to -5.5 per mil), no clear relation of isotopic composition with altitude or geographical position, and relatively depleted stable isotope value compared to the 50 years average isotopic composition of rainfall at Addis Ababa, were observed. This implies a shallow, rapidly circulating and heterogeneous groundwater systems. Furthermore, isotopic method of water budget model on lakes in the plateau region testifies that groundwater is relatively unimportant flux compared to the Ethiopian rift valley lakes. This kind of groundwater condition is highly vulnerable to prolonged drought.