Hydrogeology Of Complex Volcanic Systems In Continental Rifted Zone

Middle Awash basin, located in the complex volcanic centre of the Ethiopian Rift valley, is one of the drought prone areas marked by considerable water supply problems. Due to lack of surface waters, groundwater remains the sole resource supplying water for domestic and livestock. However, its effective use has been hampered due to the complex aquifer system of this basin. In this study, multidisciplinary method has been applied to characterize the hydrogeological system of this complex volcanic basin. The combined results from datasets show two distinct aquifer systems linked to geology and physiographic location. The Ca-alkaline rocks like basalt, ignimbrite, and trachybasalt form aquifers in the mountain regions whereas the Na-alkalne rocks which include scoria, pumice, tuff, and volcanoclastics are main aquifers in rift floor. Ground waters circulating in the highland areas are slightly mineralized and are Ca-Na-HCO3 type. On the other hand, ground waters in the rift floor are Na-HCO3-Cl types and are highly mineralized as well as contain high load of fluoride much higher than the permitted standard. Rivers hydrograph, hydrochemistry, environmental isotope, and 2D tomography are in good agreement showing fast percolation of rainfall and strong interaction between surface waters and ground waters. Result from numerical groundwater flow modeling further indicates the strong interaction between groundwater and surface waters in the form of losing and gaining.