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Publication Date

2019

Keywords

Abaya chamo, Environmental isotopes, Groundwater recharge and flow, Hydrogeological systems, Hydrochemistry, Water type

Abstract

Hydrochemical and environmental isotope (2H,18O) data of different waters in relation to geologic units and tectonics are used to characterize the different hydrogeological systems, spatial variation of water types, sources and mechanism of recharge and groundwater flow systems in aquifers. The area is tectonically active volcanic terrain of three physiographic regions (rift, escarpment and highland) having large topographic differences ranging in elevation form 1,180 to over 4,000masl. In the rift valley, there are geothermal fields. There is strong spatial hydrochemical variation between the rift and highlands in identifying four major groups of water types evolving from Ca-HCO3 at highlands to Na-Cl highly evolved thermal water. The four groups have ten subgroups indicating clear geo-hydrological patterns along the groundwater flow path representing different lithologies, residence time, degrees of rock–water interaction and aquifer composition. The result supplemented with isotope signatures also has clearly indicated five hydrogeological systems: 1. Shallow flow dilute and depleted highland and towards escarpment system, 2. Intermediate flow less depleted and dilute rift floor system, 3. Deep flow less concentrated and highly depleted rift floor system 4. Deep flow highly concentrated and relatively less depleted, and 5. Deep highly enriched and moderately concentrated rift floor hydrogeological systems. The deep groundwaters are manifested as thermal springs and deep confined and artisan wells mainly aligned along major regional rift floor faults. The hydrogeological systems are recharged by recent meteoric waters of different physiographic sectors, indirect vertical flow from upper aquifer systems, enriched lakes, and paleowaters. The major groundwater flows are originated from highland and escarpment precipitation and the recharge are strongly controlled by the axial faults in the rift valley. Though the three major flows (shallow, intermediate and deep) occur in the basin, the groundwater flow converges from all sides of the basin to the center of the rift floor. The use of hydrochemistry and stable isotopes in the study area allowed to understand the hydrogeological systems, sources of recharge to the systems, variable water types and its evolution, and the flow system. As there was no systematic way of understanding the resources in the basin, this study has provided very important knowledge on the groundwater systems and clues in utilization of the system sustainably, implications on possible location and depth of future drilling site for development prospects and added the use of hydrochemistry and isotopes as tool for the study of groundwaters in the basin.

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