Development of an improved hydrogeological and hydro-geochemical conceptualization of a complex aquifer system in Ethiopia

Comprehensive aquifer characterization requires the development of a three-dimensional (3D) geological model and estimation of hydraulic and hydro-geochemical properties. This can be used to discern the governing processes of groundwater flow and chemistry, and plan pertinent groundwater management approaches. This study evaluated the influence of geological settings and groundwater flow on the groundwater development potential and chemistry in a Quaternary aquifer system in the Raya Valley, Ethiopia. Surface geology, digital elevations, groundwater-level measurements, and data from drill logs, pumping tests and vertical electrical soundings were combined to characterize the physical properties of the aquifer system. 3D geological and hydrogeological models were developed and used to delineate subsurface formations and to quantify groundwater flow. The aquifer was characterized as a heterogeneous and anisotropic unconfined system. The available groundwater volume was estimated to be 80 +/- 1 km(3)with a maximum sustainable yield of 530,409 +/- 16,800 m(3)/day. The use of geochemical models and principal component analysis revealed that the origin and geochemical composition of the groundwater were spatially variable. Rock weathering, mineral dissolution, ion-exchange and anthropogenic activities were the major processes governing the hydro-geochemical characteristics of the aquifer, while evaporation processes caused groundwater salinity enrichment. However, even though mineral saturation of the groundwater at specific locations was highly influenced by the geologic matrix, advective groundwater transport led to areas with mixed groundwater chemistry. The observed and modelled complexity of this aquifer system suggests that such evaluations are important to design appropriate groundwater management strategies in heterogeneous and structurally complex aquifer systems.