Major ion hydrochemistry and environmental isotope signatures as a tool in assessing groundwater occurrence and its dynamics in a fractured volcanic aquifer system located within a heavily urbanized catchment, central Ethiopia

SummaryHydrochemical and environmental isotope (2H, 18O and 3H) data were used to characterize the groundwater flow, occurrence and major ion chemistry in a complex fractured volcanic aquifer system located at the edge of the Ethiopian rift. The study has been supported by conventional hydrogeological mapping. The result clearly indicates the presence of two distinct groundwater systems: (1) The shallow groundwater characterized by low ionic concentration and in places isotopically enriched younger waters; and (2) The deep thermal waters with relatively older age showing negative 18O shifts and higher ionic concentration. The deep groundwaters are manifested as thermal springs and deep confined and artisan wells mainly aligned along major regional faults. The first group of groundwater is further divided into four subgroups indicating clear geo-hydrological patterns along the groundwater flow path representing different lithologies, residence time and anthropogenic influences. The classification shows different degrees of rock–water interaction and aquifer composition. There are indications of aquifer recharge from highly evaporated surface waters. This integrated study enabled to conceptualize the groundwater flow system vertically in to three zones: namely shallow fast circulating fresh water; mixed and moderately mineralized groundwaters representing the majority of wells and the deep old and mineralized thermal waters. This study has very important implications on possible location and depth of future boreholes, well head protection and in understanding the spread of pollutants that may originate from urban areas.