Recharge elevation, residence time and renewability of groundwater in the Upper Awash valley, Ethiopia: applying environmental tracers in a highly populated volcanic basin

The Upper Awash Basin in Ethiopia, which contains the rapidly-expanding national capital Addis Ababa, is being increasingly impacted by groundwater abstraction, leading to falling water levels in many areas. Groundwater management is impaired by the lack of consensus over detailed (hydro)geological characterisation of the complex volcanic aquifers that underlie this high-relief basin. Here we use an empirical study based on environmental tracers (isotopes, trace gases and hydrochemistry) measured on 40 groundwater samples collected from a selection of borehole sites, to explore the hydrogeological functioning of the basin The stable isotopes δ18O and δ2H provide information on likely recharge elevations (1900–3500 m above sea level) and extent to which surface waters are contributing to recharge (45% of sites sampled). The trace gases CFCs and SF6 show that proportions of modern water are generally < 10%, but 14C indicates that the groundwater storage currently critical for buffering change is in most cases of the order of hundreds rather than thousands of years old, and therefore may be vulnerable to comparatively rapid modification. With little evidence for significant variation in hydrogeochemical changes with depth across individual wellfields, we conclude that recharge is usually derived locally, though indications of longer flow paths exist in some locations, principally around the town of Mojo. Hydrochemistry shows that inorganic groundwater quality remains good at present, despite the existence of some poor-quality surface waters. The methodology of this study could be applied in other high-relief basins reliant on groundwater, to characterise vulnerability to abstraction where a detailed geological model and long-term monitoring are absent.