Estimation of groundwater recharge using different methods, supported by radon measurement and lineament density analysis: Southern sections of the western Afar rift margin and associated rift floor

Abstract The aim of this study was to quantify the spatial and temporal groundwater recharge of the southern sections of the western Afar rift margin and the associated rift floor. Several methods were used, including water balance, WetSpass, Chloride mass balance (CMB) and baseflow separation, supported by radon measurements and lineament density analysis. The first three methods result in mean annual groundwater recharge rates of 114.39 mm/year, 100.88 mm/year and 92.37 mm/year, respectively. The base flow separation approach conducted at the rift margin, particularly at the outlets of the marginal grabens gives a higher recharge rate (136.7 mm/year) than the other methods, suggesting that additional groundwater may be flowing from the adjacent plateau, while the rift floor has a lower value (17.86 mm/year) due to various geological and structural features. The recharge value of the CMB approach is lower, indicating that the groundwater has a higher chloride concentration than from precipitation alone. The spatial distribution of groundwater recharge from the WetSpass model shows that the rift margin has higher groundwater recharge due to moderate to high annual precipitation. The geology consists of alluvial deposits of sand and gravel underlain by highly fractured basalt and ignimbrite as well as transverse structures with SW-NE, NW-SE and W-E orientations. The low annual rainfall and the lacustrine deposits, on the other hand, lead to low groundwater recharge in the rift floor. However, the presence of NE-SW and W-E trending transverse structures connecting the marginal grabens and the rift floor promotes groundwater flow from the former to the latter, resulting in moderate to high groundwater recharge at the rift floor. The results of this study highlight the need to use different groundwater recharge estimation techniques to understand and capture potential recharge mechanisms and obtain an acceptable estimate of recharge, especially in regions with arid to semi-arid climates and active tectonic areas such as the western rift margin and associated Afar rift floor.