Multi-proxy (CFCs, Cl, δ18O and δD) assessment of the origin, residence time and recharge of groundwater in the Bilate River Basin (BRB) of Southern Main Ethiopian Rift (SMER)

The volcanic aquifer of Bilate River Basin (BRB) situated on the southwestern side of the Main Ethiopian Rift (MER) serves as the sole reservoir of water supply for urban and rural people found in the basin. This study investigates residence time and groundwater recharge process in the Bilate River Basin. For the first time, this research work delivers information on the resident time and groundwater recharge on the basis of multi-tracer approach (CFCs, Cl, delta 18O and delta D). The delta 18O and delta D values of groundwater are drawn along or close to local Addis Ababa Meteoric Water Line (AAMWL), indicating that groundwater recharge is of meteoric origin. CFC data show that the oldest component of groundwater was recharged before 1950, while the younger component recharged in different time as of 1950. The assigned apparent age for groundwater range from 24 to more than 50 years. Binary mixing model is used to estimate the age and fraction of the younger component. Mixing model suggest that groundwater consists young groundwater that mainly recharged the aquifer between the mid-1980s and mid-1990s. The proportion of young water in the groundwater shows decreasing trend from highland towards the rift indicating the main recharge area is the highland part. The chloride mass balance employed to Bilate River Basin calculated yearly average groundwater recharge to be 217 mm/y which accounted for 20% of the mean annual areal rainfall. The estimated recharge decrease from highland to the rift floor highlighting the importance of preferential flow recharge mechanism. The analysis of apparent age, young proportion and recharge rate, can aid in understanding complex groundwater flow system and residence time of groundwater in the complex hydrogeological setting of rift volcanic aquifer.