Identification of groundwater recharge and flow processes inferred from stable water isotopes and hydraulic data in Bilate River watershed, Ethiopia

Abstract

The stable water isotopes can reflect the locations of recharge sources of groundwater. δ18O and δ2H composition reflect the hydrological mixing at land surface and the origin of moisture at the time of precipitation and infiltration; therefore, understanding the spatial variability in groundwater recharge and the flow paths is significant for the effective management of aquifers. This study investigates the groundwater recharge sources and groundwater flow paths using the stable water isotopes (δ18O and δ2H) and measured groundwater levels in the Bilate River watershed, Ethiopia. The isotopic ranges in the groundwater samples are more similar to precipitation than river water. The groundwater isotopes are distributed close to the meteoric water line of the watershed. Mass balance analysis of the stable water isotopic characteristics also shows that local rainfall is the dominant recharge source for the groundwater. The groundwater isotopes and static-water-level data analysis indicate that the watershed groundwater is mainly recharged by rainfall in the highlands during abundant precipita tion. However, the groundwater in the northeast and east of the watershed shows high enrichment resembling surface-water isotope content, and could be better associated with the mixture of groundwater and surface water due to the effect of lake water intrusion at the watershed boundary. The isotopic signature of the watershed, along with the hydraulic flow pattern, identified four recharge mechanisms: recharge by local rainfall only, recharge at higher altitudes, recharge by local rainfall and surface flows, and recharge of regional deep aquifers by rainfall and interaction with open water.