Hydrochemistry and isotope hydrology of groundwater and surface water in the Sor and Gebba watershed, southwestern Ethiopia

Abstract The Sor and Gebba watershed plays a crucial role in the water resources of the Baro-Akobo River Basin. However, various factors have had a detrimental effect on the watershed. Unplanned groundwater extraction, wetland drainage, surface water pollution, and land use changes have all intensified due to population growth, urbanization, commercial farms, industrial development, poor water resources management, and improvements in living standards. This research utilized major hydrochemical ions and environmentally stable isotopes to elucidate the interaction between groundwater and surface water in the watershed, which are reliable tracers in various hydrologic processes. During the 2020 wet season and 2022 dry season, over 25 meteoric water samples were collected from different locations for hydrochemical and isotopic analysis. The hydrochemical analysis revealed that the dominant water type is Ca–Mg–HCO 3 − . Among the anions, HCO 3 − was the most dominant, followed by NO 3 − , Cl − , and SO 4 2− , and those of cations Ca 2+ , followed by Na + , Mg 2+ , and K + . The spatial analysis of environmentally stable isotopes confirmed the interaction between groundwater and surface water in the watershed. The quality of groundwater is primarily influenced by rock-water interaction (Gibb's diagram) and, to some extent, by evaporation (Isotope study). A graph of the water samples closely aligned with the Addis Ababa-local meteoric water line indicates their meteoric origin. However, there was a slight deviation to the left due to the effects of distance and altitude. The isotopic correlation further supported the interaction between different water sources.