Integrated litho-structural, hydro-geochemical, and numerical modeling analysis to characterize the 40 springs discharge system, Arba Minch, Ethiopia

Abstract

The 40-spring discharge system (40SDS) in Arba Minch, Ethiopia, is significantly influenced by the complex geomorphology and tectonic activities of the Ethiopian Rift Valley. These springs are the major source of water supply for drinking, domestic use, and tourism, with historical discharge rates between 130 and 250 L per second (l/s). The study aimed to characterize the 40SDS through an integrated approach using litho-structural, hydro-geochemical, and numerical modeling techniques to understand the dynamics and continuing decline rate. The results showed that faults channelize groundwater from the western plateau through the escarpment to the rift floor, discharging into Lakes Abaya and Chamo. Hydro-geochemical analysis identified magnesium (Mg²⁺), calcium (Ca²⁺), and bicarbonate as dominant ions, reflecting interactions with silicate minerals, with concentrations increasing downstream due to water-rock contact. Numerical modeling indicated that the aquifer is highly sensitive to hydraulic conductivity, with 99.8% of groundwater outflow occurring through constant head boundaries. Recharge estimates from WetSpass ranged from 0 to 331.8 mm/year, with an average of 64.7 mm/year. A lag in rainfall-discharge correlation was noted, with a 1-month lag during wet seasons and a 2-month lag during dry seasons, linking recharge to western highland precipitation. Long-term data (1981–2015) indicates a continuous decline in mean annual flow due to reduced recharge and anthropogenic pressures. The springs exhibit a Ca-Na-HCO₃ water type, contrasting with nearby Mg-Na-HCO₃-Cl surface and groundwater. This decline threatens Arba Minch's socioeconomic stability, as these springs are the primary source of water for the community. Effective management strategies are crucial for the sustainable use of this essential resource.