Structural controls on groundwater salinization and borehole failure in the Upper Blue Nile Basin, Ethiopia

Abstract

Groundwater salinization causes recurrent borehole failures in the Fetam-Yisir watershed, Upper Blue Nile Basin, yet its origin remains uncertain. This study investigates whether a major west-east fault system controls this salinity, challenging conventional regional recharge models. We integrated hydrochemistry, stable isotopes (δ 18 O, δ 2 H), and multivariate statistics, specifically Hierarchical Cluster Analysis (HCA) and Principal Component Analysis (PCA), to distinguish groundwater populations and governing processes. Results reveal two distinct systems. Regional meteoric groundwater (Mg-Na-HCO₃ to Na-Mg-HCO₃; TDS <1,800 mg/L) reflects modern recharge. Conversely, a saline groundwater system (Na-HCO₃; TDS >9,000 mg/L) is strictly confined to the Bure-Baguna fault corridor. These fault-zone waters exhibit marked isotopic depletion (δ 18 O: -5.6‰ to -2.6‰) and stoichiometric cation exchange (slope ≈ -1), indicating the fault-driven ascent of deep, highly evolved groundwater from underlying carbonates. Consequently, the fault acts dually as a vertical conduit for deep fluids and a lateral barrier preventing widespread mixing. This structural model enables predictive well-siting strategies and provides a transferable framework for diagnosing fault-controlled salinization in tectonically complex aquifers. • Fault-controlled salinization explains recurrent borehole failure in the Upper Blue Nile Basin. • High-salinity Na-HCO₃ groundwater (>9,000 mg/L) is confined to the west-east Bure-Baguna fault zone. • Stable isotopes identify deep, evolved paleo-meteoric groundwater. • Stoichiometric cation exchange signatures (slope ≈ -1) strongly indicate carbonate-derived salinity modified during fault ascent. • Structural mapping and spatial hydrochemical gradients support precautionary, risk-based setbacks for well siting.