Hydrogeochemical Characterization and Water Quality Index-Based Evaluation of Groundwater for Drinking and Irrigation Use in the Arid Ewaso Ng’iro–Lagh Dera Basin, Kenya

Groundwater is the main source of water for both domestic and agricultural use in arid regions. This study assessed the hydrogeochemical characteristics and suitability of groundwater for drinking and irrigation in Kenya’s Ewaso Ng’iro–Lagh Dera Basin. A total of 129 borehole samples were collected and analyzed for pH, electrical conductivity (EC), total hardness, and major ions. The groundwater was found to be mostly neutral to slightly alkaline and ranged from marginal to brackish in salinity. The dominant water type is Na-HCO₃, with the ionic order Na⁺ > Ca²⁺ > Mg²⁺ > K⁺ and HCO₃⁻ > Cl⁻ > SO₄²⁻ > NO₃⁻. Mineral saturation indices indicate the water is undersaturated with gypsum and anhydrite, but saturated with calcite, dolomite, and aragonite. Groundwater chemistry is primarily influenced by ion exchange and rock weathering processes. The Water Quality Index (WQI) reveals that much of the groundwater is rated poor to unsuitable for drinking, largely due to high levels of sodium, EC, and bicarbonate. Similarly, the Irrigation Water Quality Index (IWQI) places most samples in the moderate to severe restriction category due to salinity and sodicity. These findings highlight the need for proper treatment before use.