Spatial variations in groundwater chemistry in Uganda: Geogenic origins and geochemical controls across diverse hydrogeological settings

Groundwater sustains about 75 % of Uganda’s population, particularly in rural and peri-urban areas. However, the baseline hydrogeochemistry of operational community water sources remains poorly understood. Using hydrochemical analysis and geochemical modelling, we assessed groundwater inorganic geochemistry and geochemical controls across five distinct hydrogeological settings in Uganda: Precambrian metasedimentary (MS; n = 30), granulitic-gneissic complex (GG; n = 21), unconsolidated sedimentary (SDM; n = 10), volcanics (VO; n = 7), and metavolcanics (MV; n = 6). Results show that Ca-HCO 3 is the predominant hydrochemical facies in most settings, reflecting limited geochemical evolution, whereas Na-HCO 3 and Na-Cl facies dominate MV and SDM, consistent with cation exchange and advanced geochemical evolution. World Health Organisation (WHO) drinking water guidelines for selected targets were exceeded in 3 % of samples (for Fe, F - , and Cl - ), 5 % for Mn, and 14 % for NO 3 - . By settings, exceedances were: (i) MS – 20 % NO 3 - , 13 % Mn, and 7 % Cl - ; (ii) GG – 14 % NO 3 - , and 5 % Fe; (iii) MV –17 % Fe; and (iv) SDM – 10 % F - , indicating more frequent guideline exceedances in MS. Major-ion ratios, including (Na + +K + )/Cl - ratios ( > 1), HCO 3 - /Na + (1–4), (Ca 2+ +Mg 2+ )/HCO 3 - ( ∼ 0.3–0.8), and (Ca 2+ +Mg 2+ )/SO 4 2‒ (>1), are consistent with active silicate weathering, cation exchange, and contributions from carbonate dissolution, with minimal influence from evaporative concentration and gypsum dissolution. Geochemical modelling shows undersaturation for calcite, dolomite, and gypsum, but quartz supersaturation. Thermodynamic modelling indicates kaolinite as the stable secondary mineral in VO, contrasting clinoptilolite in other settings, reflecting active monosiallitisation under rapid flow, well-drained conditions, and low silica accumulation. This study establishes baseline hydrogeochemical characterisation across Uganda’s diverse hydrogeologies, highlighting spatial variability and controls to inform targeted, sustainable groundwater management.