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

Abstract

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–HCO3 is the predominant hydrochemical facies in most settings, reflecting limited geochemical evolution, whereas Na–HCO3 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 NO3 −. By settings, exceedances were: (i) MS – 20 % NO3 −, 13 % Mn, and 7 % Cl−; (ii) GG – 14 % NO3 −, 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), HCO3 − /Na+ (1–4), (Ca2++Mg2+)/HCO3 − (~0.3–0.8), and (Ca2++Mg2+)/SO4 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 under-saturation for calcite, dolomite, and gypsum, but quartz super-saturation. 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.