- 1Department of Earth and Environmental Sciences and Williamson Research Centre for Molecular Environmental Science, The University of Manchester, Williamson Building, Oxford Road, Manchester, M13 9PL, United Kingdom.
- 2Department of Energy, Minerals, and Petroleum Studies, Mbarara University of Science and Technology, P.O Box 1410, Mbarara, Uganda
- 3Bangladesh Atomic Energy Commission, Paramanu Bhaban, Agargaon, Dhaka-1207, Bangladesh.
Groundwater sustains about 75% of Uganda’s population, especially in rural and peri-urban areas [1,2]. While recent progress has been made to understand groundwater quality at the time of drilling [3], the baseline hydrogeochemical characteristics of operational community drinking water sources remain poorly understood, further complicated by potential surface-groundwater interactions [4]. This study assesses the variations in major groundwater chemistry 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), volcanic (VO; n=7), and metavolcanic (MV; n=6), as well as surface water (SW; n=8). Hydrochemical facies are predominantly CaHCO3, with HCO3- as the dominant anion, reflecting limited geochemical evolution in shallow, discontinuous aquifers. However, NaHCO3 and NaCl facies dominate in MV and SDM settings, respectively, indicating cation exchange processes and more advanced geochemical evolution. The mean (Ca2++Mg2+)/(Na++K+) ratios were generally >1, except in SDM, suggesting reverse cation exchange, further supported by the ion balance plot (slope = –1; R² = 0.6). A mean (Ca2++Mg2+)/HCO3– ratio of ~1 across all settings suggests a dominant influence of carbonate dissolution. The (Ca2++Mg2+)/SO42– ratio was consistently high (>1), with a maximum of 74 for VO, indicating limited gypsum dissolution. Similarly, (Na++K+)/Cl– ratio was high (>1) across all hydrogeologies, with a maximum (17) in MV and a minimum (2.3) in SDM, suggesting minimal evaporative concentration and dominant meteoric recharge. The HCO3–/Na+ ratio [5] was low (1–4) across all settings, with the highest in VO and lowest in SDM, reflecting the influence of silicate weathering. Interestingly, mineral stability diagrams based on ion activity ratios suggest kaolinite as a stable secondary mineral in VO, in contrast to clinoptilolite in other settings. This likely reflects active monosiallitisation in volcanics, where rapid water flow, good drainage, and low silica favour kaolinite stabilisation. Geochemical modelling predicts undersaturation in calcite, dolomite, gypsum, and anorthite across all settings, while feldspars like K-feldspar and albite are supersaturated, with albite undersaturation mainly in VO settings. These findings reveal diverse geochemical processes shaping Uganda's groundwater chemistry, emphasizing the need for hydrogeologically-tailored groundwater monitoring and management.
Acknowledgements
We acknowledge the University of Manchester Faculty of Science and Engineering Dean’s Doctoral Scholarship (to DM), the Dame Kathleen Ollerenshaw Fellowship (to LAR), the International Science Partnership Fund – England project (ODA), and UKRI Future Leaders Fellowship (MR/Y016327/1 to LAR). Thanks to the Ministry of Water and Environment, Uganda for permissions, Jonny Huck for discussions, and the MAGU analytical team for lab support.
References:
[1]MWE, 2024. Water Supply Atlas: National Report.
[2]Nsubuga et al. 2014. Water Resources of Uganda: An Assessment and Review. J. Water Resour. Prot. 06, 1297–1315. https://doi.org/10.4236/jwarp.2014.614120
[3]Owor et al. 2021. Hydrogeochemical processes in groundwater in Uganda: a national-scale analysis. J. Afr. Earth Sci. 175, 104113
[4]Wilson et al., 2024. Surface-derived groundwater contamination in Gulu District, Uganda: Chemical and microbial tracers. Sci. Total Environ. 177118.
[5]Gaillardet et al., 1999. Global silicate weathering and CO2 consumption rates deduced from the chemistry of large rivers. Chem. Geol. 159, 3–30.
How to cite: Muloogi, D., Wilson, G. J., Ahmed, F. T., Polya, D. A., and Richards, L. A.: Major Groundwater Chemistry of Contrasting Hydrogeological Settings in Uganda, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-662, https://doi.org/10.5194/egusphere-egu25-662, 2025.
