Critical Zone weathering controls on the hydrogeochemistry and spatial variability of groundwater arsenic and fluoride in the Lake Victoria Basin, Northwest Tanzania

Fanuel Ligate; Julian Ijumulana; Regina  Irunde; Vivian Kimambo; Rajabu Hamisi; Jyoti Prakash Maity; Joseph Mtamba; Felix Mtalo; Prosun Bhattacharya

doi:10.63697/jeshs.2026.10033

Authors

Fanuel Ligate KTH-International Groundwater Arsenic Research Group, Department of Sustainable Development, Environmental Science and Engineering, KTH Royal Institute of Technology, SE-100 28 Stockholm, Sweden https://orcid.org/0000-0002-2883-2445
Julian Ijumulana KTH-International Groundwater Arsenic Research Group, Department of Sustainable Development, Environmental Science and Engineering, KTH Royal Institute of Technology, SE-100 28 Stockholm, Sweden https://orcid.org/0000-0002-7435-1677
Regina Irunde KTH-International Groundwater Arsenic Research Group, Department of Sustainable Development, Environmental Science and Engineering, KTH Royal Institute of Technology, SE-100 28 Stockholm, Sweden https://orcid.org/0000-0002-0501-9515
Vivian Kimambo KTH-International Groundwater Arsenic Research Group, Department of Sustainable Development, Environmental Science and Engineering, KTH Royal Institute of Technology, SE-100 28 Stockholm, Sweden
Rajabu Hamisi KTH-International Groundwater Arsenic Research Group, Department of Sustainable Development, Environmental Science and Engineering, KTH Royal Institute of Technology, SE-100 28 Stockholm, Sweden https://orcid.org/0000-0002-8957-6772
Jyoti Prakash Maity Department of Chemistry, School of Applied Sciences, KIIT Deemed to be University, Bhubaneswar, Odisha 751024, India https://orcid.org/0000-0003-4702-335X
Joseph Mtamba DAFWAT Research Group, Department of Water Resources Engineering, College of Engineering and Technology, University of Dar es Salaam, Dar es Salaam, Tanzania
Felix Mtalo DAFWAT Research Group, Department of Water Resources Engineering, College of Engineering and Technology, University of Dar es Salaam, Dar es Salaam, Tanzania
Prosun Bhattacharya KTH-International Groundwater Arsenic Research Group, Department of Sustainable Development, Environmental Science and Engineering, KTH Royal Institute of Technology, SE-11428 Stockholm, Sweden https://orcid.org/0000-0003-4350-9950

DOI:

https://doi.org/10.63697/jeshs.2026.10033

Keywords:

Critical Zone process, Arsenic and fluoride, Hydrogeochemistry, Geospatial analysis, Lake Victoria Basin

Abstract

Arsenic (As) and fluoride (F⁻) in groundwater limit access to safe drinking water across East Africa, yet process-based understanding of their co-occurrence in Precambrian basement aquifers of northern Tanzania remains limited. This study analyzed groundwater samples (n = 88) from 13 wards in Geita district, Lake Victoria Basin, adjacent to large-scale and artisanal gold mining areas. An integrated approach combining hydrogeochemistry, geospatial analysis, and geochemical modeling was used to investigate spatial variability and the hydrogeochemical processes controlling the distribution of As and F⁻ in the aquifer. Arsenic concentrations ranged from 2.5 to 280 μg/L and F⁻ from 0.13 to 2.17 mg/L; with approximately 82% and 8% samples exceeding World Health Organization (WHO) guideline values, respectively. Groundwater pH was 5.7–7.5, electrical conductivity (EC) and total dissolved solids (TDS) indicated generally acceptable salinity and dissolved As occurred predominantly as As(V) species. As and F⁻ exhibited spatial clustering and co-occurrence associated with migmatite-granitoid-metasediment complexes and volcano-sedimentary Greenstone Belt lithologies, with higher concentrations observed in mining-affected areas. These patterns are interpreted within a Critical Zone framework, where coupled lithological, hydrological, and biogeochemical processes regulate weathering reactions, generate reactive Fe-oxide and clay surfaces, and establish pH and redox conditions that control trace element mobility. Variations in pH (5.7–7.5) influenced As mobility primarily through oxidative weathering of arsenopyrite and other sulfides, forming secondary Fe(III) oxides and hydroxides. Subsequent reductive dissolution of these Fe phases under favorable geochemical conditions released sorbed or co-precipitated As into groundwater, particularly within Ca-HCO₃ type waters. Fluoride mobilization was enhanced under alkaline, Na–HCO₃ waters, low Ca²⁺ activity, elevated Na⁺ and HCO₃⁻, ion exchange, and calcite precipitation, which together favored fluorite dissolution and desorption. By linking observed spatial patterns to Critical Zone weathering processes, this study provided a mechanistic basis for As and F⁻ co-occurrence in Precambrian basement aquifers of Northwest Tanzania and offered insights for groundwater-risk assessment and the design of safe drinking-water supply strategies in mining-affected regions.

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