Critical Zone weathering controls on the hydrogeochemistry and spatial variability of groundwater arsenic and fluoride in the Lake Victoria Basin, Northwest Tanzania
Year: 2026
Type: article
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.
Language: English
Subfield: Environmental Chemistry
Field: Environmental Science
Domain: Physical Sciences
Open Access status: diamond
OpenAlex ID: W7127055699