Geochemistry, enrichment mechanisms and geomedical assessment of fluoridated groundwater in some parts of the largest semi-arid sedimentary basin in Ghana

This study investigated the hydrogeochemistry, spatial distribution, health risks, and underlying mechanisms of elevated fluoride (F⁻) concentrations in groundwater across communities situated within the Voltaian Supergroup of the semi-arid Volta Basin in Ghana. This was achieved through a medical geology framework that integrates hydrogeochemistry, GIS-based spatial analysis, geochemical modelling, unsupervised machine learning, petrography, PXRD analysis, Monte Carlo Simulation, Sobol sensitivity analysis, probabilistic health risk assessment, and community-based health surveillance to investigate the geogenic sources, spatial distribution, and health implications of F⁻ in groundwater. Thirty-one borehole water samples were collected from Mion District, Karaga District, and Gushegu Municipality in the Northern Region of Ghana. Petrographic and PXRD analyses identified minerals such as quartz, albite, biotite, zeolite, illite, and opaque minerals in the reservoir rocks. Lithological evaluation of boreholes confirmed the reservoir rocks at depth. These minerals dissolve in groundwater, enriching it with various ions. The major cations in groundwater were found in the order: Mg²⁺ > Ca²⁺ > K⁺ > Na⁺, and the major anions in the order: HCO₃⁻ > SO₄²⁻ > Cl⁻. The groundwater is alkaline, with Mg²⁺ − HCO₃⁻ as the common water type. Fluoride concentrations ranged from 0.23 to 19.5 mg/L, with an average of 4.71 mg/L, exceeding the World Health Organization's guideline value of 1.5 mg/L in about 87 % of the boreholes around the northeastern fringe of the study area. The high F⁻ levels are primarily due to natural sources and are influenced by water-rock interaction, weathering, ion exchange, and mineral dissolution and precipitation. Health risks associated with fluoridated drinking water were assessed using the US Environmental Protection Agency Model, Monte Carlo Simulation, and Sobol sensitivity analysis revealing non-carcinogenic risks such as dental and skeletal fluorosis, particularly affecting children, followed by adults, teenagers, and infants. Sobol sensitivity analysis reveals that body weight is the primary driver of the non-carcinogenic risk variability across the population, followed by water consumption and F⁻ concentration. However, age-specific patterns show that for infants and children, F⁻ levels and body weight are key contributors, while in teenagers and adults, F⁻ concentration alone dominates risk variability. Community-based health surveillance confirmed these findings. Due to the high F⁻ levels, the people now use polluted surface water for drinking and domestic purposes, leading to severe gastrointestinal infections. Therefore, urgent attention is needed by the stakeholders to curb the threat of groundwater contamination and the associated human health risks.