<p>The processes involved in the co-occurrence of As (Arsenic) and F⁻ (Fluoride) in groundwater was studied in Voltaic Sedimentary Basement rocks within the Oti River basin of Ghana. Hydrogeochemical data of a total of 133 groundwater samples were analyzed by statistical and geostatistical methods. The results indicated reducing conditions in aquifers during the dry season and oxidizing conditions in the wet season based on Eh values. In the dry season, 13% of samples exceeded the WHO limit of 1.5&#xa0;mg/L for F⁻, and 17% did so during the wet season. Over 25% of dry season As levels surpassed the WHO guideline of 0.01&#xa0;mg/L. A positive correlation between As and F⁻ levels, strengthened by higher pH, was observed. Elevated As levels in the dry season were linked to evaporative enrichment, while F⁻ increases were due to desorption from fluorine-rich rocks. Saturated indices calculations revealed that precipitation of supersaturated Ca ions and common ion effect from calcium rich minerals contributed to F⁻ presence in groundwater. There were localised elevated concentrations of both As and F⁻ in the Oti-Pendjari geological group as a result of the presence of clay minerals that facilitated the co-occurrence of As and F⁻, when compared to the Kwahu-Morago group. Generally, the liberation of As is mainly linked to evaporative enrichment and desorptive discharge in alkaline conditions while, the occurrence of F⁻ can be ascribed to desorption from fluorine-rich minerals, the precipitation of Ca²⁺ in calcium-rich minerals, and the common ion effect; leading to localized pollution in affected areas.</p>