Groundwater Quality and Fluoride Enrichment: Hydrochemical and Lithological Controls in a Crystalline Basement Aquifer of Northern Ghana

Groundwater is the principal source of drinking water in northern Ghana, where fluoride and nitrate are the major groundwater-quality concerns. Naturally elevated fluoride concentrations (>1.5 mg/L) have resulted in the closure of wells, while nitrate pollution regularly occurs in shallow aquifers due to lack of sanitation infrastructure and agricultural activities. Distinguishing natural from human-induced groundwater contamination is therefore essential for effective groundwater management. This study assessed groundwater quality with regard to fluoride enrichment and established lithology-specific natural background levels (NBLs) for major ions and selected trace elements using groundwater samples from 110 wells across three different lithologies in the Vea Catchment, northern Ghana. The results reveal high fluoride NBLs in a K-feldspar-rich granitoid formation (3.84 mg/L) and near-guideline concentrations in a hornblende–biotite tonalite (1.41 mg/L). Silicate weathering, lithology-dependent dissolution of fluorine-bearing minerals, and cation exchange were identified as the principal processes controlling groundwater chemistry and fluoride enrichment. Elevated NBLs of Sr, Li, Ba and SiO2 in the K-feldspar-rich granitoids indicate enhanced water–rock interaction whereas relationships between nitrate and Cl−, Br−, and SO42− suggest localized anthropogenic impacts. These findings demonstrate the value of lithology-specific NBLs for improving groundwater quality assessment and supporting groundwater resource management in crystalline basement aquifers.