Impacts of anthropogenic activities on groundwater quality using multiple isotope fingerprints in the Upper East Region of Ghana

This study evaluated the hydrochemistry, groundwater recharge mechanisms, and nitrate pollution sources in the Upper East Region of Ghana using an integrated approach of ion chromatography, laser spectroscopy, and stable isotope analyses (δ¹⁸O–H₂O, δ²H–H₂O, δ¹⁵N–NO₃⁻, and δ¹⁸O–NO₃⁻). A total of 90 water samples (60 boreholes, 20 hand-dug wells, and 10 surface water samples from dams) were collected and analysed. The results indicate that most groundwater and surface water samples were classified as fresh, although one borehole exceeded the 1000 mg/L TDS guideline. Approximately 46.7% of boreholes and 80% of hand-dug wells exhibited nitrate concentrations above the natural background value of 3 mg/L. The δ¹⁸O and δ²H compositions of borehole and hand-dug well samples cluster along the Global Meteoric Water Line, confirming meteoric origin, while surface water samples plot away from the line, indicating significant evaporative isotopic fractionation. Dual‑isotope (δ¹⁵N–NO₃⁻ and δ¹⁸O–NO₃⁻) and hydrochemical analyses (NO₃⁻/Cl⁻, NO₃⁻/Na⁺, Cl⁻/Na⁺ ratios) evidence indicates that nitrate in groundwater is predominantly associated with organic waste sources (manure, sewage and septic effluents), although these sources could not be unequivocally differentiated because of overlapping isotopic signatures. No clear isotopic evidence of extensive denitrification was observed; however, the absence of complementary redox indicators precludes definitive assessment of denitrification processes. The findings suggest that groundwater nitrate contamination is predominantly associated with mixed organic sources, including manure, sewage, and septic effluents, although overlapping isotopic signatures preclude definitive source attribution. Consequently, poor sanitation infrastructure is considered a likely contributing factor rather than the sole dominant driver of nitrate contamination. This study integration of hydrochemical and isotopic tracers provides robust evidence for groundwater recharge processes and nitrate source characterization in the Upper East Region of Ghana. The findings support improved sanitation management, routine groundwater quality monitoring, and targeted nutrient management to reduce nitrate contamination in vulnerable rural aquifers.