Tracing Nitrate Contamination Sources and Apportionment in North-Western Volta River Basin of Ghana Using a Multi-Isotopic Approach

Nitrate contamination in drinking water is now gaining global attention because of its potential effect on human health. In this study, nitrate concentrations and their potential sources in groundwater and surface water were investigated using hydrochemical and isotopic methods. The physical parameters were measured in-situ using multi parameter meters; major ions, stable isotopes (δ²H and δ¹⁸O of H₂O) and δ¹⁵N and δ¹⁸O of NO₃⁻ were measured using the Ion Chromatography, laser spectrometry and titanium (III) reduction method respectively. The results indicate that the dominant water type is Ca-Mg-HCO₃, followed by the Ca–Mg–Cl hydrogeochemical facies. The plot of NO₃⁻/Cl⁻ against Cl⁻ revealed that the dominant sources of NO₃⁻ in the groundwater are manure/sewage with few traces from soil inputs. Meanwhile the plot of δ²H-H₂O against δ¹⁸O-H₂O showed that rainfall is the main source of groundwater recharge with few groundwater samples showing evidence of recharge from an enriched source (Black Volta River). The plot of δ¹⁸O – NO₃⁻ against δ¹⁵N – NO₃⁻ suggests that a significant percentage of nitrate is from manure and sewage, followed by the soil zone, hence leading to nitrification and denitrification being important biological processes affecting NO₃⁻ concentrations in groundwater in the study area. The stable isotope mixing model suggests manure contributed a mean proportional contribution of about 74% of NO₃⁻ to groundwater in the study area, while soil nitrogen contributed 10%. The Nitrate Pollution Index (NPI) suggests that about 80.2% of water samples were pollution free, while anthropogenic activities resulted in about 8.3% of the pollution index. Since most samples with lower nitrate pollution indices were observed around recharged areas and increased toward the discharge points, our study suggests the possibility that recharge areas of the water was free or lowest in nitrate contamination.