Quantification of nitrate contamination sources in groundwater from the Anayari catchment using major ions, stable isotopes, and Bayesian mixing model, Ghana

Nitrate (NO3−) contamination threatens the quality of groundwater in many agricultural areas, and excessive NO3− consumption may cause adverse human health implications. Determination of NO3− occurrence and its source of contamination is essential for the effective management of groundwater quality. The characteristics of NO3− in groundwater from the Anayari Catchment were analysed using hydrogeochemical and stable isotope data. Concentrations of major ions and the isotopic values of δ2H, δ18O, δ15N–NO3− and δ18O–NO3− of groundwater and δ15N–NO3− and δ18O–NO3− of potential NO3− source materials (precipitation (P), chemical fertilisers (CF), soil nitrogen (SN), and sewage or manure (S/M)) in the area were determined and used to identify groundwater NO3− sources. The Bayesian mixing model was applied to estimate the proportional contributions of NO3− load in boreholes (BH) and hand–dug wells (HDW) in the Catchment. The NO3− levels were mostly lower than the WHO guideline limit. Correlation analyses indicate that the source of NO3− was mostly livestock and septic waste. The results from δ15N–NO3− and δ18O–NO3− showed that the contribution of NO3− from potential sources is in the order of S/M > SN > CF > P, with denitrification influencing the NO3− concentration in some of the groundwater sources. The Bayesian mixing model demonstrated that by proportion, SN and S/M were the major NO3− contributors to BH and HDW, respectively. There is a need to institute and enforce proper manure application management strategies to reduce the risk of groundwater contamination from manure nutrients.