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

Nitrate (NO 3 − ) contamination threatens the quality of groundwater in many agricultural areas, and excessive NO 3 − consumption may cause adverse human health implications. Determination of NO 3 − occurrence and its source of contamination is essential for the effective management of groundwater quality. The characteristics of NO 3 − in groundwater from the Anayari Catchment were analysed using hydrogeochemical and stable isotope data. Concentrations of major ions and the isotopic values of δ 2 H, δ 18 O, δ 15 N–NO 3 − and δ 18 O–NO 3 − of groundwater and δ 15 N–NO 3 − and δ 18 O–NO 3 − of potential NO 3 − 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 NO 3 − sources. The Bayesian mixing model was applied to estimate the proportional contributions of NO 3 − load in boreholes (BH) and hand–dug wells (HDW) in the Catchment. The NO 3 − levels were mostly lower than the WHO guideline limit. Correlation analyses indicate that the source of NO 3 − was mostly livestock and septic waste. The results from δ 15 N–NO 3 − and δ 18 O–NO 3 − showed that the contribution of NO 3 − from potential sources is in the order of S/M > SN > CF > P, with denitrification influencing the NO 3 − concentration in some of the groundwater sources. The Bayesian mixing model demonstrated that by proportion, SN and S/M were the major NO 3 − 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.