Nitrate contamination and source apportionment in surface and groundwater in Ghana using dual isotopes (15N and 18O-NO3) and a Bayesian isotope mixing model

The rising food production to meet the growing human population has led to increased anthropogenic inputs of nutrients such as NO3- in groundwater and aquatic environments. Nitrate concentrations, hydrochemistry, and isotope data (delta O-18-H2O, delta H-2-H2O, N-15-NO3, and delta O-18-NO3) from boreholes (BH), hand dug wells (HDW), and surface water (SW) were analyzed. The objectives of the study were to identify potential nitrate sources and their proportional contributions using an isotope mixing model (SIAR). The results showed that NO3- concentrations in the BH, HDW, and SW were heterogeneous and controlled by localized anthropogenic activities. The hydrochemistry and dual isotope (N-15-NO3 and O-18-NO3) identified manure/sewage as the dominant source of NO3- in the groundwater, while the SW showed a complex signature overlapping in the areas of manure/septic, chemical fertilizer, and soil nitrogen. The SIAR analysis showed that sewage/manure contributed about 66%, 68%, and 55% of NO3- in the BH, HDW, and SW, respectively. In the study area, the NO3- source contribution based on the mean probable estimate (MPE) were in the order S&M > SN > CF > P. Shortcomings and the uncertainties associated with the SIAR to guide future studies have also been discussed. The study also highlighted the use of hydrochemistry, environmental isotopes, and Bayesian isotope mixing models for NO3- source identification and apportionment. This is to enable effective planning, farming practices, and sewage disposals to safeguard groundwater quality and control the eutrophication in rivers to meet safe drinking water demand.