Tracing nitrate pollution within the Agneby subcatchment, SE Côte d'Ivoire (West Africa)

Coastal environments around the globe provide services and support to the population. However, they are frequently impacted by human-based activities in addition to seawater intrusion problems. These lowland groundwater resources are exposed to a significant amount and types of pollutants, the most significant of which is nitrate. Tracking and quantifying the sources of nitrate that pollute surface and groundwater systems can be challenging without the use of environmental tracers such as nitrate isotopes. This study explored how changes in regional flow paths impact the nitrogen concentrations and origin of pollution in coastal waters during high tides. Water samples were collected from surface (rivers, lagoon and Atlantic shore) and groundwater (wells and boreholes) systems along the east coast of Côte d'Ivoire during the boreal summer (October). Water samples were analysed for major ions, dissolved nitrogen concentrations, coliforms presence/amount, as well as dual nitrate isotopes (δ15N-NO3- and δ18O-NO3-). Bayesian isotope mixing models were conducted to estimate the contributions of potential main sources (wastewater, seawater, atmospheric deposition, and agrochemicals). In some areas, nitrate inputs were found likely coming from wastewater sources. Nitrate concentration in groundwater was high at several sites. Some groundwater samples (n = 7) exceeded the WHO drinking water limit of nitrate concentrations of 50 mg/l, and most groundwater samples had high levels of total coliforms (>500 cfu/100ml). However, great isotope variation found in both surface and subsurface water samples suggested a spatial differential impact and origin of nitrate pollution, which is in agreement with the modelling. These water resources are the primary source of water for 53% of the local population and an alternative domestic water source for 97%, which highlights the importance of determining the main pollution sources for sustainable development. We expected that groundwater may have been better protected from nitrate pollution than lagoon surface water during wet periods, but this might have not been the case.