Socio-hydrogeological approach to identify contaminant fluxes towards groundwater-dependent hydrosystems, case of the Biguglia lagoon (Corsica, France)

Coastal Mediterranean lagoons are very often groundwater-dependent hydrosystems, however their hydrogeological functioning is poorly known, damaging their management. Socio-hydrogeology allows, in an inter-and transdisciplinary way to clarify the relationships linking human activities and groundwater status. Those interactions within the watershed, combined with consumption patterns of the population, and sanitation defects can generate processes leading to pollutant fluxes with impacts on surface water, groundwater and lagoon water quality. This approach integrates both social and economic components into hydrogeological investigations.

The Biguglia lagoon watershed (Northern Corsica, France) has been chosen as a pilot site. Indeed, significant nitrate content, emerging compounds, and pesticides have already been observed in the lagoon waters, but their origin still needs to be specified, both in terms of source and dispersion modalities.

The aim of this study is to (1) assess the link between groundwater quality and the anthropogenic pressures on the watershed, (2) understand water users’ and the stakeholders ‘perception and knowledge of the watershed and the local territory, (3) identify the origin of pollutions detected in the lagoon’s water.

In this purpose, a field sampling was led in spring 2021, combining several tools useful for the knowledge improvement of the hydrogeological functioning and the tracing of anthropic pollutant fluxes. Investigations with structured interviews was administered to 32 water users and 16 local stakeholders involved in the monitoring assessment, to determine the land use evolution since 1950’s to present and aiming at identifying past and present uses of the water resource over the watershed. At the same time, a multi-tracer water sampling, combining physico-chemical parameters, major ions and trace elements as well as, stable isotopes of the water molecule was carried out on 53 points (lagoon, rivers, canals waters, groundwater), of which 21 samples were also analysed for a set of pesticides (screening of 240 molecules).

Pesticide’s analysis show that the study site is affected by agricultural pollution. Indeed, neonicotinoid insecticides, extensively used worldwide, have been found on the sampling points with significant concentrations. Those pesticides are mainly used in fruit, vegetable and cereal crops. The field survey, the questionnaire and the sampling campaign have allowed to identify and confirm the presence of these cultures on the study site. In the same way, benzotriazoles, perfluorinated acids (PFAs) and DEET (insect repellent) have also been detected. They are related to the consumption habits of the population on the watershed.

Geochemical analysis correlated with the social analysis and the land use analysis permitted to better constraint pollution sources, evidencing two main sources: sanitation defect and agriculture activity.

The socio-hydrogeological approach is essential to improve the knowledge of the Biguglia lagoon hydrosystem. The purpose of this work is to offer a new functional diagram of the area, including the space-time continuum of anthropogenic impacts within the watershed. This new knowledge will help local stakeholders towards the recovery of a good geochemical and ecological status for the lagoon brackish water body of Biguglia.