Using socio-hydrogeology to identify pollutant fluxes toward groundwater within a highly anthropized coastal hydrosystem

Coastal Mediterranean lagoons are usually groundwater dependent systems, however their hydrogeological functioning is poorly investigated, damaging their management. Socio-hydrogeology allows, in a transdisciplinary way to clarify the relationships linking human and groundwater. Those interactions within the watershed can generate processes leading to pollutant fluxes with impact 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) has been chosen as a pilot site. Indeed, significant nitrate content, emerging compounds, and pesticides have already been observed, 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 pollution detected in the lagoon’s water.

For this purpose, a field campaign was led in spring 2021 combining classical hydrogeological investigations with structured interviews. The latter involved the administration to 32 water users and 16 local stakeholders 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, hydrogeological investigations have been carried out including chemical, geochemical and isotopic (²H/¹⁸O) sampling at 53 points, of which 21 samples were also analysed for pesticides.

Pesticide’s analysis shows that the site is affected by agricultural pollution. Neonicotinoid insecticides imidacloprid and acetamiprid are extensively used worldwide, threatening ecosystems and human health, they have been found on the sampling points with concentrations between 28.8ng/l to 36.4 ng/l and 6.3ng/l to 123 ng/l. Also benzotriazole, one of the most commonly found compound in groundwater is present with concentrations between 30.7ng/l to 233 ng/l.

The purpose of this work is to offer a new functional diagram of the Biguglia hydrosystem, including the space-time continuum of anthropogenic modifications within the watershed. This new knowledge will help local stakeholders to maintain the good status of water bodies on the watershed.