Résumé
Mediterranean coastal hydro-ecosystems face high anthropogenic pressures exacerbated by climate change, making them ecologically vulnerable and threatening any ecosystem services they provide. Urbanization and agriculture development on these areas inevitably impacts the quality of interconnected water bodies – groundwater, surface water, coastal lagoon water and sea water.However, understanding water origins, mixing ratios and flow modalities towards the coastal ecosystem is complex. In such highly man-modified systems, emerging contaminants combined with isotope tracers were thought relevant to track interconnections of water bodies with high precision. Few studies focused on the pollutant legacy footprint left in groundwater, surface water and lagoon water and the role of each in the lagoon quality degradation.Our innovative approach combines hydrogeological tracers (trace and major elements, δ18O, δ2H, 3H, δ15N and 11B) with a screening of 120 pharmaceutical compounds and 200 pesticides. This approach was here applied to the “Etang de l’Or” lagoon (Languedoc coast, Southern France), characterized by its sensitivity to eutrophication and poor ecological status. A multi-tracer survey was carried out in May 2023 on 30 sampling points.Results highlight the relevance of this multi-tracing approach to determine the role of water bodies interconnections on the agricultural and domestic pollution footprints on groundwater. The “Etang de l’Or” lagoon recharge mainly depends on groundwater, supplied by both autochthonous and allochthonous inputs, notably the irrigation canal carrying water from the Rhône. The lagoon both faces short-term domestic pollution fluxes from surface water origin and historical agricultural pollution legacy stored in groundwater. This work is one of the first benchmark studies demonstrating the need to redefine protection status of coastal hydroecosystems, considering the complex water bodies connexions, to improve water management strategies.