Résumé
Hundreds of submarine springs drain inland karst aquifers, providing freshwater to the sea. Submarine karst springs can be subject to flow reversal in which alternatively freshwater outflows to the sea or saltwater inflows into the aquifer. In karst aquifers, a full flow reversal at a submarine karst spring has never been monitored. Here, we provide a full flow reversal time series of discharge, electrical conductivity and temperature measurements at the Vise submarine spring (below Thau lagoon, south of France) during which saltwater inflows into the aquifer from 28 November 2020 to 14 March 2022. We demonstrate that the three main factors triggering the flow reverseal are: the lagoon water level, the aquifer piezometric head and the lagoon water salinity. All together, they constitute a tipping point beyond which the karst system undergoes an abrupt degradation. We show that the sudden filling by saltwater of the vertical karst conduit feeding the submarine spring induces a pressure wave into the aquifer which creates a high hysteresis into the flow regime. We estimate the amount of salt mass that infiltrated into the aquifer. Using a density dependent numerical approach, we model the flow reversal mechanism, explain the phenomenon and the reason for its long duration after the onset of the event. This phenomenon increases the vulnerability of coastal fresh groundwater resources, especially in deep karst aquifers. In a changing climate context, flow reversal at submarine karst springs could be more frequent in the future due to sea level rise and a decrease of recharge, threatening inland or offshore freshwater resources.