Hydraulic and geochemical impact of occasional saltwater intrusions through a submarine spring in a karstic and thermal aquifer (Thau Lagoon area, Montpellier, France)

Submarine springs are common discharge features of karstic systems along the Mediterranean coast. In some cases, these submarine springs allow occasional seawater intrusions in the karstic aquifer causing an adverse impact on the quality of the groundwater resources. In order to manage and npreserve the water resources, it is required to have a deep understanding of this phenomenon. The aim of this study is thus to characterize the hydraulic and geochemical impacts of such local and temporary saltwater intrusions on a karstic system and to determine the recovery of the aquifer. The karstic and thermal aquifer of the Thau basin is investigated in this study since the quality of the groundwater resource has been threatened and temporarily impacted over the last 50 years by occasional saltwater intrusions (a few weeks to several months long) from the Thau lagoon through a submarine spring. The causes and impact of this process named "inversac" are not well understood. This work is based on a comprehensive review of the last 6 saltwater intrusion episodes (from 1967 to 2014) and the integration of both historical and new hydrogeological and geochemical data. Results show that the hydraulic perturbation is propagated instantly in the Balaruc-les-Bains peninsula and reaches a distance of about 5 kilometers upgradient within 9 days. The comparison of hydraulic heads during the 2010 and 2014 inversac episodes indicates an aggravation of the phenomenon with an increase in hydraulic head variations. In contrast, isotopic tracers (87Sr/86Sr, water stable isotopes) and Rare Earth Elements show that the geochemical impact of these inversac events is only observed at the local scale and is still perceptible several years after the event stopped. For example, some of the thermal wells still have not recovered their initial geochemical state 20 and 40 months after the last two inversac events (2010 and 2014), suggesting a geochemical remanence of the phenomenon in the karstic system of more than 3.5 years. In addition, this study shows that the deep karstic compartment located south of the study area is not affected by the saltwater intrusion episodes and presents a different dynamic than the rest of the hydrosystem. Overall, this work on episodic and local saltwater intrusions is a key step in understanding the dynamics of this complex karstic and thermal aquifer and will support the management of the groundwater resource