Hydraulic and geochemical impact of occasional saltwater intrusions through a submarine spring in a karst and thermal aquifer (Balaruc peninsula near Montpellier, France)

Marie-Amélie Pétré,Romain Hemelsdaël,Bernard Ladouche,Véronique De Montety,Christelle Batiot-Guilhe,Claudine Lamotte,Jean-Luc Seidel

doi:10.5194/hess-24-5655-2020

Abstract

Abstract. Submarine springs are a common discharge feature of the karst aquifers along the Mediterranean coast. In some instances, occasional and localized saltwater intrusions can occur through the submarine spring and negatively impact the quality of the groundwater resource. The hydraulic and geochemical behaviour of a submarine spring discharging into the Thau Lagoon just offshore of the Balaruc peninsula near Montpellier, France, has been characterized to determine the impacts of such phenomena to better understand the dynamics of a regional karst aquifer and improve its groundwater management. This work is based on both historical and new hydrogeological and geochemical data, illustrating six occasional saltwater intrusion events (from 1967 to 2014) in the Thau Lagoon area (southern France). Hydraulic perturbation of the aquifer is propagated instantly within the Balaruc-les-Bains peninsula and reaches a distance of about 5 km upgradient within 9 d. Comparison of hydraulic heads during seawater intrusion events in 2010 and 2014 indicates an aggravation of the phenomenon with an increase in hydraulic head variations. In contrast, isotopic tracers (87Sr∕86Sr, D/H, 18O∕16O) and rare earth elements (REEs) demonstrate that the geochemical impact of these inversac events is only observed at the local scale but is still perceptible several years after the event. For example, some of the thermal wells had not recovered their initial geochemical state 20 and 40 months after the last two inversac events (2010 and 2014, respectively), suggesting a geochemical legacy of this phenomenon within the complex karst system. By contrast, an adjacent deep karst compartment located south of the study area is not affected by the saltwater intrusion and is characterized by distinctly different hydrodynamic behaviour. Overall, this work on occasional and localized saltwater intrusions constitutes a key step in understanding the dynamics of this complex karstic and thermal aquifer and will support the management of the groundwater resource.

Highlights

Submarine springs are common discharge features of karst systems along the Mediterranean coast (Bakalowicz, 2014; Fleury, 2005; Fleury et al, 2007; Stieglitz et al, 2013)
Thermal waters rise from the deep reservoir along inferred faults in the vicinity of the Balaruc peninsula to reach the top of the Jurassic aquifer
A comprehensive review of historical inversac events undertaken as part of this study showed that there is a hydraulic connection between the Villeveyrac Basin and the Balaruc peninsula

Summary

Introduction

Submarine springs are common discharge features of karst systems along the Mediterranean coast (Bakalowicz, 2014; Fleury, 2005; Fleury et al, 2007; Stieglitz et al, 2013). Especially during periods of low discharge, flow can reverse and instead intrude into the upgradient karst aquifer, causing an adverse impact on the quality of the groundwater resource and the economic activities that depend on it (Arfib and Gilli, 2010; Drogue and Bidaux, 1986). This phenomenon, termed “inversac” in French (Gèse, 1987; Pinault et al, 2004), was previously. Occasional saltwater intrusion through a submarine spring is a largely understudied phenomenon

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