Marie-Amélie Pétré
email: marieamelie.petre@gmail.com

Bernard Ladouche

Jean-Luc Seidel

Romain Hemelsdaël

Véronique De Montety

Christelle Batiot-Guilhe

Claudine Lamotte

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

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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, Bernard Ladouche, Jean-luc Seidel, Romain Hemelsdaël, Véronique de Montety, Christelle Batiot-Guilhe, Claudine Lamotte

To cite this version:

Marie-Amélie Pétré, Bernard Ladouche, Jean-luc Seidel, Romain Hemelsdaël, Véronique de Montety, et al.. Hydraulic and geochemical impact of occasional saltwater intrusions through a submarine spring in a karst and thermal aquifer (Balaruc peninsula near Montpellier, France). Hydrology and Earth System Sciences, 2020, 24, pp.5655-5672. 10.5194/hess-24-5655-2020. hal-02991728

Introduction

Submarine springs are common discharge features of karst systems along the Mediterranean coast [START_REF] Bakalowicz | Karst at depth below the sea level around the Mediterranean due to the Messinian crisis of salinity. Hydrogeological consequences and issues[END_REF][START_REF] Fleury | Sources sous-marines et aquifères karstiques côtiers méditerranéens : Fonctionnement et caractérisation[END_REF][START_REF] Fleury | Submarine springs and coastal karst aquifers: a review[END_REF][START_REF] Stieglitz | Karstic groundwater discharge and seawater recirculation through sediments in shallow coastal Mediterranean lagoons, determined from water, salt and radon budgets[END_REF]. Water from these springs can support economic development or be exploited for drinking water supply.

In certain instances, 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 [START_REF] Arfib | Karst côtier et sources sous-marines-fonctionnement et exploitation[END_REF][START_REF] Drogue | Simultaneous outflow of fresh water and inflow of sea water in a coastal spring[END_REF]. This phenomenon, termed "inversac" in French [START_REF] Gèse | Les mésaventures des sources de l'Estavelle et de l'Inversac en Languedoc méditerranéen[END_REF][START_REF] Pinault | Characterizing a coastal karst aquifer using an inverse modeling approach: The saline springs of Thau, southern France[END_REF], was previously described by [START_REF] Albéric | River backflooding into a karst resurgence (Loiret, France)[END_REF] and [START_REF] Joigneaux | Impact of climate change on groundwater point discharge: backflooding of karstic springs (Loiret, France)[END_REF] in a related context of river backflooding into karst springs.

The factors responsible for initiating and terminating inversac events are not particularly well understood and the recovery of carbonate aquifers after a seawater intrusion is poorly described [START_REF] Han | Groundwater salinization processes and reversibility of seawater intrusion in coastal carbonate aquifers[END_REF]. Yet, it is necessary to understand the mechanisms controlling this phenomenon, as well as the long-term impact of occasional and localized saltwater intrusions, to appropriately manage the groundwater resources of such karst systems.

In this context, a hydraulic and geochemical study was undertaken to characterize the occasional and localized saltwater intrusions into the karst aquifer of the Thau basin. More specifically, our study addressed the following questions: (i) What are the hydraulic and geochemical conditions in the karstic aquifer during and after an inversac event? (ii) What are the triggering factors contributing to the occurrence of the inversac event? (iii)To what extent is the aquifer system able to recover from an inversac event and get back to its original geochemical state? (iv) What are the implications in terms of groundwater resources management?

The Thau hydrosystem in the Balaruc-les-Bains area is an example of a karst and thermal aquifer affected by occasional saltwater intrusions through a submarine spring. This complex hydrologic system has been described by [START_REF] Aquilina | Etude géochimique des eaux souterraines autour du bassin de Thau[END_REF][START_REF] Aquilina | Origin, evolution and residence time of saline thermal fluids (Balaruc springs, southern France): implications for fluid transfer across the continental shelf[END_REF][START_REF] Aquilina | Deep water circulation, residence time, and chemistry in a karst complex[END_REF], Doerfliger et al. https://doi.org/10.5194/hess-2020-292 Preprint. Discussion started: 9 July 2020 c Author(s) 2020. CC BY 4.0 License.

(2001) and [START_REF] Ladouche | Cubizolles: Étude du pourtour est de l'étang de Thau[END_REF]. However, the causes and extent of the impact of the saltwater intrusions here remain poorly understood. The karst and thermal water resources across the Thau hydrosystem are exploited both for drinking water supply and spa activities. Balaruc-lesbains (Fig. 1) is the largest spa in France with 55,000 visitors annually. In the past, water from the Vise submarine spring was diverted to the seashore and used for aquaculture activities.

Additionally, the Thau lagoon supports an extensive shellfish aquaculture and fishery. Thus, the economic stakes are high in this area and water use conflict has the potential to arise between stakeholders during dry conditions, as a consequence of an increase in water demand, and the occurrence of occasional saltwater intrusions [START_REF] Chu | Spatial and temporal dynamics of bacterial contamination in South France coastal rivers: focus on in-stream processes during low flows and floods[END_REF][START_REF] Jeunesse | Is the governance of the Thau coastal lagoon ready to face climate change impacts?[END_REF].

Indeed, the karst system has been affected by six inversac events through the Vise submarine spring over the past 50 years (Table 1), five of which (1967,1983, 2008, 2010 and 2014) have been documented and described (Ladouche et al., 2011[START_REF] Ladouche | Dem'Eaux Thau -Synthèse et valorisation préliminaire des données sur l'hydrosystème de Thau[END_REF][START_REF] Ladouche | Inversac de la source sous-marine de la Vise en[END_REF].

Human activities, such as bauxite mining upstream from the Vise spring, and intervention on the spring itself are thought to have contributed to the saltwater intrusions over the period by drawing down the hydraulic head of the karst system. Historically, these inversac events produced an increase in the water level and electrical conductivity at the observation points (thermal well and spring) in the Balaruc peninsula. A better understanding of this phenomenon is for local use purposes, but is also of strategic importance for water managers of the Issanka karst spring, a major drinking water supply in the area.

The last two inversac events in 2010 and 2014 lasted seven and five months, respectively. The 2014 inversac event led to the permanent abandonment of the Cauvy spring which provided drinking water to a local population of 30,000.

https://doi.org/10.5194/hess-2020-292 Preprint. Discussion started: 9 July 2020 c Author(s) 2020. CC BY 4.0 License. A multi-tracer approach was followed to better define the origin and contribution of the thermal, karst and saltwater end-members under different conditions. Water stable isotopes (D/H, 18 O/ 16 O) were used to quantify the contribution of the saltwater end-member, whereas strontium isotopes ( 87 Sr/ 86 Sr) were used to distinguish the origin of salinity (thermal or marine). Rare Earth Elements (REE) were used to establish the chemical signature of the thermal wells in a reference context and to evaluate the influence of the marine signature over time. Finally, the B-isotope signature (d 11 B) was used to trace water-rock interaction initiated during inversac events.

Study Area and Geological Setting

The Balaruc peninsula is located along the Mediterranean coast of southernmost France, west of the city of Montpellier, (Fig. 1a). The karst aquifer of the peninsula lies within the Middle and Upper Jurassic carbonates that outcrop to the north in the Aumelas Causse and the Gardiole Massif. These Jurassic carbonates are buried under Miocene to Plio-Quaternary deposits in the eastern part of the Thau lagoon (Fig 1b). Karstification and burial of the main Jurassic aquifer are the result of the geological history of the region described below. A Mesozoic series of Triassic to Early Cretaceous age was deposited during Tethyan rifting [START_REF] Baudrimont | Un bassin mésogéen du domaine péri-alpin: le Sud-Est de la France[END_REF][START_REF] Debrand-Passard | Synthèse géologique du Sud-Est de la France -Stratigraphie et paléogéographie[END_REF]. The Mesozoic carbonate platform underwent uplift during the Mid-Cretaceous, leading to exhumation that caused erosion of the Early Cretaceous series and strong karstification of the Jurassic carbonates

The Mesozoic series was successively deformed during the Pyrenean collision phase from Upper Cretaceous to Eocene time [START_REF] Arthaud | Contraintes, déformation et déplacement dans l'avant-pays Nordpyrénéen du Languedoc méditerranéen[END_REF][START_REF] Arthaud | M: Les structures pyrénéennes du Languedoc et du Golfe du Lion (Sud de la France)[END_REF][START_REF] Choukroune | Caractéristiques et évolution structurale des Pyrénées; un modèle de relations entre zone orogénique et mouvement des plaques[END_REF][START_REF] Choukroune | Tectonique des plaques et Pyrénées: sur le fonctionnement de la faille transformante nord-pyrénéenne; comparaison avec des modèles actuels[END_REF] and the later rifting of the Gulf of Lion [START_REF] Arthaud | M: Cadre tectonique de quelques bassins sédimentaires[END_REF][START_REF] Arthaud | M: Les structures pyrénéennes du Languedoc et du Golfe du Lion (Sud de la France)[END_REF][START_REF] Benedicto | Low-angle crustal ramp and basin geometry in the Gulf of Lion passive margin: Oligocene-aquitanian Vistrenque graben[END_REF][START_REF] Maerten | Extensional tectonics of the Oligo-Miocene Herault Basin (S France), Gulf of Lion margin[END_REF][START_REF] Séranne | The Gulf of Lion continental margin (NW Mediterranean) revisited by IBS: an overview[END_REF][START_REF] Thaler | Premiers résultats d'une recherche systématique des dents de rongeurs, par lavage de marnes de l'Oligocène, en Bas-Languedoc[END_REF]. Then, the sea level rise of the Early Miocene (Burdigalian) deposited transgressive sediments across the region (Fig. 1b). The overall sedimentary record associated with these Upper Cretaceous to Middle Miocene geological events, includes a wide range of detrital and carbonate facies deposited in marine, lacustrine and fluvial environments [START_REF] Combes | Typologie, cadre géodynamique des bauxites téthysiennes[END_REF]Marchand, 2019) (Fig. 1a).

The sea level fall related to the Messinian Salinity Crisis caused major river incision across southern France which, as a consequence, deepened the karst system [START_REF] Clauzon | Le canyon messinien du Rhône; une preuve décisive du "desiccated deep-basin model[END_REF][START_REF] Hsu | The origin of the Mediterranean evaporites[END_REF][START_REF] Ryan | Quantitative evaluation of the depth of the western Mediterranean before, during and after the Late Miocene salinity crisis[END_REF]. The Messinian paleovalley in the coastal area was then flooded during the early Pliocene when the Mediterranean was connected again to the Atlantic Ocean. Recent sea level fluctuations over the last 5 Ma and associated Plio-Quaternary deposits have generated the present day morphology of the Thau lagoon.

Structurally, the Balaruc-les-bains area is located between both the major NE-trending Cevennes and Nîmes faults, to the south of the Pyrenean Montpellier Thrust (Fig. 1a). The study area is also affected by the presence of the E-W oriented Pyrenean Thau Thrust. The complex fault network at the intersection with the Nîmes Fault allows to bring the Paleozoic basement at shallow depth (less than 2000 m according to borehole data) in the Balaruc-Sète area (Fig. 1a) https://doi.org/10.5194/hess-2020-292 Preprint. Discussion started: 9 July 2020 c Author(s) 2020. CC BY 4.0 License.

Hydrogeological conceptual model of the Thau hydrosystem

The Balaruc peninsula is located at the point of natural convergence of different types of waters including seawater from the Thau lagoon and Mediterranean Sea, cold karst groundwater sourced from the Aumelas Causse and Gardiole Massif, and mineralized hot thermal water rising from depth in the Balaruc peninsula [START_REF] Aquilina | Origin, evolution and residence time of saline thermal fluids (Balaruc springs, southern France): implications for fluid transfer across the continental shelf[END_REF]. These different types of water interact together in the Middle to Upper Jurassic carbonate reservoir. The Montpellier Thrust to the north of the study area (Fig. 1a) brings impermeable marly Lias to surface, which delimits the aquifer and acts as barrier to groundwater circulation. Groundwater flow follows a general N-S trend from the Aumelas Causse and Gardiole Massif to the Balaruc peninsula and the Mediterranean Sea. A clear hydraulic connection from the Vène spring (Aumelas Causse) to Issanka spring (western slope of the Gardiole Massif) has been demonstrated through artificial tracer testing [START_REF] Paloc | Definition de la structure géologique profonde du bassin de Montbazin-Gigean[END_REF][START_REF] Ladouche | Cubizolles: Étude du pourtour est de l'étang de Thau[END_REF].

The main outlets of the Aumelas Causse karst network are the Vise submarine spring, the Issanka spring and the Vène spring which acts as an overflow outlet. The Vise submarine spring is located on the floor of the Thau lagoon at 30 m depth and constitutes the most downgradient discharge feature of the hydrological system. The western part of the Gardiole Massif is drained by the Vise spring as well as Cauvy and Ambressac springs. https://doi.org/10.5194/hess-2020-292 Preprint. Discussion started: 9 July 2020 c Author(s) 2020. CC BY 4.0 License. At the regional scale, four levels of groundwater circulations have been recognized by previous studies [START_REF] Aquilina | Deep water circulation, residence time, and chemistry in a karst complex[END_REF]: 1) a surficial groundwater flow component originating from the recharge areas that has a residence time <20 years; 2) an intermediate groundwater flow system from the Aumelas Causse, that circulates below the Montbazin-Gigean basin and the Gardiole Massif and discharges at points along the continental shelf (residence time of 50 years); 3) a deep regional groundwater flow system directed from the Aumelas Causse to the Villeveyrac Basin where several deep drillings (Pézenas and La Castillonne, depth of 1000 m) present hot karstic waters with low mineralization (residence time of several thousand years); 4) a very deep (>2 km) paleo-karst water circulation mixed with an ancient seawater circulation both recharging the thermal reservoir that has residence times on the order of 100,000 years.

Material and methods

Hydrogeological Monitoring

Water level, specific electrical conductivity and temperature were measured at 10 locations on an hourly basis during the last three inversac events in 2008, 2010 and 2014. The time-series data used in this study come from the "national quantitative groundwater monitoring network of the AFB/BRGM (available on the ADES website: https://ades.eaufrance.fr for P4 La Balme (location 10166X0253) and CGE Tennis (location 10166X0212.) Other observation stations (F5, F6, F8, F9 and S12) are managed by Balaruc-les-Bains and the municipality of Sète and the Syndicat Mixte du bassin de Thau (El Cantou, F5-Issanka, Frescaly and Cauvy).

In order to compare the different inversac events, hydraulic head and electrical conductivity were expressed with respect to reference values observed before the inversac events, as discussed in the Results Section below. In Figures 3 and5) b and 5d), the x-axis corresponds to the number of hours elapsed since the beginning of the inversac event (t0). The y-axis represents the difference between the parameter (hydraulic head or electrical conductivity) measured during the inversac and the preceding respective reference values. The variable "delta H" is the difference between hydraulic head measured at time ="t" -hydraulic head observed at time= t0 (before the inversac event). Thus, a positive value corresponds to an increase compared to the baseline condition and vice versa. This relative comparison permits the evaluation of the inversac event perturbations with respect to the initial conditions prevailing in the system and result in a more precise valuation of the transient states of the system. https://doi.org/10.5194/hess-2020-292 Preprint. Discussion started: 9 July 2020 c Author(s) 2020. CC BY 4.0 License.

Geochemical and Isotopic Analysis

Geochemical data for the 1996-2000 period are from [START_REF] Aquilina | Origin, evolution and residence time of saline thermal fluids (Balaruc springs, southern France): implications for fluid transfer across the continental shelf[END_REF][START_REF] Aquilina | Deep water circulation, residence time, and chemistry in a karst complex[END_REF] and [START_REF] Ladouche | Cubizolles: Étude du pourtour est de l'étang de Thau[END_REF]. They are considered as representative of reference geochemical conditions of the karst system. Indeed, these data were collected three to seven years after the 1993 inversac, which is considered as a sufficient time for the system to have returned to its equilibrium state given the short, 21-day interval of this event.

New geochemical data (major ions and REE compositions) and isotope analyses (D/H, 18 O/ 16 O, Water sample for major and trace element analysis were filtered on-site with disposable PP syringe with a 0.22 µm Durapore membrane and stored in acid washed HDPE bottles. Aliquots for cations and trace elements were acidified to pH 2 with ultrapure HNO3 (1‰ v/v). Samples for H-and O-isotope analysis were collected in 15 mL amber glass vials capped with airtight lids. One liter samples were collected for B-and Sr-isotope analysis in pre-cleaned HDPE bottles and were later filtered in a cleanroom through a 0.22 µm Durapore membrane in a pressurized Nalgene filtration unit, with samples for Sr-isotope analysis acidified with 1 % ultrapure HNO3. Samples for tritium measurement were collected in 1 L HDPE bottles. All samples were stored at 4 °C before analysis. Chemical analyses were performed in the HydroSciences Montpellier laboratory at the University of Montpellier. Total alkalinity was measured by acid titration with 0.1N HCl. Major ion (Cl -, NO3 -, SO4 2-, Ca 2+ , Mg 2+ , Na + , and K + ) were analyzed by ion chromatography (ICS 1000 Dionex ® ). Precision error was < ± 5%.

After acidification with 1% HNO3 -, trace elements (Li, B, Sr, REE and U) were analyzed by inductively-coupled mass spectrometry using a Thermo Scientific ® iCAP Q at the AETE-ISO technical platform of the OSU OREME at the University of Montpellier. The use of an in-line Argon Gas Dilution system permits the direct injection of highly mineralized samples without prior dilution. Precision error was typically <5%. Fresh water reference material SLRS-6, and seawater reference materials CASS-6 and NASS-6 for trace metals (National Research Council, Canada) were analyzed every 20 samples to monitor analytical accuracy. Mean results are https://doi.org/10.5194/hess-2020-292 Preprint. Discussion started: 9 July 2020 c Author(s) 2020. CC BY 4.0 License. within the range of certified uncertainties. Precision error for all analyses was typically <5%.

The REE data are represented in profiles, after normalization to a reference geological material, the North American Shale Composite (NASC) for natural waters, which corresponds to an average sample of North American shales [START_REF] Taylor | The continental crust: its composition and evolution[END_REF].

For stable isotopes analysis, samples collected in 2010 were analyzed at BRGM Laboratories Sr-isotope analyses were made by thermal ionization mass spectrometry, at the BRGM Laboratories for the 2010 samples and at the Centre de Recherches Pétrographiques et Géochimiques in Nancy for the 2012 samples. Chemical separation of Sr was done using a Sr-Spec ion-exchange column that has a total blank <0.5 ng for the entire chemical separation procedure. Around 150 ng of purified Sr was loaded onto a tungsten filament and analyzed with an average internal precision of ±10.10 -6 (2σ) using a Finnigan MAT262 multiple collector thermal ionization mass spectrometer. Measured 87 Sr/ 86 Sr ratios were normalized to a 86 Sr/ 88 Sr ratio of 0.1194. The reproducibility of 87 Sr/ 86 Sr ratio measurements was tested through replicate analyses of the NBS987 standard (0.710240) for which the mean value was 0.710232 ± 22x10 -6 at the BRGM and 0.710262 ± 13x10 -6 at the CRPG.

Boron isotopic ratios were measured with a Neptune+, Thermo Electron inductively-coupled mass spectrometer (at the BRGM laboratories for samples collected in 2010 [START_REF] Guerrot | Accurate and High-Precision Determination of Boron Isotopic Ratios at Low Concentration by MC-ICP-MS (Neptune)[END_REF]

Results

Revised Conceptual Model for the Thau hydrosystem

The conceptual hydrogeological model of the Thau system at the local scale of the Balaruc-les-Bains peninsula, is summarized in Figure 2. 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.

Consistent with the recent data compilation and results acquired in the Balaruc-les-Bains area by the Dem'eaux Thau project [START_REF] Ladouche | Dem'Eaux Thau -Synthèse et valorisation préliminaire des données sur l'hydrosystème de Thau[END_REF], we propose that the Thau Thrust is likely to provide the main pathway for thermal water. The karst freshwater is found below this thermal lens as these cold waters are more dense. The diffuse intrusion of marine waters from the Mediterranean Sea into the Jurassic limestone corresponds to a saltwater wedge and is located below the karst waters which have a lower density. This saltwater wedge can nevertheless have a more complex geometry than that represented in Fig. 2, since this heterogeneous karst system is made up of multiple compartments. These contrasts in water density generate several hydraulic interfaces that change according to the different hydraulic heads resulting from continually-varying lagoon water levels, groundwater withdrawals and recharge from precipitation. Table 2 Water temperature and electrical conductivity from the exploratory drilling F13 at different depths. This inclined well (40°NW, N320°) is located near the F14 well (Fig. 1b)

This hydrogeological setting illustrated in Figure 2 was confirmed by the F13 exploratory well (Table 2), that displayed contrasting electrical conductivity and temperature over depth.

Under normal hydrologic conditions (Fig. 2a), groundwater from the karst aquifer discharges Table 3 Characteristics of the main wells and springs in the study area

Hydraulic impact of the inversac event

This section provides a detailed hydrodynamic analysis of the most recent inversac event in 2014 and then makes a comparison between the last three inversac events of 2008, 2010, and 2014.

The hydraulic impact of the inversac event of 2014 was observed at all Balaruc peninsula springs and observation wells (Fig. 3). From the first hours after the intrusion of lagoon water, a sharp increase in water level is observed for both karst and thermal wells, although the impact is variable for different locations. The hydraulic impact was most significant for thermal well https://doi.org/10.5194/hess-2020-292 Preprint. Discussion started: 9 July 2020 c Author(s) 2020. CC BY 4.0 License.

F9, for which a +2.2 m increase in hydraulic head was observed during the first four days of the inversac event. The magnitude of the hydraulic impact is similar for F5 and F6 wells, with experienced respective increases in hydraulic head of +1.8 m and +1.9 m respectively. A lower increase in hydraulic head of just +0.7 m during the first four days of the inversac event was recorded at the Cauvy spring, situated only 1 km from the Vise submarine spring. For this spring, the increase is also partly caused by the interruption of water pumping. The hydraulic disturbance gradually increases over a 3-months period for the CGE Tennis and P4 La Balme wells, reaching a maximum of 2 m at CGE and around 1 m at P4 La Balme piezometers after 1,500 h.

Two large rainfall events occurred in September and October 2014, some 3,400 h after the beginning of the inversac event. These resulted in recharge to the aquifer that caused a sharp increase in well water levels of +1.3 m at CGE Tennis and +1 m, at P4 La Balme.

The end of the inversac event occurred in October during the normal recession period of the groundwater levels across the region. The thermal wells experienced a rapid decrease of hydraulic head, which contrasted with the slow decrease of water levels in the CGE Tennis, P4

La Balme and F7-Issanka wells. After the inversac event, the water levels in CGE Tennis, P4

La Balme and Cauvy spring were observed in an intermediate position between the maximum inversac and reference levels. In addition, the analysis of the water levels in the Thau lagoon by [START_REF] Ladouche | Inversac de la source sous-marine de la Vise en[END_REF] indicates that the end of the 2014 inversac resulted from strong Tramontane winds caused an abrupt decrease in the lagoon water level in the vicinity of the Vise submarine spring. Thus, both a temporary condition of a low water level in the Thau lagoon and high waters conditions in the karst aquifer following heavy rainfall events appear to have contributed to the conclusion of the 2014 inversac event.

In the Villeveyrac Basin area, the evolution of piezometric levels during 2014 indicates that the hydrology of this region was not affected by the inversac event [START_REF] Ladouche | Inversac de la source sous-marine de la Vise en[END_REF].

The elevated piezometric level of some 70 m here compared to the Vène area (3 m) strongly suggests a compartmentalization of the karst aquifer. The hydraulic head of the karst well P4 La Balme was higher during the 2010 inversac (intermediate waters) than during the 2014 inversac (very low waters) reflecting contrasted hydrologic conditions during these periods (Fig. 5 a). However, the hydraulic head difference, with respect to the reference measured before the inversac (Fig. 5b), is higher in 2014 than in 2010. This suggests that the low water conditions magnify the hydraulic impact of inversac events. Similarly, electrical conductivity differences for the thermal wells F5 (Fig. 5c) and F9

(Fig. 5d) show a stronger response to the 2014 inversac than during both the 2008 and 2010 inversacs.

Geochemical impact of the inversac event

In addition to the hydraulic impact just described, inversac events also have a notable geochemical impact on the water bodies circulating within the karst system. As indicated in the discussion of the "Hydrogeological conceptual model" section, the salty and highly mineralized water of the lagoon mixes with thermal and karst waters to alter their quality. A multi-tracer approach was undertaken to determine the long-term hydrochemical impact of inversac events on the system that employed analysis of major ion contents, REE concentrations

Groundwater characteristics-Major ions and water types

Outside of the Balaruc Peninsula, karst groundwater from Issanka spring and Pézenas deep borehole (738 m) is of a Ca-Mg-HCO3 type, characterized by electrical conductivity (EC) values of about 500 μS cm -1 , in agreement with their karstic water type. The water from the deep borehole has a higher temperature (37.2 °C) than that of the Issanka spring (about 17.5 °C).

Within the Balaruc peninsula, karst water from the Cauvy spring is of the same water type, but displays elevated Cl contents and EC values of up to 222 mg L -1 and 1200 μS cm -1 , respectively.

In contrast, the thermal waters with temperatures that range up to 49.9°C are of a Na-Cl type with Cl concentrations of up to 7,900 mg L -1 . The Ambressac spring, located east of the Balaruc peninsula, is of mixed water type, with higher Cl concentration than at the Cauvy spring (up to 930 mg L -1 ). Some samples also have high sulfate concentrations that approach 585 mg L -1 .

The Cauvy spring and the thermal wells F5, F6, and F9 were the most impacted by the 2010 inversac event, displaying a sharp increase of conductivity as well as high contents of Cl and Na. For example, Cl concentrations in the Cauvy spring and F6 reached levels 8 to 11 times above values typical of normal flow conditions (i.e. 1,240 mg L -1 compared with 157 mg L -1 for Cauvy and 8611 mg L -1 compare with 759 mg L -1 for F6). For the Cauvy spring, which is used for potable water, this Cl content observed during the inversac event far exceeds the drinking water standard of 250 mg L -1 .

In a post-inversac context, Cl and Na concentrations observed for the Cauvy spring and F5, F6 and F9 wells in 2012 decreased significantly, with waters at these sampling points having returned to levels close to those of the 1996-2000 reference period. By contrast, Cl concentrations in wells F8 and F3 increased between 2010 and 2012, suggesting an increase in the relative contribution of the marine water component.

Karst water and saltwater mixing traced by water stable isotopes

As shown in Figure 6a This result indicates that thermal waters result from a mixing between karst and marine endmembers. However, stable isotopes information alone does not allow a determination of the origin of the marine end-member, which could be either lagoon water or modern or ancient seawater. Samples from the F8 thermal well show a slightly enriched stable isotopes signature compared to the other thermal wells, reflecting a greater contribution of the saltwater endmember in this area, as initially indicated by [START_REF] Aquilina | Origin, evolution and residence time of saline thermal fluids (Balaruc springs, southern France): implications for fluid transfer across the continental shelf[END_REF].

F6 samples from the reference period are located within the karst water domain, confirming the strong influence of karst waters in this well [START_REF] Aquilina | Origin, evolution and residence time of saline thermal fluids (Balaruc springs, southern France): implications for fluid transfer across the continental shelf[END_REF] under normal flow conditions. Results of the mixing calculations are included in the Supplement (Table S1). This calculation gives an indication of the magnitude of the karst water and saltwater component in of each water sample, but does not provide additional insight into the origin of groundwater salinity.

The calculated fractions of saltwater for the karst wells and spring are similar for the three tracers (δ 18 O, δD and Cl). However, for thermal wells with typical high Cl contents, the fractions obtained are distinct for the different tracers, indicating another source of Cl for these wells. Indeed, as indicated by [START_REF] Aquilina | Origin, evolution and residence time of saline thermal fluids (Balaruc springs, southern France): implications for fluid transfer across the continental shelf[END_REF], Cl in the thermal wells is thought to derive from both seawater and the thermal water end-member, which is itself partly constituted of old seawater.

https://doi.org/10.5194/hess-2020-292 Preprint. Discussion started: 9 July 2020 c Author(s) 2020. CC BY 4.0 License.

Inversac persistence revealed by REE

The 14 members of the lanthanide group of elements (from La to Lu), commonly termed rare earth elements (REE), constitute an effective tool to trace water origin, water/rock interaction and mixing processes [START_REF] Johannesson | Rare earth elements as geochemical tracers of regional groundwater mixing[END_REF][START_REF] Tweed | Behavior of rare earth elements in groundwater during flow and mixing in fractured rock aquifers: An example from the Dandenong Ranges, southeast Australia[END_REF][START_REF] Zhan | Characteristics of Rare Earth Elements in Groundwaters along the Flow Path in the North China Plain[END_REF], Gil et al. 2018). The REE are naturally present in natural waters at trace concentrations and their normalized abundances, represented as a distribution pattern of individual REE ordered by their atomic number, provide a means of visualizing the behaviour of the entire group of elements.

The REE profiles for seawater and groundwater from the thermal wells (S12, F8, F9, F5, F6)

and seawater are shown in Figure 7 for the pre-inversac reference and three post-inversac situations (Fig. 7). With the exception of the F6 well, all patterns are generally flat and show a small negative Ce anomaly. The F6 profile exhibits a more pronounced Ce anomaly and an enrichment in heavy REE. These characteristics are those of limestone REE profiles, inherited from the seawater REE profile and confirm the strong karst influence of F6 samples. the Mesozoic carbonate aquifer. The karst end-member is different from the thermal endmember by its lower Sr concentrations (100 µg L -1 ), suggesting different water residence times.

As shown in Fig. 8, all water samples plot within a ternary domain, confirming that the geochemistry from the Balaruc peninsula groundwater can be explained by a 3-member mixing.

The karst waters from the Issanka spring that drain the Aumelas Massif have a lower 87 Sr/ 86 Sr ratios and Sr contents. The Issanka isotopic signature defines an up-gradient end-member for the karst system, unaffected by mixings of the thermal and seawater that characterizes the Balaruc peninsula. Waters from the P4 La Balme piezometer had a very high 87 Sr/ 86 Sr ratio in 2012, almost certainly reflecting the influence of rainfall infiltration since this sample site is located at the outcrop of the aquifer and may be representative of surficial recharge water to the karst aquifer. The contribution of each end-member (saltwater, thermal water and karst water) to groundwater samples under different hydrogeological situations related to inversac events were estimated through 3-component mass balance equations following [START_REF] Faure | Principles of isotope geology[END_REF]: Results of the mixing calculations are in the Supplement (Table S2).

𝑓 𝑠𝑎𝑙𝑡 + 𝑓 𝑘𝑎𝑟𝑠𝑡 + 𝑓 𝑡ℎ𝑒𝑟𝑚 =
The Sr-isotopic composition of the Cauvy spring is quite variable during the reference interval, with samples aligned along the karst-saltwater mixing line in Figure 8. The contribution of the saltwater end-member to the spring discharge is negligible under normal flow conditions, with https://doi.org/10.5194/hess-2020-292 Preprint. Discussion started: 9 July 2020 c Author(s) 2020. CC BY 4.0 License.

the Cauvy isotopic signature very close (99%) to the karst end-member. However, the Cauvy spring is strongly impacted by the inversac phenomenon. Although he karst end-member remains dominant at 88.1 ± 0.2 %, the contribution of saltwater end-member increases to 8.1 ± 0.5 % and that of the thermal end-member reaches about 3.8 ± 0.3 % during the 2010 inversac event. The contribution of thermal waters to the Cauvy spring is not surprising, as it is located close to the old thermal spring (S12), but the large contribution of the saltwater end-member during inversac events implies a preferential groundwater flow axis between the spring and the Vise submarine spring. [START_REF] Pinault | Characterizing a coastal karst aquifer using an inverse modeling approach: The saline springs of Thau, southern France[END_REF] have shown that variations in the contribution of thermal waters may be related to the existence of a piston effect on the thermal reservoir, particularly following an intense recharge event.

Thermal well waters of the Balaruc peninsula are all aligned along the thermal-saltwater endmember mixing line in Figure 8, except for F3, F4 hotel, and F6 waters, each of which documents a significant contribution from the karst end-member (87.9 ± 0.3 %, 89.8 ± 0.2 % and 53.7 ± 0.6 %, respectively).

The evolution of the isotopic signature for the F5, F9 and F14 borehole waters shows a similar pattern. Thermal water 87 Sr/ 86 Sr ratios increase during the 2010 inversac event, but then decreases in 2012 to values intermediate between the inversac condition and normal state. These observations suggest that waters from this group of thermal wells were strongly affected by the 2010 inversac had not returned to their 1996-2000 pre-inversac Sr-isotope signature by the 2012 post-inversac situation.

The contribution of the Thau Lagoon end-member increased to 8.3 ± 1.3 % for the F9 waters during the 2010 inversac, the thermal end-member was 91.3 ± 0.6 %, the karstic contribution being negligible (0.4 ± 1.9 %). The contribution of the Thau lagoon end-member decreased to 5.7 ± 1.2 % during the 2012 context.

Reporting on water collected from the F8 borehole, [START_REF] Aquilina | Origin, evolution and residence time of saline thermal fluids (Balaruc springs, southern France): implications for fluid transfer across the continental shelf[END_REF] noted a seawater contribution to the thermal waters in this area of the Balaruc peninsula, which we have estimated to be 8.8 ± 1.3 % for the 1996-2000 pre-inversac reference period.

The strontium concentrations and 87 Sr/ 86 Sr ratios of F8 well waters were very similar in 2010 and in April and September of 2012. Although the contribution from the saltwater end-member during the 2010 inversac was greater than the reference condition (18.0 ± 1.6 % instead of 8.8 ± 1.3 %), and remained between 17.5 ± 1.6 % to 21.2 ± 1.8 % from April to September 2012.

The thermal well F8 does not seem to be impacted by a direct intrusion of lagoon water during the inversac event but instead influenced by seawater already within the hydrosystem. By contrast to the F6 well, the three end-member contributions are relatively stable for the waters of Ambressac spring. Indeed, water samples display a large variability, with a saltwater end-member contribution of between 1.1 ± 0.1 % and 2.7 ± 0.2 % and a karst contribution between 94.4 ± 0.1 % and 91.5 ± 0.1 % over the reference interval (1996)(1997)(1998)(1999)(2000). The thermal contribution ranges between 4.4 ± 0.1 % and 5.8 ± 0.2 % during normal times.

Boron Isotope Tracking of Water-rock interactions

A lack of knowledge of the baseline B-isotopic signature during normal flow conditions does not allow to discuss the origin of boron and its quantification for thermal borehole waters.

However, examination of δ 11 B variations as a function of B/Cl ratio clearly identifies water interactions with clay minerals. Indeed, boron is present in aqueous solution, in the form of boric acid B(OH)3 and borate ions B(OH) - 4. The distribution between these two species depends on pH, temperature and salinity [START_REF] Hershey | The ionization of boric acid in NaCl, Na-Ca-Cl and Na-Mg-Cl solutions at 25°C[END_REF], Hakihana 1977). For the boron isotopes, 11 B is preferentially incorporated into boric acid whereas 10 B has a greater affinity for the borate ion. During the water-rock interaction, B(OH) - 4 will be preferentially adsorbed onto clay minerals or organic matter, resulting in a decrease of water B concentration and a consequent enrichment of 11 B in residual water.

Figure 9 shows the Cl/B ratio as a function of B isotopic composition. δ 11 B of the Thau Lagoon is 39.1 ‰, in agreement with the signature of the modern seawater of 39.5 ‰; [START_REF] Aggarwal | The effect of instrumental mass bias on d11B measurements: a comparison between thermal ionisation mass spectrometry and multiple-collector ICP-MS[END_REF][START_REF] Aggarwal | Boron isotope analysis[END_REF]. By contrast, karst waters display a range of δ 11 B values There is a good alignment of the data points in Fig. 9, which illustrates the process of water/rock interaction and indicates that the B in the waters across the Balaruc peninsula is derived from clay sediments. That 11B/10B ratios are well above the seawater values, suggests that water from the Thau lagoon interacts with clay sediments within the karst system during the occasional intrusion into the carbonate aquifer. An exploration dive of the bottom of the Vise spring in April 2017 showed the absence of sediment on the walls and bottom of the spring.

The inversac phenomenon therefore seems to be accompanied by a significant intrusion of sediments (clayey marine mud, rich in organic matter). This phenomenon seems to contribute to initiate the process of boron isotope fractionation. This process could also occur within the silty-clay Miocene unit during the transfer of water from the lagoon to the Jurassic aquifer. This

Miocene formation overlies the Jurassic aquifer and is about 30 m thick at the Vise spring. The high isotopic enrichment observed for the F3 borehole waters can be explained by their high pH values (8.3) and thus a relatively higher proportion of the B(OH) - 4 species, leading to a higher adsorption on the solid phase and a higher enrichment of δ 11 B of the residual water.

Boron isotopes are used to identify the chemical modification of the water that enters the system and interacts with the clay matrix. There is therefore a modification of the water chemistry before it even enters the karst aquifer, and this modification is initiated during the saltwater intrusion.

https://doi.org/10.5194/hess-2020-292 Preprint. Discussion started: 9 July 2020 c Author(s) 2020. CC BY 4.0 License. The impact on water quality is local whereas the hydraulic impact is perceived at the regional scale

Results in section 5.1 showed that the hydraulic impact of an inversac event is instantly propagated throughout the Balaruc peninsula and the effects of such events can also reach longer distances into the karst system than previously described. The return to the equilibrium situation occurs within days after a significant recharge event and is much more rapid than the dissipation of the chemical impact of the inversac event. Indeed, the 2010 and 2014 inversac events lasted about 6 months and had a persistent chemical impact on the hydrosystem. It stayed perceptible on the quality of the water in certain compartments from 20 months to 42 months after the occasional inversac intrusions ceased.

Even though the geochemical conditions within the hydrosystem were not monitored during the 2014 inversac event, the 2018 data are similar to those for 2012, suggesting that the hydrosystem did not reach its geochemical reference conditions and was still under a state of influence by the latest inversac event.

Geochemical analysis indicates that the 2010 inversac had the greatest impact on waters from the F6 well and the Cauvy spring, thus confirming the existence of a preferential groundwater flowpath between the Vise spring and those points. peninsula. The geochemical impact is, therefore, restricted to within a 1-kilometer radius from the Vise spring (Fig. 10).

This slow geochemical recovery is best explained by the characteristic internal heterogeneity of the karst aquifer, which is typically described in terms of a triple porosity model comprising:

(i) matrix or primary porosity, (ii) fracture porosity and (iii) conduit porosity [START_REF] Palmer | Introduction[END_REF][START_REF] Martin | Exchange of matrix and conduit water with examples from the Floridan aquifer[END_REF]. Influx of inversac water into the matrix porosity of the karst aquifer, its storage, and then prolonged time of removal and return to fracture and conduit flow explains the persistent fingerprint of the saltwater intrusions in the Balaruc peninsula.

Wells F8 and F10 in the southern part of the Balaruc peninsula were not affected by the inversac events. This part of the hydrosystem corresponds to a deeper compartment in the karst system, where the top of the Jurassic aquifer is 136 m deep in the F8 well and 195 m deep in the F10

well, which appears to be less hydraulically connected to the rest of the peninsula. Previous studies postulated that an indirect impact of an inversac event might occur [START_REF] Ladouche | Caractérisation géochimique de l'aquifère hydrothermal de Balaruc-les-Bains lors d'un épisode d'inversac, in Dix-huitièmes journées techniques du Comité Français d'Hydrogéologie de l'Association Internationale des Hydrogéologues[END_REF]. However, the increasing mineralization of F8 waters over time from 20 mS cm -1 in 1996 to 26.5 mS cm -1 in 2018 (Figure S1 in the Supplement) suggests that waters from this well are not influenced by the occasional saltwater intrusions into the hydrosystem. This distinct evolution cannot be explained by pumping withdrawals as well withdrawals rates in the F8 thermal well were steady over the period of inversac monitoring. However, a decrease in the recharge rate from the karst system could result in an increase in the proportion of the thermal or marine end-member contributions. Recent monitoring of springs (Ambressac and S12) will address this question in the Dem'Eaux Thau project.

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.

The relationship between these two regions is clearly demonstrated as dewatering of a bauxite mine in the Cambelliès area (Fig. 1) triggered the 1967 inversac event.

Implications on groundwater management:

Results presented here illustrates the fragile equilibrium that exists between different groundwater bodies in the Thau hydrosystem. During low flow conditions, any modification of the exploitation regime of the system can cause an inversac event and cause deleterious longterm consequences.

Historically, the 1967 and 2010 inversac events led to the complete cessation of withdrawal from the Cauvy spring, followed by its subsequent abandonment because the quality of this major drinking water supply was strongly impacted by the inversac events.

The Issanka spring is another major source of drinking water in the area for a population of 40,000. This study recognized the conditions needed for the hydraulic impact of an inversac event to reach the Issanka well-field. Such new understanding can support water management operations by facilitating the prediction of the arrival of an inversac disturbance, using the criteria highlighted in section 6.1.

While the southern compartment of the hydrosystem has not been impacted by inversac events, the gradual increase in mineralization observed for the F8 thermal well has the potential to affect its quality over time. In order to sustain the water quality of this thermal well, further studies should investigate the specific dynamics of this karstic compartment and the underling mechanism explaining this observation. More specifically, the hydraulic connection between this deep compartment with the Sète thermal system south of the study area should be investigated.

https://doi.org/10.5194/hess-2020-292 Preprint. Discussion started: 9 July 2020 c Author(s) 2020. CC BY 4.0 License.

Finally, unlike classical saline intrusion, the inversac events mobilize sediments loaded with organic matter during saltwater intrusions from the Vise spring. This intrusion of sediments may have an additional impact on the water quality within the karstic hydrosystem as suggested by B-isotopic signatures. Future studies could explore the fate and transport of the organic matter in the karst system.

Conclusion

This study investigated the impact of occasional saltwater intrusions (inversac in French) into the karst hydrosystem of the Balaruc peninsula and provides an improved understanding of the hydrodynamics and recovery of the aquifer from saltwater intrusions. Differences were observed between the hydraulic and geochemical responses, both spatially and temporally. The karst, thermal and marine water bodies within different compartments of the karst system coexist within a very delicate equilibrium that is disturbed by the rapid intrusion of lagoon waters during inversac events. While the hydraulic impact of such saltwater intrusions is immediate and manifest over a distance of some 5 km within the hydrosystem, the geochemical impact focused within a 1 km radius around the Vise spring but temporally persistent.

Geochemical tracers showed that the hydrosystem did not reach its pre-inversac reference state 20 months and 40 months after the occurrence of inversac of 6-months duration. The slow geochemical recovery of the Balaruc peninsula karst system reflects the triple-porosity character of the karst aquifer. Additionally, preliminary results suggested that the modification of the groundwater chemistry is initiated by an interaction process with clay sediments and organic matter during the saltwater intrusion event and possibly during the subsequent transfer of seawater to the karst aquifer through the Miocene formation. This study demonstrates the potential to couple physico-chemical, hydrogeological and isotopic data to understand the complex inversac phenomenon and recognize preferential flowpaths during such events.

The results of this study support groundwater management across the Balaruc peninsula. The triggering factors of the saltwater intrusions were highlighted as well as the conditions needed for the saltwater intrusion to hydraulically impact the upgradient area, which is a major source of drinking water supply. While the water quality of some of wells across the Balaruc peninsula are strongly impacted by the inversac and retain a geochemical signature of the event, the thermal wells located in the southern portion of the hydrosystem are not affected by inversac events, suggesting hydraulic isolation from the remainder of the Balaruc peninsula.

  https://doi.org/10.5194/hess-2020-292 Preprint. Discussion started: 9 July 2020 c Author(s) 2020. CC BY 4.0 License.

Figure 1

 1 Figure 1 (a) Simplified geological map showing the main structures of the Montpellier region. The box locates the Balaruc-les-bains study area, between the Montpellier thrust and the Nîmes Fault. (b) Detailed geological map of the eastern Thau lagoon. Sample sites indicated by the yellow dots in both maps correspond to the locations of the thermal wells, karst springs, and other piezometric drilling sites. The location of the ancient bauxite mine (Cambelliès) is indicated by the dashed oval in the northwest of the study area.

  3 H, 87 Sr/ 86 Sr and d 11 B) were determined for samples collected in October 2010, April and September of 2012 and March and August of 2018, i.e. during the 2010 inversac, 17 and 22 months after the end of the 2010 inversac and 3.5 years after the end of the 2014 inversac. Temperature, pH, Dissolved Oxygen and Electrical Conductivity (Tref=25 °C) were measured in the field, using a portable pH meter, oxymeter and conductivity meter (WTW 3210).

  using a Finnigan MAT 252 mass spectrometer, whereas those collected in 2012 and 2018 were measured on an Elementar Isoprime stable isotope mass spectrometer at the LAMA laboratory of HydroSciences Montpellier at the University of Montpellier. Calibration was performed by repeated analyses of in-house standards of known isotopic composition in alternation with samples. Oxygen ( 18 O/ 16 O) and hydrogen (D/H) isotope ratio measurements are expressed in parts per thousand (i.e. ‰) in the familiar δ notation relative to the Vienna Standard Mean Ocean Water (SMOW) standard, where δ = ([Rsample/Rstandard -1] x 1000). Samples analyzed by BRGM have a precision of ±0.8 ‰ for δD values and ±0.1 ‰ for δ 18 O values, whereas those determined at the University of Montpellier have an overall precision of ± 0.6 ‰ for both δD and δ 18 O values. Tritium analyses were performed at Hydrogeology laboratory at the Avignon and Pays du Vaucluse University.

  and at the AETE-ISO technical platform of the OSU OREME at University of Montpellier for the other samples. The average value determined for the NIST SRM 951 standard was 4.67 ± 0.0033. https://doi.org/10.5194/hess-2020-292 Preprint. Discussion started: 9 July 2020 c Author(s) 2020. CC BY 4.0 License.

  into the Vise spring, creating a plume of freshwater in the lagoon. During an occasional episode https://doi.org/10.5194/hess-2020-292 Preprint. Discussion started: 9 July 2020 c Author(s) 2020. CC BY 4.0 License. of saltwater intrusion (or "inversac" event), the natural flow is reversed and saltwater from the Thau lagoon flows into the Vise spring and enters the karstic aquifer via the Miocene cover (Fig.2b). The equilibrium between the different water bodies inside the Jurassic reservoir is then strongly disrupted and the outflow of the karstic system through the Vise spring is prevented. The characteristics of the thermal and karstic wells and spring in the Balaruc-les-Bains peninsula are shown in Table3

Figure 2 :

 2 Figure 2: Conceptual cross-section oriented NW-SE passing through thermal wells of the Balaruc peninsula, and showing groundwater flows of the karst hydrosystem under (a) normal conditions and (b) inversac conditions.Location of the cross-section is indicated in Figure1a.

  https://doi.org/10.5194/hess-2020-292 Preprint. Discussion started: 9 July 2020 c Author(s) 2020. CC BY 4.0 License.

Figure 3

 3 Figure 3 Evolution of variations in hydraulic head (m) of wells and springs across the study area during the 2014 inversac event. The rainfall record during this time is shown at the top of the figure.

Figure 4

 4 Figure4illustrates the dynamics of the hydraulic disturbance to the karst system caused by the 2014 inversac event across the Balaruc-les-Bains peninsula. This detailed analysis demonstrates the hydrologic connection between the Vise spring and the Issanka area. This perturbation propagates rapidly and reaches a distance of 3 km upgradient in less than 24 h (i.e. a velocity of 125 m h -1 ). More specifically, the inversac causes an increase in hydraulic head of +1.5 m throughout the peninsula and +0.5 m to the north of the peninsula (Frescaly, CGE Tennis) in the first 24 h. This hydraulic perturbation then reaches the Issanka area in less than 72 h (i.e. a velocity of 69m h -1 ) and causes a +0.4 m increase in hydraulic head within 10 days.In addition, Fig.4clearly shows that the hydraulic perturbation follows a preferential pathway with a NNE orientation. A second minor flowpath follows a ENE orientation from the Vise spring towards Cauvy and Ambressac springs.

Figure 4

 4 Figure 4 Spatial propagation of the hydraulic perturbation (hydraulic head increase in m) across the Balaruc-les-Bains peninsula during the 2014 inversac event at three times after initiation: T+24 h, T+72 h, and T+216 h.

Figure 5

 5 Figure 5 (a) Hydraulic head (m) and (b) hydraulic head variation (m) of the karst well P4 La Balme during the 2010 and 2014 inversac events; electrical conductivity variation (mS cm -1 ) for the thermal wells (c) F5 and (d) F9 during the 2008, 2010 and 2014 inversac events.

  , and H-, O-, Sr-and B-isotopic compositions. The geochemical analyses reflect contrasting conditions: the https://doi.org/10.5194/hess-2020-292 Preprint. Discussion started: 9 July 2020 c Author(s) 2020. CC BY 4.0 License. 1996-2000 reference interval, the 2010 inversac event, and the 2012 and 2018 post-inversac period.

  , different waters present in the Balaruc peninsula hydrosystem are characterized by distinct δD and δ 18 O values. Normal waters within the karst system lie close to the Local Meteoric Water Line (δD = 8 δ 18 O+14; Ladouche et al., 1998), illustrating their https://doi.org/10.5194/hess-2020-292 Preprint. Discussion started: 9 July 2020 c Author(s) 2020. CC BY 4.0 License. meteoric origin. Waters sampled from thermal wells are more enriched in D and 18 O than the karst waters and are distributed along a karst-saltwater (i.e. lagoon or seawater) mixing line.

H

  -and O-isotopic signatures of groundwater from F5, F9, F8 and F6 thermal wells are shifted towards the saltwater end-member during the 2010 inversac. Then, in 2012 and 2018, their δD and δ 18 O values decreased, but still remain in an intermediate position between the inversac maximum and the reference(1996)(1997)(1998)(1999)(2000) period minimum. This result raises the question of the persistence of the chemical impact in the aquifer system following occasional saltwater intrusions.Thermal wells F14, F5 and the thermal spring S12 show similar mixing proportions for the 2012 and 2018 post-inversac times. For F5 and S12, these mixing proportions are intermediate between that of the 2010 inversac event and the 1996-2000 reference time. Groundwater samples of F3 well do not follow the same evolution, as the shift in its δ 18 O value does not appear inversac related (Fig.6b).https://doi.org/10.5194/hess-2020-292 Preprint. Discussion started: 9 July 2020 c Author(s) 2020. CC BY 4.0 License.

Figure 6 H

 6 Figure 6 H-and O-isotopic compositions, expressed as δD and δ 18 O values, of (a) karst waters, thermal waters, F8 well water, seawater, and Thau lagoon water, showing the Global Meteoric Water Line (GMWL) after Craig (1961) and the Local Meteoric Water Line (Ladouche et al., 1998 ) for reference; (b) expanded view of the thermal well water array from (a) during reference, inversac, and post-inversac conditions

  https://doi.org/10.5194/hess-2020-292 Preprint. Discussion started: 9 July 2020 c Author(s) 2020. CC BY 4.0 License.

Figure 7

 7 Figure 7 REE profiles for thermal wells at different times: the 1998 data are for the pre-inversac reference state, whereas the 2011, 2012 and 2018 profiles are for the three post-inversac situations.

  https://doi.org/10.5194/hess-2020-292 Preprint. Discussion started: 9 July 2020 c Author(s) 2020. CC BY 4.0 License.

Figure 8

 8 Figure 8 Plot of 87 Sr/ 86 Sr ratio versus Sr concentration (as 1/Sr) for springs and wells of the Balaruc peninsula for different hydrogeological situations: pre-inversac reference (green), 2010 inversac (red) and 2012 postinversac (yellow).

  https://doi.org/10.5194/hess-2020-292 Preprint. Discussion started: 9 July 2020 c Author(s) 2020. CC BY 4.0 License.Water at the F6 borehole was the most impacted by the 2010 inversac. During this time, the contribution from the saltwater end-member sharply increased from 1.6 ± 0.1 % to 35.4 ± 1.8 % between 1996 to 2010 then decreased to 0.6 ± 0.1 % in 2012, a value below the 1996 to 2000 reference interval. Waters from the F6 borehole are representative of the upper part of the Jurassic aquifer and demonstrate a significant contribution of the karst end-member (94.1 ± 0.1 % to 91.2 ± 0.2 % in 1996 and 2012). This karst contribution remains significant during an inversac context (53.7 ± 0.6 %). Thus, waters from the F6 well illustrate the range of fluctuations of the thermal component.

  from 23.1 to 52.3 ‰, but are strongly 10 B enriched relative to seawater signature, whereas, thermal waters have δ 11 B values of up to 44.3 ‰ that are similar to or higher than the seawater signature. https://doi.org/10.5194/hess-2020-292 Preprint. Discussion started: 9 July 2020 c Author(s) 2020. CC BY 4.0 License.

Figure 9 .

 9 Figure 9. Plot of δ 11 B versus Cl/B ratio for different hydrogeological contexts: 2010 (inversac times in red), 2012 and 2018 (post-inversac times in yellow and blue)

  The physico-chemical disturbances resulting from inversac event are not observed at sampling points CGE Tennis, P4 La Balme, Issanka located in the upgradient area of the Balaruc https://doi.org/10.5194/hess-2020-292 Preprint. Discussion started: 9 July 2020 c Author(s) 2020. CC BY 4.0 License.

  https://doi.org/10.5194/hess-2020-292 Preprint. Discussion started: 9 July 2020 c Author(s) 2020. CC BY 4.0 License.

Figure 10 .

 10 Figure 10. Bloc diagram of the study area showing the spatial extent of the hydraulic and geochemical impacts.

  https://doi.org/10.5194/hess-2020-292 Preprint. Discussion started: 9 July 2020 c Author(s) 2020. CC BY 4.0 License.

Table 1 .

 1 Summary of the observed occasional saltwater intrusions (inversac events) in the karst and thermal aquifer of the Thau basin.

	Date	Duration Causal Factors		End of Episode
	2014	5 months	Very low waters. No storm surge in	Temporary low water level in the
	(May-October)		the	lagoon.	Groundwater	lagoon combined with recharge
			withdrawals		in the karst aquifer
	2010	7 months	Intermediate waters		Heavy rainfall
	(June-December)
	2008	3 months	Low waters and a pump test on	-
	(January-March)		thermal well F14
	1993	24 days	Low waters. Human intervention	Heavy rainfall
	(October)		on the submarine spring and
			pumping test on the Balaruc
			peninsula
	1984-1987	20 months Low waters. Lagoon with high	Heavy rainfall
	(May-February)		water level. Human intervention on
			the griffon of the submarine spring
	1967 (October) -	17/18	Dry conditions. Bauxite mining at	Cessation	of	groundwater
	1969 (March)	months	the Cambelliès site. Large	pumping
			pumping (up to 1000m 3 h -1 ), that
			lowered the water level to -20 m asl

Linear depth (m) EC (mS cm -1 ) Temperature (°C)

	85	16.5	25
	145	1	23
	223	1	37
	237	1	37
	249	2	37
	297	3.2	37
	389	35	37

Data availability

The geochemical and isotopic data are accessible at https://doi.org/10.5281/zenodo.3893897

Supplement link

The supplement related to this article is available online at 10. Authors contribution BL, JLS and CL designed the study and data collection plan. MAP, JLS and BL conducted the field work, collected the hydrogeological and geochemical data, and performed the data analysis. MAP prepared the paper with contributions from RH and JLS. All authors reviewed and edited the manuscript.

Competing interests

The authors declare that they have no conflict of interest

Acknowledgements


Karine Molvinger

Rose-Marie Ayral

Jean-Sébastien Filhol

Integrating Lecture and Laboratory Work for a Materials Chemistry Course to Engage and Motivate Students through Highly Visual and Intriguing Syntheses

Keywords: First-Year Undergraduate/General, Second-Year Undergraduate, Hands-On Learning

An original integrated course called "Chemistry: Magical Science" at Montpellier University (France) is presented. This approach mixes tutorials and practical work by using the pretext of high-tech materials synthesis to teach students knowledge, skills, and practical techniques. It also educates students about chemical risks, bibliographic research and trains them for oral presentations. The practical work is partly directed and partly autonomous. This work involves a description of this course and its evaluation.

INTRODUCTION

Some of the objectives of science education at university level is to impart a global knowledge of science and technology, and also to train future engineers and scientists. For this, science teaching at university is usually composed of theoretical courses and practical laboratory work. At undergraduate level, a classical course organization alternates lectures and practical work throughout the semesters.

Unfortunately, these two types of teaching are often quite distinct, with evaluation of specific skills, different teachers and a chronology (the sequence of lectures and practical work) that is not necessarily optimized, for reasons of organization. Furthermore, when students are actively involved in the construction of their knowledge base, they tend to learn better: [START_REF] Bretz | What Faculty Interviews Reveal about Meaningful Learning in the Undergraduate Chemistry Laboratory[END_REF] in order for a student to invest time learning chemistry in a meaningful way, knowledge cannot simply be passed verbally from teacher to student. Therefore, students should be active participants in the learning process so that they might construct their knowledge of a subject through an interacting combination of theory and practice.

This is particularly so in the case of chemistry, which is an experimental science, since laboratory work plays a key role in the chemist training. Nevertheless, recent researches in the teaching of experimental chemistry in higher education remain rare: Obenland et al. [START_REF] Obenland | A General Chemistry Laboratory Course Designed for Student Discussion[END_REF] reviewed the history of lab teaching and showed that it is still quite controversial. [START_REF] Hawkes | Chemistry is not a Laboratory Science[END_REF][START_REF] Domin | A Review of Laboratory Instruction Styles[END_REF] Some have argued that experimental work should be removed. Obenland et al. point out that when taught as a traditional laboratory course, experiments do not encourage students to deepen their understanding of chemical concepts. They propose that students think about chemistry by discussing data, and gaining an understanding of concepts rather than by following protocols. [START_REF] Obenland | A General Chemistry Laboratory Course Designed for Student Discussion[END_REF] For many university professors, teaching chemistry without practical work is not conceivable since the laboratory is by definition the place where students are confronted with the reality of the chemical world and make connections with models and theories. [START_REF] Hofstein | The Laboratory in Science Education: Foundations for the Twentyfirst Century[END_REF][START_REF] Reid | The Role of Laboratory Work in University Chemistry[END_REF][START_REF] Tiberghien | An Analysis of Labwork Tasks Used in Science Teaching at Upper Secondary School and University Levels in Several European Countries[END_REF] Hofstein et al. point out that the studies conducted so far have not highlighted simple relationships between experimental practice and student learning. [START_REF] Hofstein | The Laboratory in Science Education: the State of the Art[END_REF] However, a recent study in Australia and the United Kingdom shows that there is a general consensus on the need to develop transferable practical skills while transmitting theoretical knowledge. [START_REF] George-Williams | What do you Think the Aims of Doing a Practical Chemistry Course are?' A Comparison of the Views of Students and Teaching Staff across three Universities[END_REF] The underlying assumption in each of these studies is that students will be better at learning chemistry [START_REF] Domin | A Review of Laboratory Instruction Styles[END_REF] only if they perceive the subject in a positive way and come to understand and internalize knowledge.

Furthermore, many authors have pointed out the increased enthusiasm of the students that are interested and motivated by experimental courses. For example, Furlan et al. [START_REF] Furlan | Removal of aromatic pollutant surrogate from water by recyclable magnetite-activated carbon nanocomposite: an experiment for general chemistry[END_REF] show that nanomaterials based experiments "allowed students to gain exposure to the modern field of nanoscience and technology and to gain direct working experience with functional nanocomposite materials". Porter 11 shows an example of courses on nanotechnology where students participate in lecture and guided discussion, in writing and laboratory exercises. Other examples of courses based on materials science are described in the literature. [START_REF] Widstrand | Designing and reporting experiments in chemistry classes using examples from materials science: illustrations of the process and communication of scientific research[END_REF] Since more than 40 years, [START_REF] Miller | Survey of long-term integrated laboratory use in undergraduate chemistry programs[END_REF] different approaches including the term "integrated" have been undertaken in order to improve undergraduate chemistry education. This term can have different meanings such as laboratory course in the same physical space [START_REF] Budner | Project-Based Integrated Lecture and Laboratory Quantitative Analysis Course[END_REF] or a program of study combining sub-disciplines of chemistry into a single laboratory sequence. [START_REF] Monga | Implementation of a Student-Customized Integrated Upper-Level Chemistry Laboratory Course[END_REF] For the first example, we can notice the work of Budner et al. [START_REF] Budner | Project-Based Integrated Lecture and Laboratory Quantitative Analysis Course[END_REF] . Their approach includes moving away from a traditional lecture/lab separation, into an integrated lecture/lab course model incorporating problem-based learning. They offer laboratory works on general chemistry concepts with a reduced guidance after a few courses.

According to the authors, students are more engaged, appreciate the combined lecture and lab but unfortunately no "dramatic increase in assessments scores" is observed. The second approach can be illustrated by the recent article of Monga et al. [START_REF] Monga | Implementation of a Student-Customized Integrated Upper-Level Chemistry Laboratory Course[END_REF] . They show the implementation of an integrated The synthesis of materials with "magic"-like properties is thus used as a bait. This awakens desire to make these materials and invokes the students' curiosity about the physical reasons for these "amazing" properties. The teaching of "Chemistry: Magical Science" tries to push students towards the research world: the ultimate goal is to encourage interest in scientific research. However, CMS uses practical work as a tool for creating knowledge. The syntheses are thus the starting point for a completely integrated course for students, which allows them to quickly gain autonomy in developing valuable skills. This type of approach has been encountered in the literature in other fields of chemistry. [START_REF] Ashraf | An integrated Professional and Transferable Skills Course for Undergraduate Chemistry Students[END_REF][START_REF] Hubbard | Student-Generated Digital Tutorials in an Introductory Organic Chemistry Course[END_REF] Therefore, in this article, we present the course and its mode of evaluation. These two points can be formulated as follows:

1) How does one devise an integrated teaching course based on a practical laboratory approach?

2) Is this integrated approach more effective qualitatively (for example, students' perceptions), but also quantitatively, in the transmission of knowledge?

CHEMISTRY: MAGICAL SCIENCE

Course overview

We represent this module by the scheme in Figure 1 (More information in the Supporting Information). each laboratory session), and the criteria for evaluation (bibliography report, laboratory notebooks and a presentation of their laboratory work called "Genius") are given. The materials to be synthesized during the practical sessions are presented (superconductors, phosphorescents, opals, ferrofluids ...)

as well as the advanced synthesis methods (sol-gel/microwave/high temperature/biphasic polymerization syntheses).

The second session (week 2) is about laboratory safety. The two-hour class focuses on the behavior that a student in a laboratory should adopt (personal protection, compliance with safety rules, etc.), the risks associated with the use of chemicals, good practice in waste disposal and also how to react in the event of an accident.

The third session (week 3) is devoted to bibliographic research. The databases and their use are presented to them. A tutorial session (week 4) allows them to query the general (google…) and scientific (web of science….) databases about one of the topics covered in the practical work (superconductor, ferrofluid ...). It will then be evaluated.

Then five laboratory sessions (3 hours each, weeks 5-9) follow including the synthesis of ferrofluids, superconductors, phosphorescents, rubies and opals. Part of the protocols materials have been published in the Journal of Chemical Education. [START_REF] Dalverny | Synthesizing and Playing with Magnetic Nanoparticles: A Comprehensive Approach to Amazing Magnetic Materials[END_REF][START_REF] Filhol | Microwave Synthesis of a Long-Lasting Phosphor[END_REF][START_REF] Leyral | Microwave Synthesis of a Fluorescent Ruby Powder[END_REF] The synthesis of magnetite nanoparticles is done using a mix of 1/3 Fe(II) and 2/3 Fe(III) with a dropwise addition of an NH3 solution. The magnetic nanoparticles are then washed, followed by a sequence of magnetic decantation and elimination of the nonmagnetic phase. The ferrofluid is obtained by magnetic decantation and drying followed by addition of a few drops of a solution of tetrabutylammonium hydroxide as a dispersant. The ferrogel is obtained by dispersion of the nanoparticles in a PVA solution that is jellified using borax. Magnetic properties are tested using a strong magnet.

Superconductor

A microwave-assisted sol-gel synthesis of YBa2CuO3O7-y is performed. The precursors used are nitrates Y(NO3)3, 5H2O, Ba(NO3)2, and Cu(NO3)2,3H2O. After addition of complexing anions, the modification of the pH by addition of ammonia allows a shift in the hydrolysis and condensation balances. The phase YBa2Cu3O7-x is obtained by drying and then annealing the gel at high temperature. The superconductivity of the pellet is checked by small magnet levitation at liquid nitrogen temperature.

Phosphorescent b The material is synthesized from a solution of the different metal nitrates (europium, dysprosium, aluminium, and strontium) in a small volume of deionized water. Then, urea is added to the solution and stirred until the urea is completely dissolved.

The powder is crushed and washed using the Büchner funnel and the sidearm flask under vacuum with both distilled water and ethanol. The solid is dried at 100 °C in a drying oven.

The resulting white powder is tested for fluorescence and phosphorescence under UV light.

A strong green fluorescence and long-lived green phosphorescence are observed.

The synthesis was adapted to produce CaAl2O4:Eu:Dy, by replacing the strontium nitrate by calcium nitrate tetrahydrate, Ca(NO3)2⋅4H2O. The doped aluminate displays a blue fluorescence and blue phosphorescence.

Rubies c

Rubies are synthesized by microwave-assisted, self-propagating combustion of a gel formed from aluminium and chromium nitrate with urea and treated similarly to the phosphorescent synthesis. The formed nano-rubies can be checked by observation of a strong red fluorescence upon exposure to UV light (380 nm). Rubies can then be grown to macroscopic size up to 0.5 carats by flux synthesis.

Opals

A synthesis of nanoparticles of defined size is carried out by two-phase polymerization. For this purpose, temperature and agitation conditions are very important.

Opals are obtained by polymerization reaction of methyl methacrylate MMA in an inert atmosphere, initiated by a radical initiator (2,2-azobis(2-methylpropionamide)). After centrifugation, the solid obtained is iridescent. The nano size can be checked directly by observation with a SEM. A specific session called "Genius" is then carried out (week 10). Before this session, students are asked to choose between several possible themes (e.g. macroscopic sapphire synthesis, superhydrophobic synthesis, silver nanoparticles synthesis…). They prepare a protocol of synthesis in their own time by doing bibliographic research and taking into account chemical risks and by checking the availability of chemicals and the equipment needed with the teacher or laboratory technicians. Classic examples include the preparation of magnetic nanocomposites [START_REF] Furlan | Removal of aromatic pollutant surrogate from water by recyclable magnetite-activated carbon nanocomposite: an experiment for general chemistry[END_REF][START_REF] Berger | Preparation and properties of an aqueous ferrofluid[END_REF] , nickel nanowires [START_REF] Bentley | Template synthesis and magnetic manipulation of nickel nanowires[END_REF] , gold and silver [START_REF] Furlan | Preparing and testing a magnetic antimicrobial silver nanocomposite for water disinfection to gain experience at the nanochemistry-microbiology interface[END_REF] nanoparticles, and quantum dots [START_REF] Winkelmann | Preparation of CdS nanoparticles by first-year undergraduates[END_REF][START_REF] Boatman | A safer, easier, faster synthesis for CdSe quantum dot nanocrystals[END_REF] . All of these syntheses can be adapted to practical work for the Genius session. The students remain in contact with the different teachers that orientate the searched and validate the protocols (in particular to prevent safety issues).

The "Genius session" lasts 3 hours and the proposed synthesis is conducted.

Finally, an oral preparation session is proposed (week 11). The instructions for their oral presentation are given to them. They present their "Genius" in groups of three or four and the speech must be balanced between each student whether it is for the presentation or for the questions. The structure to make an oral presentation is provided (objective, methods, results, conclusion without

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forgetting to quote bibliographical references). Instructions on the slides (layout, animation, colors, text ...) as well as their speech (voice, gestures, respect the time, capture the attention ...) are given to them. The evaluation grid is presented to them and instructions on the dress code are also given (suits, ties and formal clothes welcome). A detailed description of good practices in oral presentations is done in direct link with future job interviews.

Students are evaluated on a final presentation of their "Genius" session, their laboratory notebook and the bibliography report. For the final presentation (week 12), the jury is mainly composed of people external to the CMS teaching team. The examination consists of a 15-minute presentation followed by a 15-minute discussion with the jury. This is a first experience of the presentation of scientific results (in the same manner as a Master's or PhD thesis defence or at conferences).

Methodology

The "Chemistry: magical science" class is open to all first and second year students at the University of Montpellier. Nevertheless, each class has a limited size and only the first 22-28 students that apply on the local online application system are selected on a first come first serve basis. The total number of applications ranges between 100 and 200 each semester. The class is not specifically advertised to the students, as it is already in high demand, leaving those not selected frustrated. To ensure that our following statistical results are not biased (the selection process may suggest that the best students register first), we checked their scores before they registered for the course.

For first-year students, not all the students enrolled are chemists, half of them are in mathematical, physical or geological fields. They are not specialists in chemistry: "Chemistry: Magical

Science" module is open to all scientific students and not only to chemists. For the other half of firstyear students, their first semester mark (before CMS) is slightly higher than the other students in chemistry (average of 10.5/20 compared to 9/20 for the whole). Overall, first-year students do not represent a group of students with better results in chemistry.

For the second-year students, the total number of chemistry students has an average of 8.7/20, those who have followed the "Chemistry: Magical Science" course have an average of 8.9/20, so roughly equivalent. The data show that we have a reasonable random sampling of students.

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All courses and laboratory sessions were filmed. During the safety and bibliography courses, the camera focused on the teacher and the slides. For laboratory sessions, the camera focused on the teacher when he explained certain practical and theoretical points. The rest of the time of the laboratory work, it was focused on different groups of students when they were manipulating. At the end of the laboratory work, we were also able to ask them about their feelings.

For this manuscript, two kinds of student populations have been studied through surveys:

-Firstly, two groups of students from the "Chemistry: magical science" course itself: 22 first year students and 23 second year students (this includes a survey of student perception of the course):

All of these students select this course for its attractive, interesting side, to have the opportunity to learn the synthesis of original materials, to learn experimental techniques and to acquire knowledge.

In terms of the surveys, the students are solicited just after having realized the series of practical sessions as part of the "Chemistry: magical science" course. Their feelings about this practical work are therefore quite significant.

-Secondly, a survey is given to third year students in chemistry (147 people) at the beginning of the academic year, some of them have followed "Chemistry: magical science" in previous years (numbering 78 -the CMS team) and some have not (numbering 69 -the control team). It is important to note that all of the students have followed the same chemistry degree courses during the previous two years. All of the courses are mandatory, except for a few optional modules such as CMS. However, only the "Chemistry: magical science" covers the concepts of bibliography, safety in chemistry and laboratory work. Therefore, the only difference between the two samples is the follow-up of this option by the students. Thus, this survey allows us to evaluate the effects of the module in terms of knowledge in security, bibliography, and motivation. In addition, at the end of their third year, these students carry out a tutored project. We also looked at the influence of CMS on these projects.

RESULTS AND DISCUSSION

Analysis of the survey of feelings

During these different sequences of practical work, the authors (KM and RMA) served as observers and in no case participated in the teaching of the various subjects, which was managed by JSF.

However, during the sessions, discussion with students was possible. Students could express their opinions on the course. The majority of the students found the class useful and believed that it taught them new and valuable skills that they felt would help them in their chemistry education. [START_REF] Ashraf | An integrated Professional and Transferable Skills Course for Undergraduate Chemistry Students[END_REF] Then, to assess the course from the point of view of the students, a small survey was carried out at the end of the module. Students were asked about their experience of "Chemistry: Magical Science" laboratory sessions.

The results are given in Table 2, where the answers from both populations (first year + second year) for their perception of the "Chemistry: Magical Science" class are shown. The students' reactions to practical work are often positive. Firstly, they seem to appreciate "Chemistry: Magical Science". 42 out of 45 students like this module. They feel autonomous (37 out of 45), and free (38 out of 45). They seem comfortable during practical work (36 out of 45). Students dare to ask the teachers questions (40 out of 45). They have a good understanding (37 out of 45). For 34 out of 45 students, the practical work does not take too much time.

At the end of the laboratory work, we asked the students how they experienced this laboratory work. These comments are consistent with their answers to the questionnaire. After transcribing their words, we see that the students feel involved in this module, which is much more interesting for them ("These practical works are more interesting and more understandable than the ones we did last year.").

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Firstly, their interest stems from the opportunity to synthesize materials that have remarkable properties and have applications in everyday life ("The materials are impressive, especially when we see our superconductor levitating"). Secondly, the organization of the module allows them to learn many more things: the course on safety followed by practical work allows them to be more aware of the risks in situation during the lab, and the course on bibliography allows them to become autonomous in their research for preparing the Genius session.

Analysis of the survey of third year students

Through this survey, we examined different points (motivation (Table 3), bibliography (Table 4), and security/safety (Table 5)) that can be improved by this way of teaching. The results of this study compare two populations (among third-year students), one of which followed the CMS module in previous years (78 people -the CMS group) and the other populated by those who did not (69 people, the control group). This is the only difference between the two populations.

 Regarding motivation:

Questions about their future are asked to find out if there is a difference between the two populations in terms of their motivation to continue studying. In other words, are students who have followed CMS more motivated to study for a long time? even if more CMS students are planning to do a PhD. The authors hypothesize that it may be still too early for an undergraduate student to project himself so far into the future.

 Regarding the bibliographic skills:

Here we checked whether the courses on bibliography taken by the CMS students are effective. We examined whether the students (whose mother tongue is French) are comfortable with bibliographic research and with reading articles in English. a N = 78. b N = 69. c The list of terms included words such as "furnace", "beaker", "heating rate", "safety glasses", etc.

The first question was to determine if students know the names of the databases, and the second one to see if they know the names of chemical journals. A third question asked if they can understand technical English words found in articles, such as furnace, beaker, heating rate, safety glasses etc. The students who have completed the module know of more search portals for bibliography (  ) and they know more scientific journals (  ).

They are also able to translate more words from English to French (  ). All of these results concerning the skills related to bibliography are statistically significant. The questions asked here indicate whether the information given on safety is profitable to CMS students. Questions about the meanings of pictograms in chemistry and safety rules were asked.

N = 78. b N = 69.
Knowledge of pictograms is a little better for the "Chemistry: magical science" group (but the results are not statistically significant,   ). Students who have taken the module react better to an accident (  ). These results show that the skills gained from studying this module seems to be effective and more deeply ingrained, even one or two years after the end of this training. The students who followed it are better able to answer questions related to bibliography and safety in chemistry than those who did not follow it.

Furthermore, at the end of their third university year, the students are involved in a tutored project where they must do a mini research project (including bibliographic research and experimental work). The grades for this project work for the students who have taken the CMS module seem better than for those who have not, whether one considers the average for the We hope that this kind of teaching can be more widely used, so that more students can have access to it in the future.

ASSOCIATED CONTENT ASSOCIATED CONTENT

Supporting Information

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.XXXXXXX. [ACS will fill this in.] Organization of the module. Protocols of the syntheses of the materials (ferrofluids, phosphorescent and ruby) (docx).

Figure 1 .

 1 Figure 1. Scheme of the organization of the module of "Chemistry Magical Science"

a

  See ref 19. b See ref 20. c See ref 21.



  Regarding safety / security:

16 CONCLUSION

 16 bibliographic research (average value 14.5/20 for the CMS versus 13/20 for the others), or the practical part (average value 15.5/20 for the CMS versus 14/20 for the others). According to the teachers, CMS students are more comfortable with this project as they have already carried out this type of work before, albeit on a smaller scale. This paper describes uses of an integrated learning environment based on a laboratory approach for a large variety of skills related to general chemistry course sequence. In this teaching course, experimentation, collaborative student involvement, lecture, technology and individual attention can be maximized for more effective learning.Students receive a solid background in bibliographic skills and laboratory safety. The syntheses realized during the practical work lead to original materials, which have remarkable properties. Students also learn to write, to communicate their results both in writing and orally.Moreover, training for oral presentations is given in the module. The analyses of two different surveys point out the effect of this integrated module in terms of knowledge in laboratory safety, bibliography and in terms of positive feelings towards the CMS module. The results obtained from the analysis of surveys show that the population who have completed the module of "Chemistry: magical science" is more knowledgeable about these topics and thus more motivated towards future studies in the field of chemistry.

Table 1

 1

contains a brief description of the synthesis of each of the materials (See the Supporting Information also). These laboratory sessions were chosen to illustrate key concepts, to see things for real, to search for the relevant synthesis protocol in the literature, to introduce new equipment or methodologies to the students, to train them in specific practical skills, to manage safety issues, to teach experimental design and finally, to develop collaborative, logical reasoning and interpretative skills.

[START_REF] Reid | The Role of Laboratory Work in University Chemistry[END_REF]

Table 1 . Brief Methods for the Synthesis of the Materials in the Course Chemistry: Magical Science

 1

	Materials	Brief Description of the Synthesis
	Ferrofluids/
	Ferrogel a

Table 2 . Comparative Results of the First-and Second-Year Student Survey Items on Experience Perceptions

 2

	Statements for Response	Distribution of Responses by Category, N = 45
		Strongly	Rather	Rather	Strongly
		Agree	Agree	Disagree	Disagree
		N	%	N	%	N	%	N	%
	1 I feel more autonomous during the "Chemistry: Magical	21	47 16	36	6	13	2	4
	Science" sessions than in "classical" laboratory work
	2 I'm not comfortable when I'm in "Chemistry: Magical Science"	3	7	6	13	6	13	30 67
	practical classes
	3 I dare to ask more information of the teacher on what is to be	22	49 18	40	3	7	2	4
	made during "Chemistry: Magical Science" practical classes
	4 The teacher leaves me more free during "Chemistry: Magical	22	49 16	36	5	11	2	4
	Science" practical classes
	5 I have a good understanding of what I am doing in	15	34 22	49	6	13	2	4
	"Chemistry: Magical Science" practical classes
	6 I like "Chemistry: Magical Science" practical classes	34	76	8	18	2	4	1	2
	7 "Chemistry: Magical Science" practical work takes me too	3	7	8	18	16	35	18 40
	much time

Table 3 . Comparative Results of the Third- Year Students' Survey Items on Motivation

 3 Master's degree. This result is statistically significant with a   The second question concerning a potential PhD thesis thereafter, is not statistically significant (  ,

	Statements for	Students Answering "Yes"
	Response	CMS Group a Control Group b
		N	%	N	%
	1 Would you like to	76	97.5	57	82.5
	continue your
	studies in a
	Master's degree?
	2 Do you plan to	15	19.0	11	16.0
	prepare a Ph.D.
	thesis in
	chemistry?
	a N = 78. b N = 69.
	More students who have followed the "Chemistry: magical science" module are willing to
	continue to a

Table 4 . Comparative Results of the Third- Year Students' Survey Items on Bibliographic Skills

 4

	Statements for	Students Answering Correctly
	Response	CMS Group a Control Group b
		N	%	N	%
	1 Cite different	35	45	12	17
	bibliographic
	search portals.
	2 Cite the names of	21	27	7	10
	scientific journals
	used in chemistry.
	3 Provide an English	30	38	14	20
	translation of the
	following scientific
	terms. c

Table 5 . Comparative Results of the Third- Year Students' Survey Items on Safety and Security

 5

	Statements for	Students Answering Correctly
	Response	CMS Group a Control Group b
		N	%	N	%
	1 Complete the	48	62	40	58
	pictogram table.
	2 What should you	78	100	65	94
	do if a part of the
	body comes into
	contact with a
	chemical product
	such as
	hydrochloric acid?
	a

ACKNOWLEDGMENTS

The authors thank the teaching staff who contribute to the effective running of this practical course: Laurent Bernaud, Anne-Laure Dalverny, Géraldine Leyral and Florence Rouessac.

They also thank Naseem Ramsahye for proofreading the manuscript.

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AUTHOR INFORMATION

Corresponding Author *E-mail: karine.molvinger-verger@umontpellier.fr