Mélanie Quenet
email: melanie.quenet@gmail.com

Hélène Celle-Jeanton

Olivier Voldoire

Julie Albaric

Frédéric Huneau

Jean-Luc Peiry

Elisabeth Allain

Alexandre Garreau

Beauger Aude

Aude Beauger

Coupling hydrodynamic, geochemical and isotopic approaches to evaluate oxbow connection degree to the main stream and to adjunct alluvial aquifer

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Wetlands were subject of various scientific considerations in recent decades and benefit from specific international convention especially as waterfowl habitat (Ramsar Convention, 1971). They are indeed particularly recognized to preserve the ecological function of hydrosystems (ecological niches, refuge areas…). However, during the last decades, the general intensification of pollution has led to the contamination of most of the water bodies with negative effects on aquatic ecosystems, human health, productive activities, water system reliability and operating costs for water use [START_REF] Gleick | Water in Crisis: Paths to Sustainable Water Use[END_REF][START_REF] Bates | IPCC Technical Paper VI, Climate Change and Water[END_REF][START_REF] Hulton | Global costs and benefits of drinking-water supply and sanitation interventions to reach the MDG target and universal coverage[END_REF][START_REF] Sutton | Comparison of genesis and geostatistical methods for characterizing the heterogeneity of alluvial media: Groundwater flow and transport simulations[END_REF]UNEP, 2016). In the meantime, a world loss of about 64 to 71 % of natural wetlands is to deplore since 1900 because of human activities [START_REF] Davidson | How much wetland has the world lost? Long-term and recent trends in global wetland area[END_REF]. Locally, some sectors are especially affected with for instance percentage of wetlands lost reaching 91% for the California State between the 1780's to the 1980's [START_REF] Dahl | Wetlands losses in the United States, 1780's to 1980[END_REF].

Actions have been therefore implemented in order: 1) to preserve or restore wetlands, 2) to complete the knowledge on their functioning and 3) to communicate about the necessity of maintaining such environments. These are the objectives, for instance, of the 3 rd National Action Plan for Abbreviations: GW = groundwater; RB = right bank; LB = left bank; T = water temperature Wetlands in France (PNMH, 2014) and of the United States Geological Survey (Fretwell et al. 1996).

As a special feature of wetlands, oxbows are generated by spatial and temporal dynamics of rivers within their floodplains. That kind of perifluvial environments are of a major ecological importance for the habitat and diversity of fauna and vegetation. Oxbows indeed promote reproduction and provide refuge areas for biotic communities, especially fishes [START_REF] Bornette | Ecological complexity of wetlands within a river landscape. 57 Abbreviations: GW = groundwater; RB = right bank; LB = left bank; T = water temperature[END_REF][START_REF] Ghosh | Catch per unit efforts and impacts of gears on fish abundance in an oxbow lake ecosystem in Eastern India[END_REF][START_REF] Yang | Remote Sensing of Hydrological Changes in Tian-e-Zhou Oxbow Lake, an Ungauged Area of the Yangtze River Basin[END_REF]. However, since an oxbow is a floodplain annex, it can be supplied by local precipitation, main stream's water, alluvial groundwater from the bank between the river and the oxbow and from the bank delineating the alluvial plain [START_REF] Rollet | Expertise hydro-géomorphologique en vue du diagnostic fonctionnel des habitas, de la restauration du transit sédimentaire et des lônes[END_REF]. Therefore, the whole ecosystem of oxbows depends largely on hydrodynamic conditions and more peculiarly on the mixing between water masses through hydrological connectivity to the main stream and/or the alluvial groundwater [START_REF] Dahm | Nutrient dynamics at the interface between surface waters and groundwaters[END_REF][START_REF] Amoros | Connectivity and biocomplexity in waterbodies of riverine floodplains[END_REF]. It is now widely recognized that maintaining oxbows in riparian areas contributes to control floods occurrence, to stabilize river base flows during dry periods, to enhance the water quality (by nutrient retention) and to preserve ecosystems [START_REF] Winter | Ground water and surface water; a single resource (USGS Numbered Series[END_REF][START_REF] Alard | Zones humides de la basse vallée de la Seine[END_REF][START_REF] Bullock | The role of wetlands in the hydrological cycle[END_REF] Abbreviations: GW = groundwater; RB = right bank; LB = left bank; T = water temperature [START_REF] Bullock | The role of wetlands in the hydrological cycle[END_REF][START_REF] Larocque | Role of the geomorphic setting in controlling groundwater-surface water exchanges in riverine wetlands: A case study from two southern Québec rivers (Canada)[END_REF]. However, obtain a complete hydrological knowledge about oxbows' dynamic remains challenging. Various monodisciplinary approaches implying mostly hydrodynamic but also hydrochemistry [START_REF] Carrel | La Morte du Sauget, un ancien méandre du Rhône: bilan hydrologique et biogéochimique[END_REF][START_REF] Bengen | Qualité des eaux de trois bras morts de la Garonne : variabilité spatio-temporelle = Water quality in three ancient arms of the river Garonne : spatio-temporal variability[END_REF][START_REF] Le Coz | Réponse hydraulique d'un bras mort au signal hydrologique de la rivière[END_REF][START_REF] Archie | Clustering oxbow lakes in the Upper-Tisza Region on the basis of stable isotope measurements[END_REF][START_REF] Hudson | Hydrologic connectivity of oxbow lakes along the lower Guadalupe River, Texas: The influence of geomorphic and climatic controls on the "flood pulse concept[END_REF] have been tested separatly.

However, such focused investigations can lead to neglect or underestimate contributions of connected water masses to the whole functiunning of the oxbow which may produce unaccurate conceptual models to water managers.

Multidisciplinary approach carried out on a representative and spatially distributed sampling network (boreholes and surface water points), covering both the selected wetland and the connected water masses of the whole hydrosystem, constitute an integrated solution to insure the most complete hydrological understanding of the system and therefore contribute effectively to wetlands and rivers preservation actions (restoration, rehabilitation, remeandering…).

The present study is carried out on the Auzon Oxbow, one of the fluvial annexes of the Allier River (Massif Central, France, Figure 1a), located in the upper Allier River basin.

Ecological concerns surround that specific oxbow especially since it constitutes trout's reproduction and refuge areas. Abbreviations: GW = groundwater; RB = right bank; LB = left bank; T = water temperature Consequently, local fishing associations carried out conservation operation as the dredging of alluvial plugs to maintain its connectivity to the main stream [START_REF] Beauger | Distribution of Diatom, Macrophyte and Benthic Macroinvertebrate Communities Related to Spatial and Environmental Characteristics: The Example of a Cut-Off Meander of the River Allier (France)[END_REF]. As an experimental site of the SOAHAL Observatory (Système d'Observation d'une Annexe Hydraulique de l'ALlier), a complete monitoring of the hydrosystem, including surface and groundwater with a dense spatial resolution (20 sampling sites for 0.4 km²) has been organized from 2014. The objectives are to provide effective diagnostic tools to evaluate the connection degree of the Auzon Oxbow to the Allier River, the main stream, and to the adjunct alluvial aquifer and thus to establish a complete hydrodynamic description of the Auzon Oxbow hydrosystem. To achieve that goal, the strategy was to combine individual investigation approaches through a multidisciplinary analysis on both hydrodynamic and hydrochemistry.

Study area

General settings

The study was performed on the Auzon Oxbow [START_REF] Mohammed | Isotopic and geochemical identification of main groundwater supply sources to an alluvial aquifer, the Allier River Valley (France)[END_REF]. Abbreviations: GW = groundwater; RB = right bank; LB = left bank; T = water temperature Geologically, the drainage basin of the Allier River is composed of 80% by crystalline formations (Hercynian crystalline bedrock and Cenozoic volcanic rocks), the remaining part of the watershed is made of calcareous Oligocene fluvio-lacustrine sediments [START_REF] Korobova | Element partitioning in sediment, soil and vegetation in an alluvial terrace chronosequence, Limagne rift valley, France: A landscape geochemical study[END_REF]. The study area is located in the small subsidence basin of Brioude. That tectonic depression constitutes the western end of an E-W direction extension grabens network due to the tectonic phase in distention which took place in Western Europe during the Cenozoic period [START_REF] Vanderhaeghe | Saint-Germain-Lembron[END_REF]. The Brioude basement is composed of a Hercynian metamorphic and magmatic bedrock overlaid by the sedimentary formation of Limagne: during the Oligocene and Miocene periods, detrital sedimentation in continental environment (alternation of clays, sands and marls with some carbonates levels) filled the basin. Since Quaternary, the alluvial deposits of the Allier River are completing the sequence on the underlying Oligocene marls [START_REF] Lasnier | Brioude[END_REF][START_REF] Korobova | Element partitioning in sediment, soil and vegetation in an alluvial terrace chronosequence, Limagne rift valley, France: A landscape geochemical study[END_REF]; they extend over a width of between 0.5 and 2 km and a thickness of about 10 m at the Auzon Oxbow site (alluvial plain of Figure 1b). Due to their good hydrodynamic properties (Transmissivity = 10 -3 à 10 -4 m²/s, Storage coefficient = 8 to 10%) and hydrochemical characteristics (moderate concentration, neutral pH and calcium-bicarbonate water-Abbreviations: GW = groundwater; RB = right bank; LB = left bank; T = water temperature type) [START_REF] Mohammed | Investigating the behavior of alluvial systems, thanks to the classical, isotopic and emerging tracers : case study of the alluvial aquifer of the Allier River[END_REF][START_REF] Mohammed | Isotopic and geochemical identification of main groundwater supply sources to an alluvial aquifer, the Allier River Valley (France)[END_REF], the alluvial aquifer of the Allier River is one of the major water resources of the area. This aquifer is therefore intensively exploited for drinking water supply and agricultural purposes as it is the case on the study site.

Figure 1

Thanks to its geomorphology and location, Auzon Oxbow is considered as an active floodplain oxbow [START_REF] Archie | Clustering oxbow lakes in the Upper-Tisza Region on the basis of stable isotope measurements[END_REF].

It was formed by the migration of an old channel of the Allier River, which occurred during two flood events in 1988 and 1989. A former gravel pit located in the point bar has facilitated the cutoff process leading to the main channel abandon. Upper part of the channel was rapidly in-filled by sands and gravels; since then, only the downstream end remains connected to the main stream [START_REF] Beauger | Impact de la capture d'un chenal fluviatile par une ancienne gravière sur la distribution des macroinvertébrés benthiques[END_REF] conferring to the Auzon Oxbow the first stage of development of oxbows [START_REF] Fontes | The neck cutoff oxbow lake cycle along the Lower Mississippi River, in River meandering[END_REF]Wren et al., 2008;[START_REF] Hudson | Hydrologic connectivity of oxbow lakes along the lower Guadalupe River, Texas: The influence of geomorphic and climatic controls on the "flood pulse concept[END_REF]. As so, it can be supplied by water of four different origins: local precipitation, main stream's water through the confluence, groundwater inflowing from the bank between Allier River and the oxbow and finally, groundwater from the bank delineating the alluvial plain (Rollet et al., -iii) and the downstream zone (DZ), which is connected to the Allier River by its downstream extremity [START_REF] Beauger | Distribution of Diatom, Macrophyte and Benthic Macroinvertebrate Communities Related to Spatial and Environmental Characteristics: The Example of a Cut-Off Meander of the River Allier (France)[END_REF] (188 m length, 17 m width and maximum depth of 1.6 m).

Highly subject to flooding, the right bank (RB) is used for punctual grazing while the left bank (LB) is a low terrace dedicated to cereals production. However, close to the Auzon Oxbow, crops are smaller and agricultural practices relatively soft compared to the western plateau where they are much more intensive (Figure 1b). That neighboring plateau presents an aquifer that is assumed to partly feed the LB alluvial groundwater. Auzon Oxbow has been monitored since 2007 [START_REF] Beauger | Impact de la capture d'un chenal fluviatile par une ancienne gravière sur la distribution des macroinvertébrés benthiques[END_REF] in terms of geomorphology, sedimentation and ecology (study of diatom, macrophyte and benthic Abbreviations: GW = groundwater; RB = right bank; LB = left bank; T = water temperature macroinvertebrate communities). To complete ecological investigations, the monitoring has been extended to surface and groundwater hydrodynamic and hydrochemistry. In this purpose, near-surface piezometers (from 0.92 to 1.85 m deep) were built on the oxbow's shore of both banks (Figure 1c) in 2012 [START_REF] Beauger | Distribution of Diatom, Macrophyte and Benthic Macroinvertebrate Communities Related to Spatial and Environmental Characteristics: The Example of a Cut-Off Meander of the River Allier (France)[END_REF] as well as deep observation boreholes catching the whole aquifer thickness in 2014. In addition, monthly monitoring is performed at the following sites in the area (Figure 1): i) 6 surface water observation points distributed along the entire Auzon Oxbow from its upstream to its downstream ends (B2 to B7); ii) 1 surface water observation point in the Allier River (Allier); iii) 6 observation boreholes (10 m deep covering the entire alluvial aquifer thickness), 2 drilled in the Right Bank (RB: PZ1 and PZ2) Abbreviations: GW = groundwater; RB = right bank; LB = left bank; T = water temperature and 4 in the Left Bank (LB: PZ3 to PZ6); iv) 7 near-surface piezometers (0.85 to 1.92 m deep), located next to the oxbow on both RB and LB (P2 to P9); v) Le Monteil spring (Figure 1b)

Sampling sites

draining the neighboring plateau aquifer, which is assumed to partly feed the alluvial groundwater. A total of 837 water samples have thus been taken in clean sterile polypropylene straight containers with polyethylene caps of 180 mL, transported in cooler to GEOLAB laboratory in Clermont-Ferrand (CNRS UMR 6042, University Clermont-Auvergne) and then stored at 4°C. Anions and cations were analyzed within 3 days in GEOLAB Laboratory: anions (F -, Cl - NO2 -, NO3 -, PO4 3-, SO4 2-) using the Thermo Fisher Scientific Dionex DX120 ionic chromatography with a Ionpac AS23 4*250 mm column and the Dionex 7 anions standard solution; Abbreviations: GW = groundwater; RB = right bank; LB = left bank; T = water temperature cations (Li + , Na + , NH4 + , K + , Mg 2+ , Ca 2+ ) using the Thermo Fisher Scientific Dionex ICS1100 ionic chromatography with a Ionpac CS12A 4*250 mm column and the Dionex 6 cations standard solution. The error margins were of ± 4-6% and ±2-4% respectively for anions and cations. These analyses are reliable since all the 839 ionic balances (837 monthly + punctual samples 3 and 103) are under 10% with even 95.1% under 5%.

Sampling and analytical methods

Hydrodynamic monitoring

From July 2014 to March 2017, only groundwater from the six observation boreholes has been analyzed for stable isotopes. Since April 2017, all sites have been sampled for δ 2 H and δ 18 O analyses, so the database accounts for 360 isotopic analyses. Waters have been collected with no air bubbles, in 20 mL borosilicate glass vials closed by polypropylene caps.

Isotopic analysis were performed at the University of Corsica, France (CNRS UMR 6134 SPE): both 2 H and 18 O of the water molecule were characterized using a liquid-water stable isotope analyzer DLT-100 (Los Gatos Research) according to the analytical scheme recommended by the IAEA (IAEA, 2009;[START_REF] Penna | On the reproducibility and repeatability of laser absorption spectroscopy measurements for ??2H and ??18O isotopic analysis[END_REF]. Isotopic data were reported in the standard delta notation in part per thousand relative to Vienna Standard Mean Ocean Water (VSMOW; [START_REF] Clark | GW = groundwater; RB = right bank; LB = left bank; T = water temperature[END_REF]). The accuracy is 1‰ for δ 2 H and 0.1‰ for δ 18 O. Abbreviations: GW = groundwater; RB = right bank; LB = left bank; T = water temperature

Geophysical investigations

Electrical Resistivity Tomography (ERT) is a geophysical method providing images of electrical resistivity ρ (inverse of conductivity) of the subsurface. ERT is generally used to characterize the lithology and the geometry of the ground, to assess its saturation, or even to monitor soil pollution [START_REF] Loke | Tutorial : 2-D and 3-D electrical imaging surveys[END_REF]. The technique consists of injecting current in a pair of electrodes set into the ground and measuring the resulting potential difference between another dipole of electrodes which yields information about ρ.

A 2D ERT survey was carried out on the SOAHAL Observatory on the 02/21/2018 in order to investigate potential underground connections between Auzon Oxbow and Allier River. The data collection was done by using a SYSCAL R1+ (Iris instruments), with a Wenner-Schlumberger electrode configuration [START_REF] Loke | Rapid least-squares inversion of apparent resistivity pseudosections by a quasi-Newton method1[END_REF]. An array of 72 electrodes was installed, distributed every 3 m, along a NNW-SSE line (Figure 1c). The apparent resistivity data were inverted with RES2DINV software [START_REF] Loke | Tutorial : 2-D and 3-D electrical imaging surveys[END_REF] using leastsquares inversion optimization. Inversion converged to RMS error of 3% after 5 iterations.

Results and discussion

Abbreviations: GW = groundwater; RB = right bank; LB = left bank; T = water temperature

End-members characterization and interactions

Hydrodynamical data

Allier River discharge presents an average value of 25 m 3 .s -1 for the studied period (07/22/2014 -11/24/2017). The most important flood impulse reached 328 m 3 .s -1 (11/23/2016) while two other flood peaks have been registered equal or superior to 150 m 3 .s -1 (156 m 3 .s -1 on the 11/29/2014 and 150 m 3 .s -1 on the 04/06/16; Figure 2d). These events are quite moderate compared to the large floods that Allier River can historically experience (10Y-flood and 50Y-flood: 590 and 840 m 3 .s -1 respectively; Banque Hydro, n.d.). Annual discharge variations are observed with globally low flow (LF) periods during the warmest months (from July to October) with an average flow rate around 14 m 3 .s -1 while high flow (HF) periods including important flood events occur along the rest of the year with an average flow rate around 32 m 3 .s -1 . This LF-HF distribution over time is more consistent with variations of precipitation recorded in the upstream mountainous area [START_REF] Mohammed | Isotopic and geochemical identification of main groundwater supply sources to an alluvial aquifer, the Allier River Valley (France)[END_REF], whose Naussac weather station is the witness (Figure 2a), than the local ones (represented by Sainte-Florine weather station on Figure 2a). However, the maximum flood peak of 328 m 3 .s -1 is actually related to high Abbreviations: GW = groundwater; RB = right bank; LB = left bank; T = water temperature precipitation recorded at both stations, suggesting a generalized rainfall event linked to a Cevénol episode (a climatic phenomenon more and more frequent these last few years with rainy events that mainly affects the Cevennes' mountain range and piedmont which often cause severe floods in the Massif Central Region) (Jubertie, 2006;[START_REF] Gay | Transfert de particules des versants aux masses d'eau sur le bassin Loire-Bretagne[END_REF]. On the contrary, flood peak occurring the 01/26/2017 seems to be only due to high altitude precipitation: rainfall event is actually only recorded at Naussac weather station.

Finally, flood peak recorded the 08/30/2017 is caused by a stormy event affecting Saint-Florine weather station only.

These examples emphasize that discharge variations recorded for Allier River are related to a large set of meteorological conditions affecting the entire Allier basin, including rapid winter temperature elevation with or without rainfall causing snow melting (Jubertie, 2006).

Figure 2

Water level variations at the Auzon Oxbow/Allier River confluence (solid black lines in Figure 2b and Figure 2c) are consistent with discharge variations of the Allier River recorded at Agnat-Pont d'Auzon station (Figure 2d). It shows Abbreviations: GW = groundwater; RB = right bank; LB = left bank; T = water temperature the accuracy of the in situ surface water level monitoring which was therefore crossed with water table monitoring for piezometric mapping (triangular interpolation). Figure 3a and The HF piezometric map illustrates the hydrodynamic situation during the third most important flood event of the studied period. This flood was linked to a large rainfall event which affected the whole Allier River catchment (Figure 2a). The map can be used to identify potential hydrodynamic changes compared to LF situation. Then an inversion of the hydraulic gradient is induced by high surface water level recorded at the Auzon Oxbow/Allier River confluence; it affects GW on both Auzon Oxbow banks. However, in the RB, inversion is reduced to a GW flow inversion (from PZ2 to PZ1) since the Allier River discharge just began to decline while the RB is still in charge.

On the LB, inversion seemed to extend from surface water to PZ5 passing through PZ4. Actually, during that HF event as generally for all HF recordings, PZ4 levels varied synchronously with the surface water level up to inversion of hydraulic gradient while the other LB boreholes remained stable, PZ5 included. PZ3 piezometric level starts to increase only 24h after the rise of surface water level, the response delay of PZ5 and PZ6 is about 26h. Besides, PZ5 piezometric level stays lower than the one of PZ6, inducing a piezometric depression.

The delay between the increase in surface water level and that of the more inland LB boreholes suggests that the hydraulic gradient inversion is limited to PZ4. This hydrodynamic feature can be explained by high disparities existing within the alluvial 57 Abbreviations: GW = groundwater; RB = right bank; LB = left bank; T = water temperature deposits (from silts to gravels; Teles et al., 2004;[START_REF] Ounaïes | Quantification of vertical water fluxes in the vadose zone using particle-size distribution and pedology-based approaches to model soil heterogeneities[END_REF][START_REF] Sarris | Using solute and heat tracers for aquifer characterization in a strongly heterogeneous alluvial aquifer[END_REF]. These 3D heterogeneities lead to variations in water flows distribution within the aquifer.

To summarize, hydrodynamic data highlight that during LF, Auzon Oxbow is supplied by GW. During HF, an inversion of the hydraulic gradient is recorded and Auzon Oxbow contributes to the recharge of GW. RB is entirely concerned by this recharge while only the oxbow closest borehole (PZ4) is affected in the LB (Figure 3b). The important time delay observed between surface water flood peaks and GW recharge at PZ3, PZ5 and PZ6 (from 24 to 26h) indicates that LB GW is mainly recharged by the southern GW system.

Physico-chemical data

For the 21 sample sites, physico-chemical parameters measurements (water temperature -T; pH; electrical conductivity -EC) as well as ions concentrations and water stable isotopes determinations are presented in Table 1 with average (arithmetic means), minimum, maximum and standard deviation values. EC vs T data of the entire SOAHAL observatory are reported in Figure 4a while EC time-series of Auzon Oxbow DZ (B2bis), IZ (B4), UZ (B7) and of each end-Abbreviations: GW = groundwater; RB = right bank; LB = left bank; T = water temperature member are quoted in Figure 4b: Allier River, PZ1 (RB GW), PZ3-PZ4 (LB GW with an increasing distance to the Auzon Oxbow from PZ4 to PZ3).

The pH values are comprised between 6.5 and 8.8 (Table 1), which is consistent with natural water pH usually ranging from 6.5 to 9.5. The more neutral values have been recorded in GW and are assumed to be due to the buffering capacity of soils. EC and T values were the most interesting physicochemical parameters to differentiate water end-members (Figure 4):

-Allier River EC is the lowest one within the study site (110 ± 17 µS.cm -1 ; Table 1), without any significant variations during the year (Figure 4b). On the contrary T experiences high amplitudes (from 0.9 to 21.8°C; Table 1) as an impact of air temperature annual variation.

-PZ1 and PZ2 show the lowest EC values of the boreholes (with 290 and 491 µS.cm -1 respectively; Table 1). EC annual variations are relatively low (Figure 4b) while T amplitudes can reach more than 12°C (Table 1). The RB (P2, P4, P6, and P9) and LB (P7) nearsurface piezometers (Figure 1c) also present low EC values and high T amplitudes (Figure 4a and Table 1). Abbreviations: GW = groundwater; RB = right bank; LB = left bank; T = water temperature -PZ3 to PZ6 are characterized by the highest EC values (mean value of the four boreholes: 981 µS.cm -1 ) while T experienced the smaller variations (around 7°C; Table 1). EC time-series show a quite stable signal with no clearly identified seasonal variations (Figure 4b). P3 and P5, the two remaining LB near-surface piezometers (Figure 1c) present similar EC and T patterns (Figure 4a and Table 1).

-Le Monteil spring shows a high EC value (925 µS.cm -1 , Table 1) and low amplitude of T (6.7°C; Table 1) which is consistent with the LB GW (Figure 4a). Chemical difference between Allier River (as well as RB GW) and LB GW is due to the geological composition of the watershed. Actually, the Ca-Na/HCO3 water type of the Allier River can be related to the volcano-metamorphic basement: upstream of the study site, the Allier River incised the basement while downwards, the riverbed widens within the Brioude basin where an enrichment in calcareous nodules between Vieille Brioude and the study site has been documented [START_REF] Lasnier | Brioude[END_REF]. Carbonate dissolution occurrences are reported in the Allier River and the 57 Abbreviations: GW = groundwater; RB = right bank; LB = left bank; T = water temperature shallow GW associated [START_REF] Négrel | Strontium isotope geochemistry of alluvial groundwater: a tracer for groundwater resources characterisation[END_REF]. Sodium minerals, subjects to weathering, are reported as common in the Allier terraces, as augite, green and brown hornblende and nepheline (Rudel, 1963;[START_REF] Pastre | Altération et paléoaltération des minéraux lourds des alluvions pliocenes et pleistocenes du bassin de l'Allier (Massif central, France)[END_REF][START_REF] Veldkamp | Weathering of alcali basalt gravel in two older Allier river terraces, Limagne, France[END_REF], which explain the Na-enriched signature of the Allier River and so that of the RB GW. The Ca/HCO3 water type of LB GW has been already described by several authors [START_REF] Négrel | Strontium isotope geochemistry of alluvial groundwater: a tracer for groundwater resources characterisation[END_REF][START_REF] Vanderhaeghe | Saint-Germain-Lembron[END_REF][START_REF] Mohammed | Isotopic and geochemical identification of main groundwater supply sources to an alluvial aquifer, the Allier River Valley (France)[END_REF] and corresponds to many alluvial aquifers in France [START_REF] Roux | AQUIFERES ET EAUX SOUTERRAINES DE FRANCE-COFFRET 2 TOME[END_REF] and over the world [START_REF] Chkirbene | Hydro-geochemical evolution of groundwater in an alluvial aquifer: Case of Kurokawa aquifer, Tochigi prefecture, Japan[END_REF][START_REF] Andrade | Hydrogeochemical controls on shallow alluvial groundwater under agricultural land: case study in central Portugal[END_REF][START_REF] Huang | Spatial and temporal variability of the chemistry of the shallow groundwater in the alluvial fan area of the Luanhe river, North China[END_REF]. Indeed, because the mineral phases that composed alluvial aquifers do not generally impose a very marked geochemical type and because of short residence times within the aquifer, alluvial GW presents frequently a Ca/HCO3 water type [START_REF] Roux | AQUIFERES ET EAUX SOUTERRAINES DE FRANCE-COFFRET 2 TOME[END_REF].

Auzon Oxbow as a result of the mixing between endmembers

General features of Auzon Oxbow chemistry

In a first approximation, taking into account their origin, perifluvial oxbows and main stream are supposed to have a similar chemical composition (Négrel et al., 2003). However, despite its still active connection with the Allier River, the Auzon Oxbow presents a mean EC value (300 µS.cm -1 ) higher than Allier River one (110 µS.cm -1 ) but lower than GW (784 µS.cm -1 in mean for observation boreholes of both banks). This indicates that the Auzon Oxbow water results from a mixing between the low concentration water of the Allier River and the more concentrated one of GW, whatever the bank (Table 1, Figure 4). Figure 6 showing constant EC. This was a short-time process since Auzon Oxbow recovered its pre-flood concentration the month after (Figure 4b), indicating that the oxbow restitutes quickly to the main stream what it absorbs during flood. This time-constrain phenomenon has already been quoted for oxbows by [START_REF] Carrel | La Morte du Sauget, un ancien méandre du Rhône: bilan hydrologique et biogéochimique[END_REF] for the Morte du Sauget, one of the Rhône oxbows and for oxbows of Ain River [START_REF] Le Coz | Réponse hydraulique d'un bras mort au signal hydrologique de la rivière[END_REF]. PZ4 records also an EC decrease in April 2016, in accordance with the inversion of the hydraulic gradient showed in HF piezometric map (Figure 3b).

Identification of punctual arrivals of GW within Auzon Oxbow

To precise the downstream-upstream evolution within the Auzon Oxbow, a fine cartography of EC has been performed on the 11/23/2017, during a low flow stage (Allier River daily discharge: 6.7 m 3 .s -1 ). Figure 7 presents the results of this campaign and puts in evidence a difference between the upstream zone characterized by low EC values, mainly in the range 0-250 µS.cm -1 , and IZ and DZ parts of the Auzon Oxbow with values comprised between 250 and 400 µS.cm -1 . This general pattern is disturbed by local arrivals of water. A high EC is observed along the downstream zone left bank (point 103 = 735 µS.cm -1 ; Figure 7) and testifies for a high concentration groundwater arrival. On the contrary, the whole upstream part is characterized by low EC in accordance with low concentration water incomes, some of which were identified as coming from the bed of the oxbow. Points 3 (as an example of the latter) and 103 have been sampled for ions analyses. 2. Results show that:

-Point 103 presents a Ca/HCO3 signature pretty close to that of LB GW (Figure 8) and especially PZ4 (Table 2, Figure 8). PZ4 shows higher NO3 -(13.1 mg.L -1 ) probably due to a contamination of alluvial GW, but not transmitted or degraded before the transfer LB GW to Auzon Oxbow.

-The low concentration water arrival located at the UZ end of the oxbow (Point 3) presents a Ca-Na/HCO3 water type similar to Allier River one (Table 2, Figure 8). This is in accordance with an upstream supply by the Allier River. The water arrival of Point 3 was coming from the bed of the oxbow which indicates that this supply is realized through underground inflows. The location of these low concentration water arrivals directly close to the paleochannel visible in the landscape, suggests that an old and deeper channel connects Auzon Oxbow to Allier River. The potential recharge by underground paleochannels, because of their high hydraulic conductivity, has already been observed (Rathore et al., 2010;[START_REF] Archie | Clustering oxbow lakes in the Upper-Tisza Region on the basis of stable isotope measurements[END_REF]. Abbreviations: GW = groundwater; RB = right bank; LB = left bank; T = water temperature Table 2 Figure 8 Therefore, to verify the present hypothesis, an ERT sounding has been performed on the 02/21/2018 (high flow period, Allier River daily discharge = 57.3 m 3 .s -1 , see part 3.3).

The location of the profile is indicated in Figure 1c. Oriented SSE-NNW, the profile was designed perpendicularly to the visible paleochannel and close to the upstream end of the Auzon Oxbow. Indeed, electrical resistivity sounding has already been used to investigate subsurface paleochannel geometry, architecture and hydrodynamic [START_REF] Sinha | Geo-electric resistivity evidence for subsurface palaeochannel systems adjacent to Harappan sites in northwest India[END_REF] as well as fresh GW discharge through a paleochannel [START_REF] Kolker | Pathways and processes associated with the transport of groundwater in deltaic systems[END_REF]. The resulting ERT profile indicates a general decrease of the electrical resistivity with depth (Figure 9). Two main zones can however be observed. In the first one, from surface to 8-10 m depth, resistivity varies laterally. The southern part of the profile, along with the 48 first lateral meters on Figure 9, presents a relatively low resistivity (ρ = 150 to 350 Ω.m -1 ) compared to the rest of the section (ρ > 600 Ω.m -1 ). It can be interpreted as a sand formation saturated Abbreviations: GW = groundwater; RB = right bank; LB = left bank; T = water temperature with freshwater [START_REF] Sinha | Geo-electric resistivity evidence for subsurface palaeochannel systems adjacent to Harappan sites in northwest India[END_REF]. This is consistent with the presence of the paleochannel of the Allier River (Figure 1c), which higher porosity induces lower resistivity [START_REF] Archie | Clustering oxbow lakes in the Upper-Tisza Region on the basis of stable isotope measurements[END_REF].

In addition, there is lateral resistivity change within the paleochannel which most likely reflects the progressive filling of the system and a potential connection to the river (blue circle on Figure 9). The second zone starts below 10 m depth with low resistivity values and corresponds to the Oligocene marl substratum [START_REF] Lasnier | Brioude[END_REF][START_REF] Korobova | Element partitioning in sediment, soil and vegetation in an alluvial terrace chronosequence, Limagne rift valley, France: A landscape geochemical study[END_REF].

Figure 9

This paleochannel supplies the Auzon Oxbow with low concentration water and explains the low EC and the water types observed in the upstream part of the Auzon Oxbow and in the near-surface piezometer P7 (Figure 1c). The repartition of Auzon Oxbow points along the mixing line of the Figure 6, with B7 closer to the Allier River end-member than B2 is thus explained. For information, a simple estimation of contribution percentages of Allier River and LB GW to the oxbow (at its different sampling locations) can therefore been deduced from the Figure 6 data. Indeed, considering that the chemical Abbreviations: GW = groundwater; RB = right bank; LB = left bank; T = water temperature characteristics of the Allier River end-member point (30.2 mg.l - 1 of HCO3 -and a Na/Ca ratio of 0.95) represent 100% of contribution from the main stream and that those of LB GW (302.6 mg.l -1 of HCO3 -and a Na/Ca ratio of 0.23) represent 0%, it is easy to deduce the contributions of those end-member to the oxbow at the different sampling sites. So, it was logically estimated that B2, the closest location to the confluence and B7, the closest location to the paleochannel arrival, were mostly and equally fed by the Allier River with respectively 63 and 64 % of contribution. The rest of the oxbow would be, according to these strictly indicative estimates, as much fed by the river as by the alluvial groundwater (49 % of Allier River contribution for B3, 50 % for B4 and 51 % for B2bis and B6).

Those estimates would imply that alluvial groundwater contributions to the Auzon oxbow are not negligible and that, in terms of contribution share, the river contributes as much by the confluence as by the upstream paleochannel to the oxbow water supply. The 2 H-18 O stable isotopes of the water molecule constitute one of the best water tracer since it provides information about water origins as well as water mixing processes through the hydrological cycle [START_REF] Fontes | The neck cutoff oxbow lake cycle along the Lower Mississippi River, in River meandering[END_REF][START_REF] Clark | GW = groundwater; RB = right bank; LB = left bank; T = water temperature[END_REF]Kendall and McDonnell, 1999). For the study of wetlands, they allow approaching their functioning [START_REF] Hunt | Using Stable Isotopes of Water and Strontium to Investigate the Hydrology of a Natural and a Constructed Wetland[END_REF][START_REF] Clay | Using stable isotopes of water to infer wetland hydrological dynamics[END_REF] and more peculiarly to characterize the isotopic content of the different supplying sources or the Auzon Oxbow [START_REF] Archie | Clustering oxbow lakes in the Upper-Tisza Region on the basis of stable isotope measurements[END_REF] according to the temperature-, amount-, altitudinal-and continental-effects [START_REF] Rozanski | Les minéraux lourds des terrasses Quaternaires de Limagne d'Auvergne et leséruptions montdoriennes[END_REF].

For coherent interpretations, data from April to November 2017, period of the concomitant investigation of all sites, have been selected among the database. Allier River (-8.0 δ 18 O‰, -52.0 δ 2 H‰) and RB GW (-8.1 δ 18 O‰, -53.2 δ 2 H‰) present the most depleted isotopic content. The LB GW (-7.7 δ 18 O‰, -51.5 δ 2 H‰) and Le Monteil spring (-7.3 δ 18 O‰, -50.0 δ 2 H‰) are characterized by more enriched signatures. As previously referred by Négrel et al. (2003) and by [START_REF] Mohammed | Isotopic and geochemical identification of main groundwater supply sources to an alluvial aquifer, the Allier River Valley (France)[END_REF], Allier River displays a clear seasonal variation (Figure 10a) with depleted values during winter and more generally during HF period (around -8.5 δ 18 O‰) than for summer (around -7.5‰), mainly due to the temperature effect on Abbreviations: GW = groundwater; RB = right bank; LB = left bank; T = water temperature during summer, due to evaporation process (Geyh and Mook, 2000;[START_REF] Mohammed | Isotopic and geochemical identification of main groundwater supply sources to an alluvial aquifer, the Allier River Valley (France)[END_REF]. Auzon Oxbow isotopic data are well plotted along a mixing line between Allier River HF and isotopically enriched waters. Auzon Oxbow DZ (B2 and B2bis) presents the more enriched and evaporated signal close to that of the LB GW; the heavy isotopes content globally decreases up to the UZ (B7; Figure 10b). On a temporal scale, evolution of isotopic data in the Auzon Oxbow tends to homogenize and the 3 identified zones DZ (B2bis), IZ (B4) and UZ (B7) present, in winter period, the same isotopic content (Figure 10a). This behavior is partly explained by previous observations: DZ is subject to a double supply through its connection to the Allier River at the confluence and LB GW inputs (Point 103). The later explains the more enrich isotopes contents and its important variations through time. B7 has been shown to be supplied by Allier River that inflows to the Auzon Oxbow through the subsurface paleochannel. However, B7 presents a behavior completely different from the one of Allier River and isotopic values of B7 match only the Allier River ones during winter high flow period (Figure 10a). This observation provides precision about the recharge of the Auzon Oxbow from the paleochannel that occurs only seasonally, during HF periods, as for the RB GW. Indeed, according to the hydrodynamic monitoring, the RB GW Abbreviations: GW = groundwater; RB = right bank; LB = left bank; T = water temperature recharge through inversion of the hydraulic gradient during HF which is also in agreement with the depleted isotopic values observed for RB GW.

Figure 10

Conceptual model of the Auzon Oxbow

The multidisciplinary analysis between hydrodynamic, geochemical and isotopic approaches carried out at the dense SOAHAL Observatory allows establishing a reliable conceptual model of the hydrodynamical functioning of the Auzon Oxbow (Figure 11). As expected for that kind of perifluvial environments [START_REF] Rollet | Expertise hydro-géomorphologique en vue du diagnostic fonctionnel des habitas, de la restauration du transit sédimentaire et des lônes[END_REF], interactions between the oxbow and both the main stream and the alluvial aquifer were identified. Since the Auzon Oxbow is disconnected from the main stream in its upstream end for the last 30 years, the Allier River supplies the oxbow generally through the downstream confluence (surface water flows on Figure 11). However, during major flood events the main stream can submerge the right bank and then supply the entire oxbow area. Geochemical data show another interaction between the Allier River and the Auzon Oxbow, through the upstream underground paleochannel, recognized both via geochemical Abbreviations: GW = groundwater; RB = right bank; LB = left bank; T = water temperature analysis and geophysical investigations. A detailed isotopic analysis has shown that this Auzon Oxbow area has a similar isotopic signature than Allier River during high flow periods only. This observation implies that the paleochannel is active from November to June only. This is in agreement with the results of hydrodynamic and geochemical approaches which identified existing pathways from surface water to alluvial groundwater during the same periods (high flow GW flows on Figure 11).

Figure 11

The coupling approach also testifies for a supply of the oxbow by the alluvial groundwater, this especially affects the water composition in its downstream part. As this part of the oxbow is the site of its confluence to the Allier River, Besides, while studies attest for the importance of oxbow connection degree to the main stream on the water quality [START_REF] Carrel | La Morte du Sauget, un ancien méandre du Rhône: bilan hydrologique et biogéochimique[END_REF][START_REF] Bengen | Qualité des eaux de trois bras morts de la Garonne : variabilité spatio-temporelle = Water quality in three ancient arms of the river Garonne : spatio-temporal variability[END_REF], it is mostly based on the type of surface connection observed between the two water bodies (oxbow isolated from the river or connected through one or two sides as Ward et al. ( 2002) described).

Those observations indeed inform about the extent of the hydrological connection that impacts the water chemistry of oxbows [START_REF] Tockner | The Danube restoration project: species diversity patterns across connectivity gradients in the floodplain system[END_REF][START_REF] Glińska-Lewczuk | Water quality dynamics of oxbow lakes in young glacial landscape of NE Poland in relation to their hydrological connectivity[END_REF] especially for instance for metals contaminations [START_REF] Ciazela | Tracking heavy metal contamination in a complex river-oxbow lake system: Middle Odra Valley, Germany/Poland[END_REF]. However, the present study show that consequent underground connection can also occur between the main stream and the oxbow through paleochannel as already suggested by [START_REF] Archie | Clustering oxbow lakes in the Upper-Tisza Region on the basis of stable isotope measurements[END_REF], increasing so their apparent connection degree and impacting the water chemistry of the oxbow. Therefore, the possibility of underground connectivity to the main stream should be further considered in oxbows Abbreviations: GW = groundwater; RB = right bank; LB = left bank; T = water temperature

Conclusions

In a near future, wetlands like oxbows are assumed to become more and more determinent as nature-based solutions to respond to hydrological (river discharge control) and ecological (excess nutrients removal function) concerns. Therefore, the dynamic of these specific hydrosystems must be fully understood. However, the establishment of complete and usefull functionning models is complicated by the different connection degrees that can have these specific wetlands with the surrounding water masses. The task is all the more difficult since it is usually undertaken through individual investigation approach which can neglect or underestimate some supply components. The objectives of the present study were so to provide effective diagnostic tools to evaluate oxbow connection degree to the main stream and to the adjunct alluvial aquifer in order to be able to establish a complete hydrodynamic description of such hydrosystems.

The Auzon Oxbow conservation is the subject of local/regional great concerns especially for the local fishing associations. In The alluvial groundwater of the left bank was identified to supply the oxbow during LF especially in its downstream part.

List of Figures

(

  sexagesimal GPS standard longitude/latitude coordinates at the confluence: 03°21'41''E/45°22'05''N; elevation: 400 m), a fluvial annex of the Allier River, a gravel bed meandering stream located about 60 km southeast of Clermont-Ferrand, Abbreviations: GW = groundwater; RB = right bank; LB = left bank; T = water temperature France. The Allier River originates at La Maure de Gardille (1423 m.a.s.l) and joins the Loire River at the Bec d'Allier near Nevers, after 410 km of course from South towards North within a 14,310 km² watershed (Figure 1a). Allier River discharge at the station K2430810 of Agnat (Pont d'Auzon), located downstream of the study area at about 1500 m to the north (03°21'26''E/45°23'09''N; elevation: 360 m), ranges from 9.7 to 246.0 m 3 .s -1 with an annual average at 30.3 m 3 .s -1 (calculated over 26 years data; ("Banque Hydro," n.d.)). The Auzon Oxbow is located in the first alluvial plain encountered downstream of the Allier Gorges. The study area presents a temperate continental climate with relatively hot summers (mean air temperatures up to 24°C in July) and cold winters (mean temperatures around -3°C in February; "Météo-France" n.d. ). Annual precipitation is moderate within the Allier basin (692 mm on average), the valley being in a shelter position behind the western Massif Central where Atlantic humid air masses produce orographic rainfall; most of the events occur between May and October (423 mm in cumulative precipitation; "Météo-France" n.d.). Mountainous areas of the upper Allier catchment are more humid with a total precipitation around 2000 mm; minima are registered in the low Allier plain and account for around 570 mm/yr

  Abbreviations: GW = groundwater; RB = right bank; LB = left bank; T = water temperature 2005). The 560 m length of the oxbow can be divided into three geomorphological parts (Figure 1c): -i) the upstream zone (UZ), which is disconnected from the main stream and is similar to a pond (197 m length, 13 m width and maximum depth of 1.8 m) ; -ii) the intermediate zone (IZ), which is a former geomorphological riffle (176 m length, 0.3 to 8 m width and maximum depth of 0.4 m) ;

  57 Abbreviations: GW = groundwater; RB = right bank; LB = left bank; T = water temperature before sampling and measuring. An intensive campaign was performed on the 23 rd of November 2017 in the Auzon Oxbow in order to assess spatial distribution of electrical conductivity (EC): 124 measures have been performed in the near surface of the oxbow (0.2 m depth) along the UZ, the IZ and on the upper part of the DZ, using a WTW multi 340i and a DGPS Trimble Geo7x whose data were post-processed with Pathfinder software. Two sites of the EC campaign (Points 3 and 103) were also sampled for major ions analyses. EC, pH and temperature have been measured in situ using a WTW multi 340i. HCO3 -concentrations have been determined directly in the field by using HACH Digital Titrator, sulfuric acid (0.1600 N and 1.600 N) and Bromocresol Green-Methyl Red Indicator (Hach method 8203).

Figure

  Figure 3b present LF (08/04/2015) and HF (04/06/2016)

Figure 3

 3 Figure 3

  Figure 5) in order to define the geochemical water-types of each end-member. Water types are distributed between two groups with a noticeable difference in the cations content. The first group has a Ca-Na/HCO3 type and Abbreviations: GW = groundwater; RB = right bank; LB = left bank; T = water temperature includes Allier River, PZ1-PZ2 (RB GW), P2-P4-P6-P9 (RB nearsurface piezometers), P7 (LB near-surface piezometer) and then the second group presents a Ca/HCO3 signature and corresponds to PZ3 to PZ6 (LB GW), P3 and P5 (the two remaining LB near-surface piezometers) and Le Monteil Spring.Then, the GW signal shows a clear distribution between RB and LB. RB GW are predominantly supplied by Allier River whereas LB GW chemical facies is consistent with the one of the plateau aquifer (Le Monteil). This is in agreement with hydrodynamic data. P7 can be quoted as an exception to this RB/LB GW distribution, as this LB near-surface piezometer presents physico-chemical and geochemical characteristics close to the RB GW ones.

Figure 5

 5 Figure 5

  confirms this trend and highlights a clear evolution of the Auzon Oxbow between a Na-pole characterized by Allier River and a more calcic end member delineated by LB GW. RB GW water type is close to the Auzon Oxbow one and seems to result also from the mixing between Allier River and LB GW. The proximity of LB GW and Le Monteil spring confirms the contribution of the plateau aquifer to the alluvial GW. This general feature is however not constant within the hydrological cycle: records of April 2016 Abbreviations: GW = groundwater; RB = right bank; LB = left bank; T = water temperature (highlighted by the grey area of Figure4b) show a decrease of Auzon Oxbow EC which reaches the value of Allier River. This specific period corresponds to one of the major floods that have affected the Allier River during the study, with a discharge flow of 150 m 3 .s -1 registered on the 04/06/16 (Figure2d). During this period, the Allier River penetrated further into the Auzon Oxbow. This surface water arrival, characterized by a low concentration, affected the Auzon Oxbow up to B6, B7

Figure 6 57

 6 Figure 6

Figure 7

 7 Figure 7

4. 2 . 3 .

 23 Use of δ 2 H and δ 18 O of the water molecule for temporal insight 57 Abbreviations: GW = groundwater; RB = right bank; LB = left bank; T = water temperature

  groundwater supply could have been underestimated otherwise. That connection with the adjunct alluvial aquifer is active during low flow (Figure 11). LB GW are mainly supplied by the southern part of alluvial aquifer and the neighboring agricultural plateau (permanent GW flows on Figure 11), except during high floods when small gradient inversions can drive pathways from surface water to groundwater. Abbreviations: GW = groundwater; RB = right bank; LB = left bank; T = water temperature The active connection existing between the Auzon Oxbow and the Allier River through the underground paleochannel would not have been identified and specified without the geochemical and isotopic approaches. would have produced an incomplete understanding of the Auzon Oxbow hydrosystem.

  fact, hydrodynamic and hydrochemical features of AuzonOxbow have a direct impact on its ecological functions as reproduction and refuge areas for emblematic fishes such as trouts. This study so proposed the coupling of hydrodynamic, Abbreviations: GW = groundwater; RB = right bank; LB = left bank; T = water temperature geochemical and isotopic approaches to produce a model which can serve as a reference state for further operations of conservation and management of the Auzon Oxbow. The conceptual model thus produced attests to a greater degree of connection than expected between the Auzon Oxbow and both the main stream and the alluvial aquifer. Indeed, despite their surface upstream disconnection for the last 30 years, the study proved that Allier River supplies the oxbow through the downstream confluence and through the upstream underground paleochannel. Besides, the model attests to a non-negligible supply of the Auzon oxbow by the adjunct alluvial aquifer.The current model can however be improved by completing the present hydrodynamic monitoring set with hydrological stations within the oxbow: one at the upstream ends of its downstream zone to compare data with the current hydrological station of the confluence; and one at the upstream end of the oxbow, close to the paleochannel water arrival. Flowrates measurements must indeed be performed in order to be able to quantify the water fluxes identified in this paper. Thus, the Auzon Oxbow role to manage flood events in the Allier River system would be evaluated. Besides, the impact of the Allier River arrival through the paleochannel on the water level of the upstream zone of the oxbow would be Abbreviations: GW = groundwater; RB = right bank; LB = left bank; T = water temperature assessed. Furthermore, such reliable conceptual models can be used for further applications: strategic wetlands operations or water quality managements responding to environmental and/or ecological issues. Indeed, such hydrodynamic knowledges on hydrosystems constitute an essential requirement to understand the transport of polluting substances by water. The conceptual hydrodynamic scheme coupled with the contaminants observations can support the assessment of the origin(s) and fate(s) of pollutants within wetlands hydrosystems and the identification of potential remediation processes occurences. The present conceptual model of the Auzon Oxbow hydrosystem is currently used to assess the role of that wetland regarding to excess nutrients and emerging molecules dynamics. The study here proved that coupling several investigation approaches such as hydrodynamic (surface water and groundwater levels), geochemical (physico-chemical parameters and ionic concentrations) and isotopic (δ 2 H-δ 18 O of the water molecule) monitorings actually efficiently supports the identification of water supply sources to the wetland through space and time. Indeed, while the hydrodynamic approach gives general informations about surface water/grounwater interactions, the geochemical and Abbreviations: GW = groundwater; RB = right bank; LB = left bank; T = water temperature isotopic approaches complete the overview by identifying the origin of water, imaging the hydrochemistry of the oxbow and following the evolution of hydrodynamical and chemical parameters within time. Spatial and temporal disparities in the oxbow connection degree to the main stream and to the alluvial groundwater are proved to largely depend on river discharge during low and high flows. Besides, relevance of considering the degree of underground connection between the oxbow and the river was highlighted. The used of the multidisciplinary approach proposed here to image oxbows is then very promising. Abbreviations: GW = groundwater; RB = right bank; LB = left bank; T = water temperature

Figure 1 :

 1 Figure 1: a) Location of the Auzon Oxbow; b) Location of the

Figure 2 :

 2 Figure 2: Time-series of a) Daily precipitation at Sainte Florine

Figure 3 :

 3 Figure 3: Piezometric maps a) of the Low Flow (LF) event of the

Figure 5 :

 5 Figure 5: Chemical water types of the 21 sampling sites given by a

Figure 6 :

 6 Figure6: HCO3 -vs Na + /Ca 2+ (mg.l -1 ) for Allier River, RB GW (mean

Figure 7 :

 7 Figure 7: Cartography of EC performed on the 11/23/2017.

Figure 8 :

 8 Figure 8: Piper's diagram established from data in mg.l -1 of the 21

Figure 9 :

 9 Figure 9: ERT profile crossing the active paleochannel (blue circle)

Figure 10

 10 Figure 10: a) δ 18 O time-series for Allier River and Auzon Oxbow

Figure 11 :

 11 Figure 11: Conceptual model of the Auzon Oxbow: identification

[07/22/2014 -11/24/2017]

	Business Center software (version 3.8.) and adjusted according
	to the dimensions of the boreholes in order to calculate level
	variations through time in the same reference of altitude
	(Cunningham and Schalk, 2011). The reliability of the
	continuous piezometric levels monitoring was checked and
	validated through manual measurements carried out during
	the 40 sampling campaigns: correlation coefficients between
	data recorded and manually measured (r² from 0.92 to 0.99)
	testify for a good representativeness of the piezometric level
	In order to assess the hydrodynamic behavior of the time-series.
	oxbow, the potential water sources entering the system have 3.2. Water chemistry monitoring
	been investigated in terms of hydrodynamic and Forty sampling campaigns have been performed on the 21
	hydrochemistry. Surface water level of the Allier River at the sampling sites on a monthly basis from July 2014 to November
	confluence with the Auzon Oxbow and water table in the 6 2017, except in February 2017 when the alluvial plain was
	observation boreholes have been recorded on an hourly basis flooded. A few data are also missing for P7, because of small
	with an in situ Level Troll 500 probe. All water level data have floods or vandalism. Prior to sampling, bottles have been
	been corrected from the barometric pressure variations. The rinsed two times by using the water to be sampled. Surface
	hydrological station was located on LB prior to the 11/05/2015 waters and Le Monteil spring were directly sampled using
	(A1, Figure 1c) and then moved to RB (A2, Figure 1c) because sampling vials. Groundwater was sampled using a Cole Parmer
	of a too important erosion of the left bank during previous Masterflex I/S pump for near-surface piezometers, while it was
	floods. Geographical coordinates and elevation of each a 12 V standard PVC narrow diameter submersible pump for
	pressure sensor were measured using the centimetric Trimble the observation boreholes. For each groundwater sampling,
	R10 DGPS whose data was post-processed with Trimble stagnating water present into the borehole was renewed
	Abbreviations: GW = groundwater; RB = right bank; LB = left
	bank; T = water temperature

Table 1 Figure

 1

	4
	4.1.3. Hydrochemical data
	Hydrochemical data of Table 1 have been plotted on a
	Piper diagram (

Table 1 :

 1 Average, minimum, maximum and standard deviation (SD) values of physico-chemical parameters, 24 ions concentrations and δ 18 O-δ 2 H data for the 40 campaigns carried out at the 21 samples sites of the 25 study zone. 26 NO3 -NO2 -PO4 3-F -Na + Ca 2+ Mg 2+ K + NH4 + Li +

	3-	F -	Na +	Ca 2+ Mg 2+	K +	NH4 +	Li +	δ 2 H	δ 18 O

57

Abbreviations: GW = groundwater; RB = right bank; LB = left bank; T = water temperature

Table 2 :

 2 Physico-chemical parameters and ions concentrations data for the 11/23-11/24/2017 campaign. 30

Acknowledgments

The study was funded by the European Regional Development Fund (ERDF 2014(ERDF -2020) ) through the CPER project "Les phytosanitaires du champ à l'assiette". The authors also thank the Auvergne-Rhône-Alpes Region for its financial support.

Besides, the study was carried out in an interdisciplinary setting under the aegis of the FR Environnement. Thanks also to the farmers of the study area who authorized the observation boreholes installation on their fields. 57 Abbreviations: GW = groundwater; RB = right bank; LB = left bank; T = water temperature