S15

In a main tributary of the semi-arid Elqui valley called the Estero Derecho in North-Central Chile, peatlands role is essential because vegetation that grow on is a key resource for biodiversity and livestock, especially at the end of the summer where the only water source is rock glaciers and aquifers connecting glaciers to streams. Thus, characterizing the structure and functioning of the peatlands, called bofedales there, is important for vegetation development as well as improving the understanding of its hydrogeological role. The subterranean geometry and stratigraphy of one peatland at about 3000masl elevation was characterized in north-central Chile using Electrical Resistivity Tomography (ERT), Ground Penetrating Radar (GPR) and core extraction. Two sounding locations, two transversal and one longitudinal profile allowed a 3D interpretation of the bofedal’s internal structure. A conceptual model of the current bofedal system is proposed. Geophysical results combined with porosity measurements were used to estimate the bofedal water storage capacity.  The second objective is to better understand the water exchanges between the peatland and the streamflow using discharge measurements and water stable isotopes. Water and isotopes budgets highlight a streamflow loss towards the peatland groundwater reservoir at the end of spring season. In addition, ten δ¹⁸O and δ²H surveys were used to characterize the distinct wetland water sources and their temporal variations. Using hydrological data at the watershed scale, implications regarding the hydrological role of bofedales in the semiarid Andes were then briefly assessed. At the catchment scale, bofedal water storage capacity, evapotranspiration losses and annual streamflow are on the same order of magnitude. High-altitude peatlands are therefore storing a significant amount of water and their impact on basin hydrology should be investigated further.

Figure 1. Topography of the Estero Derecho valley following the Elqui River from the ocean to the highest point at the headwaters.

Figure 2. Inclined aerial photography with vertical exaggeration. Yellow stars correspond to watersample locations. P1–P4 are the geophysical profiles.

Figure 3. Schematic cross-section of the studied peatland showing water surface fluxes , groundwater fluxes , and atmospheric fluxes

How to cite: valois, R., schaffer, N., and macdonell, S.: Water storage capacity and hydrological role of andean peatlands in the semi-arid Chile, IAHS-AISH Scientific Assembly 2022, Montpellier, France, 29 May–3 Jun 2022, IAHS2022-368, https://doi.org/10.5194/iahs2022-368, 2022.

Palmiet wetlands in South Africa perform a range of supporting, and regulating ecosystem services, including hydrological benefits such as flood attenuation and sustaining baseflows. Despite their importance in regulating the highly seasonal and somewhat flash flood type nature of the region’s hydrology, there remains uncertainty around the reasons for their occurrence in such a limited geographical extent (Eastern and Western Cape). Prionium serratum (Palmiet) is a robust perennial plant that is endemic to wetlands in the sandstones and quartzites of the Table Mountain Group (TMG). Palmiet wetlands are collapsing across the distributional range of this species, largely as a consequence of gully erosion that may or may not have been initiated by human activities.

Despite large investments by the State in restoration, serious knowledge gaps in our understanding of Palmiet wetland structure and function remain. This project identified that palmiet wetlands are sustained by significant amounts of sub-surface water (groundwater and interflow) moving through preferential flow paths in the alluvial fans, which are in turn sustained by groundwater discharge from the surrounding sandstones of the TMG. This conceptual model indicates that a consistent water supply is a palmiet system driver and a key component of palmiet wetland formation. The wetlands clearly retain a significant amount of water, leading to the maintenance of prolonged flows, and a larger baseflow, however the wetland is present due to the existence of the low flows and is not the primary driver of prolonged flows in the system. The work further identified the palmiet plants ability to control water loss through stomatal control, with highly variable but predictable responses to the localised climate systems apparent.

Geomorphology is another clear system driver. This aspect of the study showed that gully erosion leads to longitudinal slope reduction and valley widening in ways that can lead to wetland formation. Dating of sediment from these gullies identified sediment ages of 470 and 7060 BP indicating that these gullies were a part of system functioning prior to the introduction of European farming. The erosion was identified as key to establishing new wetland habitats downstream of the eroded wetland reach.

How to cite: Tanner, J., Smith, C., Ellery, F., and Glenday, J.: Investigating hydrological and geomorphological functioning of the mysterious endemic Palmiet wetlands in South Africa., IAHS-AISH Scientific Assembly 2022, Montpellier, France, 29 May–3 Jun 2022, IAHS2022-691, https://doi.org/10.5194/iahs2022-691, 2022.

The management of hydropower plants influences the variations in the discharge of the rivers, introducing for example weekly fluctuations. This variability can in turn influence the interaction between surface water and groundwater. In our work, we analyse an alpine area in Trentino in northern Italy, where four different river sections are considered. The four reaches (Noce, Adige before the confluence with the Noce, Adige after the confluence with the Noce, Avisio) are differently affected by reservoir operation. We hence study the influence of surface water management on the aquifer by using a transient model built with Modflow-OWHM for the hydrological years 2009/10 and 2016/17, where the year 2016/17 was characterised by particularly low amount of precipitation. The results were interpreted using continuous wavelet analysis and wavelet coherence analysis. In particular, we focused on the non-stationary weekly variations observed in the groundwater table. Such fluctuations are very pronounced near the river sections affected by reservoir operation. Moreover, the comparison of the behaviour in 2009/10 and in 2016/17 shows that the weekly variations are stronger in dry years than in wet years. Nevertheless, the proportion of the aquifer that shows a strong weekly variability remains similar. The water budget along the four river sections does not change significantly in the two hydrological years considered. However, our wavelet analysis shows that in 2016/17 a strong weekly variability is observed in surface water-groundwater exchange while in 2009/10 this dynamic is not observed.

How to cite: Chiogna, G., Basilio Hazas, M., Marcolini, G., Singh, T., and Wohlmuth, B.: A wavelet analysis of surface water – groundwater interaction in reaches affected by hydropeaking, IAHS-AISH Scientific Assembly 2022, Montpellier, France, 29 May–3 Jun 2022, IAHS2022-711, https://doi.org/10.5194/iahs2022-711, 2022.

Travel time distribution allows for an effective representation of all the physical processes occurring in hydrologic system and provides a stronger link between catchment-scale water flow and water quality models. The travel time distribution TTD has been extensively used in hydrological modeling and several approaches, with different level of complexity, have been developed in past.

The aim of our research is to investigate the travel time distribution in river catchments, specifically we would disentangle the relative contributions of the hillslopes and the river network in determining the TTD at the catchment outlet. Catchment is represented as a set of hillslopes connected to the river network; the effective recharge, moves first within the hillslopes and then is conveyed by the river network up to the outlet control section. Our theoretical framework combines a Boussinesq hillslope flow model with a GIUH-based formulation of transport along the river network. The TTD is characterized by the first two spatial moments of the travel times of the water particles crossing, at a given time, the catchment outlet section. Despite the assumptions required by the analytical development, the proposed approach is built on key physical processes and can be used to analyze the complexity governing the water age distributions and the travel time in catchments.

The analysis of a 10-year long series of synthetic recharge indicates that:

• the travel time moments strongly depend on the hillslope flow dynamics, as a consequence transmissivity is a key parameter;
• transport within river exerts a secondary effect;
• the sequence of wet and dry periods, is also of primary importance in determining the travel time moments and their temporal fluctuations;
• the variability of the travel times at the outlet, at a given time, is generally very large, with coefficient of variation typically larger than unity.

How to cite: Fiori, A., Zarlenga, A., and Cvetkovic, V.: Flow and transport at the catchment scale: hillslopes vs river network who plays the leading role?, IAHS-AISH Scientific Assembly 2022, Montpellier, France, 29 May–3 Jun 2022, IAHS2022-377, https://doi.org/10.5194/iahs2022-377, 2022.

The alluvial aquifer of the transnational Roya watershed is exploited for drinking water to supply both French and Italian coastal cities. The Interreg project CONCERT-EAUX (2017-2020) aimed to improve the understanding of the functioning of this hydrosystem. Several wells were equipped in order to measure water level, temperature and conductivity temporal and spatial distribution within the aquifer. Two surface and groundwater sampling campaigns were also carried out to perform chemical and isotopic water analysis.

Near the Porra wellfield, vertical distribution of parameters proved the homogeneity of the alluvial formation in the first layer, and very different properties in the underlying less permeable alluvions. In the upper layer, temperature shows seasonal variations, whereas it is very stable in the underlying aquitard. The existence of advective transfers between the Roya River and the alluvial aquifer was proved both by the thermal transfer analysis and modelling, and by the isotope study. The CFCs and SF6 concentrations in groundwater confirmed that the aquifer is recharged with recent water. Thanks to meteorological and water level daily data, the aquifer recharge was estimated applying two different approaches. The results proved that for an annual average rainfall, the infiltration of the effective precipitation represents only 15 to 20% of the total aquifer recharge. The main part of the aquifer recharge comes directly from the Roya River.

A hydrogeological numerical model including the exchanges between the river and the aquifer has been developed using the finite difference model MARTHE©. Run in steady-state conditions, it allowed completing the water budget. It also highlighted the sections of the river which most contribute to the aquifer recharge.

All these elements helped us to understand the behaviour of the hydrosystem following the Alex storm in October 2020. An important volume of sediment has been transported by the river from the upper valley to the downstream reach. This lead to the clogging of the riverbed, which strongly reduced the aquifer recharge. Today, although trenches have been excavated immediately upstream of the wellfield to promote the artificial recharge of the aquifer by the Roya River, groundwater level still decreases due to pumping.

How to cite: Lanini, S., Ladouche, B., Dewandel, B., Ibba, M., Bailly-Comte, V., and Genevier, M.: Impact of the Alex storm on the exchanges between the Roya River and its alluvial aquifer, IAHS-AISH Scientific Assembly 2022, Montpellier, France, 29 May–3 Jun 2022, IAHS2022-55, https://doi.org/10.5194/iahs2022-55, 2022.

Fractures are fundamental discontinuities to understand the exchange and the dynamic of water, matter and energy between the different parts of the critical zone. But except from boreholes and direct observation at rocks outcrops, imaging fracture network from the surface remain a challenging task in geosciences. Classical geophysical methods and their associated imaging methods are in a large extend adapted to retrieve the properties and (part of) the heterogeneities of the rock matrix and not of the discontinuities.

The aim of the study is to present a stochastic imaging method based on the direct simulation of Electrical Resistivity Tomography (ERT) dataset from Discrete Fracture Network (DFN). ERT is one of most used geophysical method in critical zone studies. And one can find numerous examples in literature of interpretation of ERT images as fracture zone, even if the inversion do not have the resolution to retrieve the fracture geometry or properties.

First the direct approach is described (from Roubinet et al., 2014) to simulate ERT dataset. Then from simple cases such one vertical or horizontal fracture, the inversion is detailed with emphasis on the convergence rate, resolution and depth of investigation. Further step towards the inversion of realistic DFN are discussed: how prior information can be used to enhance the convergence rate and how the non-unicity of the solutions can be retrieved from the stochastic inversion scheme. The inversion of DFN from ERT dataset is a first step towards coupled inversion such as pumping tests or tracers tests and ERT.

How to cite: Champollion, C. and Roubinet, D.: Discrete Fracture Network geophysical imaging  from Electrical Resistivity Methods, IAHS-AISH Scientific Assembly 2022, Montpellier, France, 29 May–3 Jun 2022, IAHS2022-390, https://doi.org/10.5194/iahs2022-390, 2022.

The creation of geological and sedimentological models is a non-unique process due to the many choices that must be made during their conception. The choice of the geological and sedimentological depositional concepts, the decision regarding the modeling strategy (stochastic or deterministic), the type of algorithm used for the simulations (MPS, SGS, Kriging, TGS…) are all factors that influence the final models and the corresponding uncertainty quantification.

Regarding the coastal sedimentary Roussillon aquifer (south of France), different sets of geological models have been created and are used for the hydrogeological simulations; homogeneous permeability model, depth relative permeability model, and stochastic models. 

The homogeneous and depth relative models are created based on the large structural envelopes delimiting the main aquifers. The stochastic models are also based on these envelopes but simulate inside complex sedimentary facies and permeability fields using the multiple-point statistics algorithm DeeSse. Different geological concepts were tested depending on the different representations and interpretations of the possible spatial distribution of the sediments. Each model has its own strengths and weaknesses. It is by combining these different approaches that we created an ensemble of geological models deemed to cover some conceptual uncertainty range.

These models are then used to feed hydrogeological simulations to create an ensemble of hydrogeological models and outputs that are used to characterize the hydrogeological uncertainty and help increase the robustness of the prevision. To do so, we developed an integrated process to convert the geological model to physical model and directly used them in hydrogeological simulations. The models run on MODFLOW, using the floPy python interface which helps the automation of the process. These models can then be calibrated in a semi-automatic manner using various inversion methods such as PEST, POPEX, or using ensemble Kalman filters.

How to cite: Dall'Alba, V., Schorpp, L., Lanini, S., Issautier, B., Lasseur, E., Renard, P., and Caballero, Y.: Ensemble of geological models of a complex coastal aquifer for uncertainty estimation and groundwater modelling in the framework of the DEM'EAUX ROUSSILLON project., IAHS-AISH Scientific Assembly 2022, Montpellier, France, 29 May–3 Jun 2022, IAHS2022-458, https://doi.org/10.5194/iahs2022-458, 2022.

En Côte d'Ivoire, plusieurs études ont été déclenchées sur les ressources en eau (Jourda, 1987 ; Janicot, 1990 ; Leduc et al., 2001). Cependant, ces études ont principalement commencé sur la partie émergée du bassin sédimentaire et rarement sur la partie immergée. Les compagnies pétrolières opérant dans cet environnement utilisent actuellement des techniques de forage dirigée et/ou de fracturation hydraulique pour augmenter la production. Cette pratique est très gourmande en eau car elle nécessite environ dix fois plus d'eau que le forage classique. Cela pose le problème de la gestion de cette ressource, qui n'était pas estimé dans le passé en raison de son caractère naturel, et qui est devenu partie intégrante de l'exploration pétrolière. Cette étude se propose donc de caractériser les aquifères du bassin immergé afin de mieux comprendre l'hydrogéologie de cette zone. La méthode utilisée au repos d'une partie sur la détermination de la nappe phréatique à partir de valeurs de diagraphie différée. D'autre part, elle repose sur des mesures des gradients de pression des formations géologiques. Des calculs géostatistiques sont ensuite effectués afin d'estimer la nappe phréatique sur une base régionale. Les résultats indiquant que seuls le Maestrichtien et l'Albien apparaissent en continu dans tous les puits forés. Ces puits traversent différents aquifères stratigraphiques allant du Maestrichtien à l'Albien. Ces aquifères sont sableux dans les puits A, D, G et H et gréseux dans les puits B, C, E et F. Leur profondeur est généralement comprise entre -2292 m et -2975 m selon la méthode des graphes de pression. Deux zones de rejet ont été identifiées, notamment dans les puits C et G. Les écoulements sont SW - NE et NW - SE à l'ouest du puits C et NE - SW et SE - NW à l'est du même point. Au puits G, les écoulements sont NW - SE et SW - NE d'une partie et NE - SW et SE - NW d'autre part, respectivement à l'ouest et à l'est de cette zone.

MOTS CLÉS : aquifère, puits de pétrole, bassin sédimentaire, diagraphie, gradients de pression.

How to cite: Kouamé Auguste, K., Adama, C., Francis William, K., and Eric, K.: Hydrogeology of oil wells in the coastal sedimentary basin of the Abidjan margin (Côte d’Ivoire), IAHS-AISH Scientific Assembly 2022, Montpellier, France, 29 May–3 Jun 2022, IAHS2022-278, https://doi.org/10.5194/iahs2022-278, 2022.

This study proposes a simple isotopic framework for the identification of riverbank filtration wells, using time-series of water’s stable isotopes and electrical conductivity at the watershed scale. Riverbank-filtration (RBF) is a widely used managed aquifer recharge method where the infiltration of surface water is induced by pumping groundwater at proximity of a river or lake, and where the quality of the raw water is controlled by the interaction between ground and surface water bodies. As inventories of RBF sites are rare, they limit the development of specific source protection strategies. In the framework of a project funded by the Ministry of the Environment and the Fight against Climate Change of Québec (MELCC, Canada), 40 municipal wells located at less than 500 meters from a surface water body were sampled for 18 months on a monthly to weekly basis. The underlying hypothesis was that a significant contribution of infiltrated surface water to a pumping well would propagate the temporal variations of the tracers observed in surface water. Results highlighted that 25% of the wells pumped a significant contribution of infiltrated surface water: 15% established a continuous connection during the whole hydrological year, while the other 10% revealed a seasonal connection linked to spring floods. All those wells were located less than 120 meters from the surface water and drilled less than 40 meters deep in a granular aquifer. As the number and strength of spring floods may increase in the future, the seasonally connected wells are particularly vulnerable to global change and might prefigure increasing climate forcing on drinking water supply. The remaining 75% were either evidenced as groundwater only (50%) or lacked continuity in the data acquisition (25%). A simple abacus based on the standard deviation of the tracer distribution was then proposed to facilitate the interpretation and make the methods accessible to all. Based on an affordable and easy to perform sampling protocol for water managers, this framework helps in the assessment of specific risks associated to mixed sources of water, and in anticipating variations in quality of the abstracted drinking water.

How to cite: Labelle, L., Baudron, P., and Barbecot, F.: Large-scale identification of riverbank filtration wells using an isotopic and geochemical approach, IAHS-AISH Scientific Assembly 2022, Montpellier, France, 29 May–3 Jun 2022, IAHS2022-501, https://doi.org/10.5194/iahs2022-501, 2022.

Understanding the interactions between river water and groundwater is crucial for the better management of water resource problems. The present study was carried out to assess the river water and groundwater interaction in parts of the lower Cauvery river basin. The samples were collected from river water and groundwater from May 2018 to August 2021, thrice in a year. The samples were analyzed for EC, pH, major, minor ions, trace elements, and isotopes. The hydrochemical facies shows that the water is of mixed Ca-Na-HCO₃ type followed by Na-Cl type in both river water and groundwater. The dominance of cations in river and groundwater is of order Na>Ca>Mg>K whereas the dominance of anions in river water was in the order of Cl>HCO₃ >SO₄ whereas it was HCO₃>Cl>SO₄ in groundwater. The rock water interaction was the dominant geochemical process followed by the evaporation process in both river water and groundwater. The temporal variation of major ions between river water and groundwater in a few locations indicates that they follow a similar pattern, thus indicating their interrelationships. δD and δ¹⁸ O isotopes shows that the river water and groundwater are in a mixing zone between recharge water and evaporation line during the wet season. During the dry season, few samples have been isotopically enriched due to the effect of evaporation, which indicates that the groundwater contributes to the river water. Therefore, it is evident that during the wet season, the river water recharges the groundwater zone and groundwater discharges into the river water during the dry season. Hence, this present study helps in better understanding of the river water and groundwater interaction through geochemical methods.

Keywords: River water, groundwater, interaction & isotopes.

How to cite: Lingaiah, K., Ramesh, R., and Lakshmanan, E.: Identification of river water and groundwater interaction using geochemical and isotopic indicators in the lower basin of the Cauvery river, IAHS-AISH Scientific Assembly 2022, Montpellier, France, 29 May–3 Jun 2022, IAHS2022-688, https://doi.org/10.5194/iahs2022-688, 2022.

The Thau hydrosystem lies at the interface between terrestrial and marine (lagoon) environments along the Mediterranean coast. Together with this set up, the karstic nature of this hydrosystem makes it subject to localized seawater intrusion phenomena in the Balaruc-les-Bains peninsula. This phenomenon is linked to a sudden flow reversal at the submarine Vise spring, during which saltwater from the Thau lagoon invades the karst aquifer in its discharge area. This process and the nature of the aquifer have strong impacts on the local field of hydraulic heads (sudden rise up to 2m high) as well as on groundwater quality (rise of salt concentrations close to seawater content). The inversion phenomena has already been detected 7 times over the last 50 years. Yet, our current understanding of the triggering mechanisms remains limited, notably because of a lack of precise monitoring of groundwater fluxes at the lagoon – aquifer interface. To address this issue a new monitoring network (observatory) was installed at the submarine Vise spring and across the hydrosystem. Here, we present the observatory and we show unique first results from the detailed monitoring of a flow reversal phenomenon, which started in November 2020. Submarine spring flow, electrical conductivity and groundwater head time-series gathered both offshore and on land at different depth allowed following: the sudden rise of the water level in the wells and changes of temperature and salinity associated with the transport of salt water in the karstic aquifer. Overall, this observatory provides valuable data needed to characterize and understand the processes triggering the flow reversal phenomenon in a hydrosystem at the terrestrial – marine interface.

How to cite: Hakoun, V., Ladouche, B., Lamotte, C., Dewandel, B., Maréchal, J.-C., and Séranne, M.: A closer look over the Thau hydrosystem, a new monitoring network to characterize and prevent seawater intrusion in the submarine Vise spring (Balaruc-les-Bains, France), IAHS-AISH Scientific Assembly 2022, Montpellier, France, 29 May–3 Jun 2022, IAHS2022-637, https://doi.org/10.5194/iahs2022-637, 2022.

The Balaruc hydrothermal system is fed both from surrounding karstic carbonates with fresh water outpouring in the nearby Thau lagoon with the Vise source, and at depth along deep regional faults with local springs up to 50°C. This hydrothermal system was cored and logged in 2020-2021 down to 765m depth at Balaruc-les-Bains, 200m to the NE of the Vise source. During the project, the Vise source underwent a reversal in November 2020, with the Thau lagoon salty water being drained deep into the subsurface, shedding light into the complex processes affecting the Balaruc hydrothermal system. The DEM’EAUX THAU project is aiming at a better understanding of this complex geological and hydrological system for a more sustainable use of this resource.

A set of downhole geophysical data and borehole wall images was recorded from near surface to 756m depth in 4 vertical holes, only a few meters apart at surface. While mm-scale images reveal the detailed geological structure, petrophysical data (acoustic velocities, electrical resistivity and natural gamma) contribute to better define the penetrated structure, yielding porosity and permeability. Acoustic velocities provide a base to analyze the vertical (VSP) and walk-away seismic profiles shot to replace these holes in the regional geological structure. In turn, core petrophysical measurements are being made to support these analyses and, in particular, to provide a dm-scale description of the subsurface pore fluid salinity.

In addition, the physical and chemical properties of the borehole fluid were characterized with an Idronaute probe, showing the impact of the Vise source reversal from measurements before and after. Similarly, electrical resistivity profiles were recorded over time and during downhole pumping tests, emphasizing the hydraulic vertical connectivity. On that basis, pore fluids dynamics are being described from time-lapse downhole logging measurements and the emplacement of permanent downhole geophysical observatories consisting in (i) an optical fiber for temperature and (ii) a flute for electrical resistivity of the formation. In the future, a second optical fiber already in place will be used for acoustic probing of subsurface fluid flow from Digital Acoustic Scanning (DAS).

How to cite: Pezard, P., Henry, G., Brun, L., Le Ber, E., Raad, F., Séranne, M., Widhen, F., Schleifer, A., Bellezza, C., Meneghini, F., Brillouet, N., Neyens, D., and Lamotte, C.: Petrophysical characterisation of a karstic carbonate aquifer, with implications for saltwater and brine intrusion into hydrothermal resources. The DEM'EAUX THAU project, Balaruc, France., IAHS-AISH Scientific Assembly 2022, Montpellier, France, 29 May–3 Jun 2022, IAHS2022-484, https://doi.org/10.5194/iahs2022-484, 2022.

More than 80 million m3 per year are pumped into the Roussillon plain coastal aquifer, covering 850 km² and located between the Pyrenean massif to the west and the Mediterranean Sea to the east, south of France. This is a multilayer aquifer of more than 350 m thick, made up of sandy layers embedded in low-permeability clayey material from the Pliocene and topped by alluvial formations from the Quaternary. Its groundwater resource is primarily used for the supply of drinking water, but also contributes to the irrigation of some 13,000 hectares.

For more than 40 years, this aquifer has been undergoing a general decline in its piezometric level due to pumping and water demand is expected to increase (growing irrigation areas and climatic demand). Moreover, given its flat topography, the Roussillon plain is likely to suffer sea water intrusions and marine submersion, due to the sea level rise, which could reach 1 m by 2100.

This context shaped the Dem'Eaux Roussillon project, which brought together nearly ten partners from the Occitanie region (research units, consultancies and local authorities). Its objective was to characterise the behaviour of the groundwater resource in this aquifer, in order to be able to project its future situation, in the context of climate change, rising sea levels (risk of saline intrusion) and changes in water use. A detailed characterisation of the geological reservoir highlighted the need to consider the offshore extension of this coastal aquifer. The analysis of the piezometric evolution at the scale of the Roussillon plain over the last 50 years allowed the spatialized characterization of the hydrodynamic parameters and the understanding of the vertical drainage processes that control the hydraulic equilibrium between the Quaternary and the Pliocene water tables. Two high-resolution hydro-geophysical observatories have been set up to quantify these processes and improve understanding of saline intrusions processes. Finally, a conceptual model presenting the main features of the main processes controlling the groundwater evolution and the sea water intrusion risk was obtained ready to launch a numerical modelling work.

How to cite: Caballero, Y., Duvail, C., Chazot, S., Bouchette, F., Dobricean, O., Balouin, Y., Berne, S., Bourrin, F., Champollion, C., Dall'Alba, V., Dewandel, B., Garin, P., Issautier, B., Ladouche, B., Lanini, S., Lofi, J., Montginoul, M., Neverre, N., Pezard, P., and Renard, P.: Transdisciplinary characterisation of a complex coastal aquifer, for a sustainable exploitation of its groundwater resources in a Mediterranean context. The DEM'EAUX ROUSSILLON project., IAHS-AISH Scientific Assembly 2022, Montpellier, France, 29 May–3 Jun 2022, IAHS2022-33, https://doi.org/10.5194/iahs2022-33, 2022.

The Plio-Quaternary aquifer of the Roussillon plain (multilayer aquifer of more than 350 m thick, made up of sandy layers embedded in low-permeability clay material from the Pliocene and topped by alluvial formations from the Quaternary) was extensively studied in the framework of the Dem’Eaux Roussillon project. Relying on a conceptual model of the whole groundwater system describing the main factors controlling the hydraulic behaviour of groundwater flow (recharge areas, surface-groundwater interactions, sea water intrusion risk,…) of Quaternary and Pliocene aquifers, a numerical modelling exercise of both onshore and offshore reservoirs was performed.

Two parallel modelling approaches sharing a common description of the reservoir geometries, the boundary conditions and the pumping withdrawals locations were carried out, using two (not so) different modelling tools. Results comparison illustrates the uncertainty associated to numerical methods and calibration strategies applied for each modelling tool. Climate change projections were used to explore the future evolution of the groundwater resource and results of both modelling tools were compared. An analytical formulation of the increase of the hydraulic head of the sea due to swell and waves close to the coastline was provided. The analysis of its influence on the groundwater dynamics simulated by the models highlights the potential influence of storms and future sea level rise on the risk of saline intrusions for coastal sedimentary aquifers along the Mediterranean basin.

How to cite: Lanini, S., Schorpp, L., Dall'Alba, V., Renard, P., Dewandel, B., Ladouche, B., Bouchette, F., and Caballero, Y.: Groundwater modelling of a complex multilayer coastal aquifer under climate change in the framework of the DEM'EAUX ROUSSILLON project., IAHS-AISH Scientific Assembly 2022, Montpellier, France, 29 May–3 Jun 2022, IAHS2022-395, https://doi.org/10.5194/iahs2022-395, 2022.

The Vise spring is the main outlet of a Jurassic karst aquifer located close to Montpellier city, South of France. The Vise spring is submarine, occurring at the bottom of the Thau lagoon at a depht of 30 m. The lagoon, made up of brackish water is connected to the Mediterranean sea. The fresh water from the karst aquifer as a whole, and especially from the spring, contributes to the qualitative state of the Thau lagoon which is well known for shellfish farming activities.

During the last fifty years (from 1967 to 2014), six occasional saltwater intrusions (called “inversac” in French) occurred, inversing the water flow at the submarine spring during a period varying from a few weeks to a few months. This backflooding process at the spring induces a very large saltwater intrusion into the karst aquifer. Given that this aquifer provides several highly important ecosystem services (drinking water supply for the coastal villages, fresh water to the Lagoon, thermal water to Balaruc spa and health resort), a large program of groundwater monitoring has been recently launched particularly at the submarine spring (discharge, temperature, quality) and several neighbouring borewells.

In november 2021, a seventh backflooding event  started and is observed with the new monitoring system. From an initial flow rate of about 60 l/s from the aquifer to the lagoon through the spring, the flow inverted to about 350 l/s from the lagoon to the aquifer in a few minutes on the 28^th November 2020 at 9:40 AM. This sudden backflooding created a sudden water level rise of about 2.5 meters into the karst confined aquifer. One year later, the saltwater intrusion into the aquifer is still high, equal to 150 l/s.

A physical mechanism is proposed to explain the sudden inversion of flow and its long duration after that the event has started. The propagation of a piezometric wave through the aquifer is simulated using simple analytical solutions. A preliminary meshed model of the aquifer and its interactions with the lagoon has been developed for identifying the main processes. First attempts for proposing alert indicators are also discussed.

How to cite: Maréchal, J. C., Ladouche, B., Lamotte, C., Dewandel, B., Hakoun, V., and Perrcohet, P.: Flow reversal and saltwater intrusion at the submarine spring of a Mediterranean karst aquifer : explanation and modelling, IAHS-AISH Scientific Assembly 2022, Montpellier, France, 29 May–3 Jun 2022, IAHS2022-530, https://doi.org/10.5194/iahs2022-530, 2022.

The detailed hydrogeophysical characterization of coastal aquifers provides a base to study both submarine groundwater discharge and salt water intrusion processes. With the objective to investigate the response of a coastal aquifer to a series of boundary conditions, a new experimental site was developed through a clastic aquifer located 30 km north of Barcelona (Spain). This hectometer scale site is located 50 m from the seashore and equipped with 17 nearby shallow holes, with depths ranging from 15 to 28 m.   

A series of innovative downhole geophysical measurements and experiments have been deployed over the past few years, either in a time-lapse or stationary manner. These measurements are performed through PVC due to the unconsolidated nature of the sediment. Also, the granitic mineralogy prevents clays identification from a direct use of gamma ray profiles. In time lapse, high frequency electrical resistivity induction measurements show that preferential flow paths are identified in a fast and reliable manner. Also, changes in depth of the fresh to salt water interface (FSWI) are precisely described, either in response to marine tides, or to short lived but often intense Mediterranean rain event. Changes on the order of 1.70 m are obtain in a few hours of heavy rain. Overnight as well as seasonal changes such as months of dryness are also illustrated due to the local variability of pore fluid salinity and temperature, even over short periods of time such as tens of minutes.

For a more continuous description of downhole processes over time and depth, downhole geophysical observatories were emplaced in early 2017. Intense rain events such as that of October 19, 2017 or January 21, 2020 reveal the high frequency interplay between sea water intrusion and fresh water outpour into the sea.

How to cite: Pezard, P., Henry, G., Brun, L., Gee, R., Folch, A., Goyetche, T., Martinez, L., Carrera, J., and Luquot, L.: Innovative downhole geophysical methods, petrophysical characterization and high frequency downhole monitoring of seawater intrusion dynamics in a coastal aquifer. The Me Distraes project, Argentona, Spain., IAHS-AISH Scientific Assembly 2022, Montpellier, France, 29 May–3 Jun 2022, IAHS2022-679, https://doi.org/10.5194/iahs2022-679, 2022.

Catchment fluxes are the main vector of nutrients to marine ecosystems. Most of the time research focuses on regional catchments and their nutrient contribution toward estuaries, deltas or lagoons. Fluxes coming from coastal watersheds with smaller size are under-investigated, especially micro-estuaries, in spite of their predominance in many coastal areas and particularly in the Mediterranean.

The catchment-micro-estuary continuum has a strong seasonal variation dynamic controlled by transversal water and nutrients fluxes through the watershed, estuary and coastal zone.

Identifying and quantifying the water, inorganic and organic fluxes is unavoidable to understand the hydro-ecological behaviour of the micro-estuaries. For this purpose, the Fiume Santu watershed located on the Mediterranean Island of Corsica (France) has been investigated with a hydro-ecological approach based on the crossing-use of isotopic, chemical and nutrient tracers.

The study was carried out in a humid period in February 2018 and investigations involved a multi-tracer methodology, namely major ions and traces elements, stable isotopes of water, carbon isotope ratios and concentrations of DIC as well as measurements of DIM and DOM. Eight sampling sites were chosen along the estuary from top to bottom: four along the river, three for groundwater and one in open-sea water.

Results highlight hydrological processes in the down part of the catchment which are constrained by the tidal phenomena stopping temporarily the freshwater and nutrient circulation. The catchment fluxes, the chemical and biological intrinsic processes (remineralization, sediment uptake and release) and the marsh inputs constitute sources of nutrients along the stream and induce non-conservative budgets. The role of groundwater and subsurface water is significant in terms of nutrient fluxes even their discharge contribution is neglected.

This first mutual tracer-approach in the hydrological functioning and the ecological behaviour is unique in terms of the quantification of nutrient fluxes in a pristine coastal site and is the first step toward a transposition of the methodology to other Mediterranean sites.

How to cite: Garel, E., Dufresne, C., Santoni, S., Huneau, F., and Malet, N.: Role of the riparian groundwater in a pristine Mediterranean micro-estuary catchment to quantify nutrient fluxes, IAHS-AISH Scientific Assembly 2022, Montpellier, France, 29 May–3 Jun 2022, IAHS2022-646, https://doi.org/10.5194/iahs2022-646, 2022.