Hysteresis of submarine karst springs reveals tipping points in flow reversal and saline intrusion phenomena Jean-Christophe Maréchal, Bernard Ladouche, Claudine Lamotte, Benoit Dewandel, Vivien Hakoun, et al. Communications Earth and Environment, 2025 Submarine springs that drain inland karst aquifers may be subject to abrupt flow reversals in which alternatively freshwater outflows to the sea or saltwater inflows into the aquifer. Here we provide a full flow reversal long-duration data time series at the Vise spring (below Thau lagoon, south of France) during which saltwater inflows into the aquifer. We demonstrate that the driving parameter of the hydrosystem regime shifts is the hydraulic gradient between the aquifer and the lagoon, controlled by water density changes. We reveal the existence of two tipping points: (i) below a hydraulic gradient threshold, the hydrosystem suddenly degrades from a normal regime to saltwater intrusion, and (ii) a much larger hydraulic gradient is necessary for the recovery of the hydrosystem. The high hysteresis of the hydrosystem, due to the vertical karst conduit filling by saltwater, is responsible for the long duration of the degradation and saltwater intrusion. In a changing climate context, flow reversal at submarine karst springs could be more frequent and longer in the future due to sea level rise and a decrease in recharge, threatening inland or offshore freshwater resources.
Impact of the storm Alex on water exchanges between the Roya River and its alluvial aquifer Sandra Lanini, Bernard Ladouche, Benoit Dewandel, Melissande Ibba, Vincent Bailly-Comte, et al. Proceedings of the International Association of Hydrological Sciences, 2024 The alluvial aquifer of the transnational Roya River watershed is an important water resource for drinking water supply. Through successive European projects, a monitoring network has been implemented over the alluvial plain to improve the understanding of the functioning of this aquifer. For instance, studies highlighted the predominant role of surface water in the recharge of the aquifer. Following the storm Alex and the resulting exceptional flood event in the Roya valley in October 2020, a general decrease of the piezometric levels was observed in the alluvial aquifer. Changes in the river morphology and in the granulometry of the hyporheic zone have impacted surface water – groundwater exchanges and reduced the aquifer recharge.
XLKarst, an Excel tool for time series analysis, spring recession curve analysis and classification of karst aquifers V. Bailly-Comte, B. Ladouche, J. B. Charlier, V. Hakoun, J. C. Maréchal Hydrogeology Journal, 2023 Karst aquifers are complex hydrogeological systems that require numerous in-situ measurements of hydrological and physico-chemical parameters to characterize transfer processes from the recharge area to the karst spring. Numerous graphical, statistical or signal processing methods have been developed for decades to interpret these measurements, but there is no simple and standardized tool that can be used for this purpose, which is necessary for a rigorous comparison of results between case studies. This Technical Note presents XLKarst, which has been developed to provide a simple and easy-to-use tool to process a selection of proven methods that characterize the functioning of karst systems. This tool allows (i) time series analysis based on correlation and spectral analysis and, for flow measurements, the use of other statistics and base flow separation, (ii) calculation of the cumulative distribution function to build a spring flow probability plot, and (iii) analysis of spring flow recession and expression of the results in a karst system classification scheme. These methods are first described by providing the key elements of their use and interpretation in the scientific literature. Then, an application to the Fontaine de Nîmes karst system (southern France) is used to highlight the complementarity of the methods proposed by XLKarst to describe the hydrodynamic behavior of a karst system based on daily data of rainfall and discharge over 22 years.
Middle Risle River Critical Zone Observatory: monitoring karst-process evolution and its impact on surface water and on aquatic ecosystems Pierre-Yann David, Didier Pennequin, Jean-Baptiste Charlier, Bernard Ladouche, Baptiste Meire, et al. Geological Society Special Publication, 2023 Rivers in karstic environments are known to be greatly influenced by surface water–groundwater interactions, with significant localized inflows during floods from springs, or with losses that can dry up rivers. The Middle Risle River is frequently affected by the development of sinkholes in a chalk karst area (Normandy, France). In the 2010s, two new major sinkholes in the riverbed caused a complete loss of water into the underlying phreatic aquifer, causing the river to dry up over several kilometres. The resulting changes in hydrogeological processes and surface water–groundwater interaction greatly affected water quality, water use and water-dependent ecosystems, causing a political crisis in this river-dependent touristic valley. To understand these phenomena and improve crisis management, the Middle Risle Critical Zone Observatory was set up to enhance monitoring, surveying and/or modelling of groundwater and river levels, river and spring flow, water temperature and conductivity, and ecosystem characteristics (fish, macro-invertebrates and vegetation). The results showed notable impacts on fish, macro-invertebrates and vegetation, some plants proving to be reliable indicators of surface-water–groundwater interaction. The dynamics of local hydrogeological processes were assessed and linked to the measured effects on ecosystems and water supply. Inverse modelling based on an analytical solution of the diffusive wave equation assessed lateral flow during floods, quantifying the spatial–temporal variability of surface-water and groundwater exchanges. It also highlighted the important role of karst zones in both storage and flood-peak attenuation processes, thereby protecting downstream villages against floods.
Long-term evolution of a carbonate reservoir submitted to fresh, saline and thermal waters interactions- Jurassic carbonates in the coastal area of the Gulf of Lion margin (southern France) Florian Widhen, Michel Séranne, Grégory Ballas, Pierre Labaume, Erwan Le-Ber, et al. Bsgf Earth Sciences Bulletin, 2023 Securing and managing underground water resources requires a good knowledge of the structure, texture and connections of the reservoir, in order to develop realistic and reliable hydrogeological models. On the coastline of the Gulf of Lion Margin (S. France), the Balaruc-les-Bains deep karst reservoir is subjected to interactions between fresh, marine and deep thermal waters, respectively. Water resource usage for drinking, spa resort, and fish-farming raises important economic and social issues. These were addressed by an integrated research program, involving drilling of an exploratory borehole across the Jurassic carbonate reservoir. This contribution analyses the 750 m cores, in order to (i) characterise the architecture and evolution of the karst reservoir and (ii) investigate the paleo-fluids circulations, witnessed by calcite and dolomite mineralization in the fractures, karst cavities, and as cement of tectonic beccia. The structure of the reservoir is characterised by the superposition of several aquifers separated by marly intervals. At shallow level, the initial grainstone is incompletely dolomitized in metre-thick intervals, while limestone in the 210–340 m interval was completely dolomitized at an early stage. Dolomite has been subjected to penetrative extensional cataclastic deformation, while the preserved limestone is affected by normal faulting, resulting from NNE–SSW extension. Distinct types of karsts have been documented, from the top of the reservoir (paleo-lapiaz filled with Burdigalian marine marls), down to 500 m depth (paleo-endokarst filled with continental silts). The upper reservoir (75–150 m) is intensely karstified, and includes 0.1 to 1 m-wide cavities, where present day water fluxes are documented. Analyses of calcite and dolomite crystallisation under natural light and cathodoluminescence indicate precipitation from distinct fluids: formation water in chemical equilibrium with the host rock, water rich in oxides and hydroxides, ascending hydrothermal fluid and corrosive water of meteoric origin. Alternate dolomitization and calcitization observed in the upper reservoir suggests alternate flows of karstic freshwater and marine salt-water. Vertical, metre-long and centimetre wide open cracks are presently used for large water flows; several generations of syntaxial calcite growth provide evidence for varying chemistry of the circulating fluids. Structural cross cutting relationships allowed us to establish a relative chronology of events, which can be correlated with the regional geodynamic evolution. The study reveals that the present-day reservoir architecture results from the superimposition of structures formed during the Early Cretaceous extension, Maastrichtian-Eocene Pyrenean shortening, and Oligocene rifting of the Gulf of Lion. The reservoir was also shaped by successive karstification episodes and marine transgressions. Although the present-day hydrological system is controlled by, and reactivates structures inherited from a long-term evolution, it is characterised by frequent turn-overs of the water flow, tuned by high-frequency external forcings such as sea-level changes driven by Pleistocene glacio-eustasy, or varying precipitation rates.
A lumped parameter model to evaluate the relevance of excess air as a tracer of exchanged flows between transmissive and capacitive compartments of karst systems Vianney Sivelle, Laïna Pérotin, Bernard Ladouche, Véronique de Montety, Vincent Bailly-Comte, et al. Frontiers in Water, 2022 The objective of this study was to evaluate the relevance of using excess air (EA) for the characterization of drain/matrix exchange in karst systems using a rainfall discharge model coupled with the simulation of EA measured at the outlet of the studied system. The conceptual model assumes a linear relationship between the formation of EA and the increase of hydrostatic pressure in the capacitive part of the aquifer. The simulated EA at the spring consists of the mixing of water circulating in the different compartments of the aquifer, with their own EA signature. The analysis is performed taking as an example the Durzon karst system (Larzac, France). The modeling is applied using daily rainfall discharge time series and 18 EA measurements at the main outlet of the karst system within 3 hydrological cycles. The main modeling results show that EA variations measured at the karst spring can be explained by recharge processes and exchange between conduit and matrix. EA measurements at the spring thus contain valuable information about the flow dynamics within the aquifer. Furthermore, results show that the use of EA measurements, despite their sparse temporal resolution, allows for reducing uncertainties in the estimation of some parameters of the reservoir model used for the simulation of karst spring discharge.
Improving hydrogeological understanding through well-test interpretation by diagnostic plot and modelling: a case study in an alluvial aquifer in France Thibaut Garin, Bruno Arfib, Bernard Ladouche, Julio Goncalves, Benoit Dewandel Hydrogeology Journal, 2022 The study of groundwater resources using pumping test data is usually carried out with the Theis solution, which enables the hydraulic parameters of porous aquifers such as the transmissivity and storage coefficient, to be estimated from the water-level drawdown. However, the data fitting can fail and provide only an indication that the pumped aquifer has a complex structure. Here, a diagnostic plot on log-derivative drawdown is used to identify flow regimes and thus aquifer heterogeneities, leading to plausible conceptual models. Nevertheless, the diagnostic plot is insufficient and must be accompanied by further modelling because of the nonuniqueness of the drawdown log-derivative signal. The proposed approach is applied to an alluvial plain in France, known to be complex because the deposition processes change over time, resulting in channel belts limited by low-permeability deposits in the floodplain or three-dimensional (3D) interconnected structures. Six analytical models were used to simulate drawdown and its derivatives during a three-day transient pumping test. The diagnostic performed on the pumping well showed that four conceptual models, with highly contrasted hydrodynamic behaviours, may correspond to the diagnostic. The joint use of pumping-well and observation-well data allowed the only appropriate model to be identified—a dual-permeability model characterizing a multilayer aquifer. The conceptual model matched the geological observations in boreholes and corroborates the fluvial sequence stratigraphy of the alluvial plain. The pumping test used here is a tool to explore the 3D architecture of the fluvial reservoir at the scale of the depositional sequence in the floodplain.
