Anthropogenic activities and climate change exert significant pressures on the quality and availability of water resources in karst environments, which supply drinking water to about 9.2% of the world's population. Increasing temperatures and changes in precipitation regimes will strongly impact water recharge processes. Understanding the karst hydrodynamic behaviour in the present context of climate change constitutes a major challenge for a sustainable management of karst groundwater. This study focuses on the Mediterranean area, where up to 90% of the drinking water supply depends on carbonate aquifers. The spring discharge and/or water level of six karst systems in the Mediterranean area (France, Italy, Lebanon, Slovenia, Spain and Tunisia) are simulated using precipitation-discharge reservoir modelling tools. The studied karst systems are well known and have different characteristics in terms of climatic conditions, hydrogeological properties and available data. Using different model structures, the hydrological models are first calibrated and validated over a historical period and then used to simulate spring discharge time series under various climate projections (up to 2100). To account for uncertainties in climate projection, 12 coupled GCM/RCM climate models are considered with two emission scenarios (RCP 4.5 and RCP 8.5) proposed in the framework of the CMIP5 initiative. The analysis of the forecasted spring discharge and water level time series focuses on (i) the long-term trends in the hydrological functioning of karst systems, (ii) the effects of climate change on spring discharges (intensity and duration of extreme events), and (iii) the study of uncertainties related to the exceedance of the known functioning ranges of the systems. Further discussion is also dedicated to model uncertainties in relation to model parameters and structure, climate models, and the estimation of potential evapotranspiration in future climate. This research has been conducted within the KARMA (Karst Aquifer Resources availability and quality in the Mediterranean Area) project into the PRIMA (Partnership for Research and Innovation in the Mediterranean Area) EU program.

How to cite: Cinkus, G., Sivelle, V., Jourde, H., Mazzilli, N., Tramblay, Y., Andreo, B., Barberá, J. A., Bouhlila, R., Doummar, J., Fernández-Ortega, J., Gargouri-Ellouze, E., Lorenzi, V., Petitta, M., Ravbar, N., Slama, F., and Goldscheider, N.: Impact of climate change on groundwater level dynamics and karst spring discharge of several karst systems in the Mediterranean area, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12285, https://doi.org/10.5194/egusphere-egu23-12285, 2023.

Lumped parameter modeling in karst hydrology has been widely developped in the last decades. Uncertainty in model conceptualization, which often leads to one unique model structure, is frequently neglected. This issue is particularly important for karst hydrology, where hydrological systems are highly heterogenous and information about the structure is difficult to obtain. In this work, we delopped a Bayseian multi-model framework that allows to calibrate simulteanously a set of model structure and associated parameters. This constitutes a significant step forward compared with classical calibration approaches that allow (i) to provides an ensemble of predictions considering both structural and parametric uncertainties, and (ii) to avoid epistemic error due to model structure selection, wich is generally influenced by the subjective conceptualization of the karst hydrological system by the modeler. The methodology is illustred with a Bayesian inference procedure among a large range of lumped parameter model structure considered for the simulation of discharge at fontaine de Vaucluse karst spring (southern France).

How to cite: Sivelle, V., Cousquer, Y., Jourde, H., and Mazzilli, N.: A Bayesian multi-model framework for structure selection and parameter estimation for lumped parameter modeling in karst hydrology., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5147, https://doi.org/10.5194/egusphere-egu23-5147, 2023.

Modelling solute mixing and transport processes is one of the key steps to effectively managing karstic groundwater resources, particularly under the threat of climate change and risk of contamination. For that reason, a considerable body of literature has been devoted to understanding and describing solute mixing and transport processes in karst aquifers. However, due to the strong multiscale heterogeneity (from microscale to aquifer scale), modelling solute mixing and transport processes in karst aquifers remains a challenging task. This presentation critically reviews the current state of knowledge and fundamental challenges in the modelling of solute mixing and transport processes in karst aquifers, thereby collocating and synthesizing the existing body of knowledge in the literature. To provide a holistic and objective picture of the state-of-the-art of the solute mixing and transport modelling, we performed a bibliometric analysis on the relevant literature for karst groundwater studies (over 2800 scientific papers). Further, with a meta-analysis of scientific papers focusing on the quantitative tracer tests, we evaluated the field-based transport parameters that are typically served for the solute mixing and transport models.

The review unveils the fundamental modelling hinges underlying a successful modelling practice for the solute mixing and transport processes in karst, thereby discussing to what extent and in what ways we are dealing with these challenges. The major modelling challenges are defined as follows: (i) Model conceptualization based on data collection and system understanding (e.g., How well is the problem of interest defined? To what extent is the domain of interest described?), ii) Model selection considering the choice of a dominant physicochemical process (e.g., How well is the process of interest represented by a set of governing equations over the problem domain?), iii) Time-variability of solute mixing and transport processes (e.g., To what extent do the parameters represent the process of interest under the different time-scales?), iv) Model parametrization considering the parameter non-uniqueness and transferability (e.g., How realistic are the model parameters? To which extent are they transferable over the same aquifer?), v) Uncertainty quantification in model results (e.g., How robust are the model results? How much are we (un)certain about our model?). Finally, we address potential research directions and knowledge gaps by encouraging the community for building a protocol for solute transport modelling in karst aquifers, as well as providing more transparent and reproducible results.

How to cite: Özdemir Çallı, K., Çallı, S. S., Bittner, D., Chiogna, G., and Hartmann, A.: A critical review of solute mixing and transport approaches in karstic groundwater modelling and key challenges, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-200, https://doi.org/10.5194/egusphere-egu23-200, 2023.

Recharge is an important factor controlling dissolution processes during speleogenesis of karst aquifers. In former studies simplified assumptions were considered, where a maximum recharge rate is assumed while its fluctuation is ignored. Under the latter assumption, the karst genesis is clearly divided into two successive processes characterized by either a hydraulic head limitation (hydraulic control) or a flow rate limitation (catchment control). In this study, we consider a karst system evolving according to a maximum recharge rate linked to the seasonal variation of precipitation, which may lead to speleogenesis processes under hydraulic control or catchment control from the beginning of the karst genesis. We found that, without considering the recharge fluctuation, the enlargement of fractures as well as the dimensions of dissolving area under a long-term evolution tends to be underestimated. Moreover, in the cases of a large catchment area, the time required to reach the final dissolution patterns tends to be underestimated (i.e., earlier breakthrough), while it tends to be overestimated in the cases of a small catchment area. In addition, the flow focusing during the karst genesis may be interrupted during dry seasons when the recharge regime is under catchment control. This may cause a stagnation in the evolution of flow channeling or even a less localized flow field. This study highlights the importance of recharge fluctuation in modeling karst genesis, which may have important engineering implications for the management of karst aquifers or the leakage risk prediction at dam sites.

How to cite: Jiang, C., Jourde, H., and Wang, X.: The role of seasonal variation of precipitation/recharge for different climates in karst genesis behaviors, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12506, https://doi.org/10.5194/egusphere-egu23-12506, 2023.

In the community working on karst hydrosystems, the needs for subsurface solute and contaminant transport characterization is widely acknowledge.  In former studies, several researchers addressed these needs with different approaches such as laboratory experiments, field tests, and groundwater flow and transport simulations. The main objective of such approaches is to improve knowledge of transport processes in karst hydrosystems, and propose solutions to limit the downstream hydrogeological risks (contamination of water resources). In this study, we performed a solute transport experiment in different karst aquifers, in the terminal karst conduit near the spring. We injected a dye in the karst conduit and we monitored the restitution of the tracer at three different zones downstream. In each zone, five probes were placed at different locations (middle, up, down, left and right parts of the cave) along the cross-section of the karst conduit.

Experimental data allowed to reconstruct a transient spatial distribution of concentration for each zone and a general evolution of solute plume. It also provided information about dye mixing along the karst conduit. As a next step, these results are compared to simulated results to investigate the effect of karst conduit geometry, turbulent flow and velocity profiles on concentration profiles, mixing processes and the evolution of solute plume along the conduit. Preliminary results showed that the consideration of the complex karst conduit geometry and morphology has an important effect on transport processes, with a behavior notceably different from the one obtained with numerical simulations on simplified karst conduit geometries.

How to cite: Aliouache, M., Fischer, P., Brunet, P., Lapierre, L., Ropas, B., Vasseur, F., and Jourde, H.: Solute transport experiments and modelling in terminal conduit of karst hydrosystems, Southern France, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12938, https://doi.org/10.5194/egusphere-egu23-12938, 2023.

Fragility of karst derive from a variety of reasons, starting from peculiarity of the geological, hydrogeological, and ecological features, and the facility to transform and negatively impact the environment through many anthropogenic activities. This makes karst terrains among the most endangered areas in the world, as repeatedly demonstrated in many karst areas, also with severe impacts on natural resources. Apulia, the southern-east portion of Italy, is an almost entirely karst region, where expansion of the urbanized areas, development of agricultural practices, and lack of awareness of the importance of karst resources over the last decades have determined an acceleration in the degradation of the karst environment. This is quite in contrast with the long history of the region, where past cultures and civilizations were able to live sustainably with the karst environment, without destroying or polluting its precious natural resources, in primis groundwater.

For instance, the agricultural practices marked a very bad time starting from the 1980s, when intense stone clearing (performed with the goal to obtain new land for cultivation) strongly changed the original karst landscape, which was characterized by bare karst, with slightly incised karst valleys and dolines, and a high number of caves. As a result of such an intense conversion of land cover, karst landforms were highly disturbed, many of them were canceled, and surficial erosion and loss of soil had to be often registered on the occasion of the main rainstorms. Alta Murgia, one of the main karst regions of Apulia (also included within the National Natural Park) was particularly affected, and obliteration of many karst features had to be recorded. Furthermore, the extraction activity carried out in quarries, to extract limestone rocks used for building and ornamental purposes, resulted in destruction of karst caves, against the regional laws which prescribed the need in exploring any cave found during quarrying, aimed at ascertaining any likely interest of the underground karst. Overall, these anthropogenic activities caused an high negative impact on the Alta Murgia karst, which only recently has started to become worth of specific studies from the scientific community. Quantification of the loss of soil, and of the karst landforms is not easy, but in some portions of Alta Murgia has definitely been significant.

Within projects dedicated to creating a greater awareness about local populations of the importance of living in karst, and of respecting such a natural landscape which hosts fundamental natural resources, we present in this contribution some examples of preliminary evaluations of the landscape changes observed, and of their negative impacts on karst.

References

Parise M., 2016, Modern resource use and its impact in karst areas – mining and quarrying. Zeitschrift fur Geomorphologie, vol. 60, suppl. X, p. 199-216.

Parise M. & Pascali V., 2003, Surface and subsurface environmental degradation in the karst of Apulia (southern Italy). Environmental Geology, vol. 44, p. 247-256.

Pisano L., Zumpano V., Pepe M., Liso I.S. & Parise M., 2022, Assessing Karst Landscape Degradation: a case study in Southern Italy. Land, vol. 11, 1842.

How to cite: D'Ettorre, U. S., Liso, I. S., Pisano, L., Zumpano, V., and Parise, M.: Degradation and loss of soil in karst terrains, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5640, https://doi.org/10.5194/egusphere-egu23-5640, 2023.

The objective of this work is to understand the sources of nitrate in Te Waikoropupū Springs (‘the springs’) in New Zealand, and thereby contribute to their preservation. Previous work has shed light on the recharge sources of water to the springs (Stewart and Thomas, 2008), and nitrate mass balance based on this recharge model reveals the nitrate values in the recharge waters when they reach the springs. The major recharge source (from pristine forest on karst uplands) delivers much more nitrate to the springs than expected from measurements on water in its recharge area. This excess nitrate is attributed to nitrification (following mineralisation of organic-N) within the oxic karst groundwater system as the water flows to the springs.

Nitrification (bacterial conversion of ammonium to nitrate) is widespread in soils, unsaturated zones and oxic groundwater systems. Evidence showing natural production of nitrate by nitrification in the current system is given by: (1) Total nitrogen measurements: If organic-N is converted to nitrate-N along the flowpath from the Karst Uplands to the springs, there must be input of 5 kg/ha/yr of organic-N from the recharge area of 170 km². This is within the range found by McGroddy et al. (2008) for first order streams from pristine forests in NZ. (2) ¹⁵N and ¹⁸O measurements: Nitrification produces nitrate with low values of the isotope ratios as observed for the springs, whereas denitrification would cause high values which are not observed. (3) Scientific literature: Recent papers have reported nitrification as a previously unrecognised source of nitrate in oxic karst systems. For example: Musgrove et al. (2016) showed that groundwater nitrate concentrations in the Edwards Aquifer were higher than those in the surface water recharge. They concluded that nitrification within the aquifer is the source of the extra nitrate in the groundwater.

References

McGroddy, M.E., Baisden, W.T., Hedin, L.O. 2008. Global Biogeochemical Cycles 22, GB1026.

Musgrove, M., Opsahl, S.P., Mahler, B.J., Herrington, C., Sample, L.L., Banta, J.R. 2016.  Science of the Total Environment 568, 457–469.

Stewart, M.K., Thomas, J.T. 2008. Hydrology and Earth System Sciences 12(1), 1-19.

How to cite: Stewart, M., Hickey, C., Moreau, M., Thomas, J., and Young, R.: Natural production of nitrate by groundwater nitrification in New Zealand karst springs, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2979, https://doi.org/10.5194/egusphere-egu23-2979, 2023.

The network of fractures and conduits crossing a karst massif drains water and air from the atmosphere deep into the massif, exchanging heat at the boundaries between rock, air and water. The thermal characteristic of the rock together with thermal processes including convection, evapo-condensation, radiation and conduction, concur to fix the cave’s temperature. The thermal length, the distance at which the external temperature fluctuations are damped, and the energy balance of the cave system depend on its geometry and the fluxes therein. Comprehensive knowledge of what modifies these thermal characteristics is of interest for e.g. low-enthalpy geothermal exploitation, mineralisation in water supplies and also for paleoclimatic studies on speleothems.

In Longeaigue cave (Val-de-Travers, Jura mountains, CH), we deployed several sensors measuring airflow and temperature along the main conduit network. The cave is mainly dry and has a lower and upper entrance leading to an intense airflow controlled by the chimney effect. The temperature oscillations observed throughout the cave are chiefly related to external temperature and airflow variations. Results from 8 monitoring stations reveal that 90% of the energy brought in by the air during ventilated periods is exchanged within the first tens of meters from the cave entrances. However, temporary interruptions of the airflow occur during periods of flooding related to rainfall and snowmelt. This situation can take place several times per year. Our observations demonstrate that the transient nature of this airflow modifies the temperature signals in the cave, affecting the cave energy balance in a differentiated way according to seasonal hydrological conditions. With the increasing winter temperatures, we anticipate a progressive shift towards a summer ventilation regime enhanced by limited summer rainfall. A positive feedback is observed on the energy balance of the cave. It is therefore of crucial importance to consider the presence of subsurface ventilation for the thermal characterisation of karstic environments, which can modify the biochemical, physical and thermal characteristics of seeping water and, in turn, impact on the interaction with the encasing rock.

How to cite: Pastore, C., Luetscher, M., Doumenc, F., Sedaghatkish, A., Weber, E., and Jeannin, P.-Y.: Hydrological control on cave ventilation and its effect on the heat balance of Longeaigue cave, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8112, https://doi.org/10.5194/egusphere-egu23-8112, 2023.

The Cosquer cave is a paleolithic decorated cave, in a coastal karst linked to the sea. Stability of climatic parameters in caves is known to be one main condition for conservation of art. Hydroclimate data are measured since several years at a 5 minutes time step: karst air pressure, water level in the karst, atmospheric pressure and sea level. Data shows an unusual behaviour for a karst: the karst air pressure is nearly always higher than the atmospheric pressure. As a result, water level in the karst is below the sea level. Some rock art figures present on walls near water level undergo wash out and fading but limited thanks to the karst pressurization. A stop of this mechanism due to rising sea-level, an increase of the massif permeability or changes in climatic conditions would lead to the loss of arts near water bodies.

The cave air overpressure is related to the rock permeability that should be low. The pressure time series show that three main processes drive the cave pressure. The daily variations of the sea tide provide an assessment of the cave air volume above the pools water level. Although the cave air is confined by the rock and the seawater, there are external air inflows during short pressurization events, that can be deduced from pressure data. Then, the low cave air pressure decrease over the summer season is explained by air outflow through the rock. A bulk permeability is then calculated using Darcy law, assuming a gas permeability in a non-saturated medium. Three theoretical cases are evaluated: an equivalent porous medium, a single fracture, and a single karst conduit. The time series give an observation database to assess future changes in the pressure behaviour of this decorated paleolithic cave, and to detect water level increase and adjust conservation choices.

How to cite: Hugo, P., Pierre, H., Bruno, A., and Stéphanie, T.: Seasonal pressurization of a coastal karst: the paleolithic decorated Cosquer cave (SE France), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12973, https://doi.org/10.5194/egusphere-egu23-12973, 2023.