Displays

About 50% of the Slovene drinking water demand are covered by karst aquifers. Consequently, appropriate protection and sustainable management of these aquifers are essential. Due to the hydrologic complexity and variability of karst systems, predicting potential contaminations and aquifers’ responses to changes in climate conditions, remains a challenge for karst research. In this study, the dynamics of potential solute contaminant transport in the Unica springs catchment, located in the southwest of Slovenia, are characterized and simulated with a semi-distributed karst model. The catchment encompasses autogenic and allogenic recharge across an area of about 820 km². The autogenic karst massive hosts one of Europe’s largest confluences of subterranean river branches and is strongly karstified. To take into account the temporal and spatial characteristics of the catchment, the model is linked with a GIS-based approach to assess spatiotemporal karst vulnerability. The validation of the model is performed by discharge observations and tracer experiments. The resulting simulations enable us to identify hot spots and hot moments of high contamination risks. By simulating solute contaminant transport during extremely dry and extremely wet years, we show that contaminant transport slows down under dry conditions. This can be explained by fast flow pathways not being activated or hydraulically connected under low flow conditions, which results in a retention and dilution of the contaminant in the aquifer. Our new approach improves the understanding of potential contaminants' transport behavior in a large complex karst system and justifies the consideration of spatiotemporal hydrologic variability in solute transport forecasting. It proposes a good basis for a better evaluation, management and protection of water resources in karst areas.

How to cite: Scheller, M., Blatnik, M., Kogovšek, B., Liu, Y., Mayaud, C., Petrič, M., Ravbar, N., and Hartmann, A.: Modeling spatial and temporal hydrologic variability of karst vulnerability at a large Slovenian karst aquifer, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1009, https://doi.org/10.5194/egusphere-egu2020-1009, 2020.

Lumped karst hydrological models often suffer from the over-simplified structures. In recent days, hydrochemical has been used as an auxiliary information to define realistic karst model structures. For karst aquifers, the hydrochemical dynamics of karst springs contain important information about the internal behavior of the karst aquifer that can be conducive to model identification and calibration. In this presentation, electrical conductivity (EC), as a substitution for Ca²⁺ concentration of the spring discharge, is evaluated for its potential for model structure identification and reduction of simulation uncertainty. A new framework to integrate EC in karst modeling is developed at a small, well-instrumented karst catchment near Guilin City, China. A set of different hydrologic models were enabled to consider the linear dissolution process of Ca²⁺ and its transport to investigate the models’ abilities to reproduce the behavior of spring discharge and EC. We found that most hydrologic models obtain similar performances concerning spring discharge, however, their performances in simulating spring EC show obvious differences. The combination of observed spring discharge and EC could identify a more realistic model structure that was in accordance with the observed perception of this karst aquifer’s functioning. Using the identified “most realistic” model, we use sensitivity analysis to show that spring EC only improves the identifiability of one hydrologic parameter mostly due to too complex EC dynamics during recharge events and therefore has limited potential to reduce discharge simulation uncertainty. Consequently, our new framework to include EC in karst models opens new doors for more realistic simulation but an explicit treatment of uncertainties remains necessary due to EC’s limited potential to reduce simulation uncertainty. The next step of our work is to integrate appropriate nonlinear dissolution process of carbonate rock in the model to further improve the simulation of spring EC (Ca²⁺).

How to cite: Chang, Y., Liu, L., Jiang, G., Hartmann, A., and Wu, J.: Can the electrical conductivity of karst spring discharge improve the identification of model structures and reduce simulation uncertainty?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3781, https://doi.org/10.5194/egusphere-egu2020-3781, 2020.

Density-induced CO₂ dissolution - approaches to test a new hypothesis on a process relevant for epigenetic karstification   

A process which has not yet been discussed as relevant for epigenetic karstification in phreatic zones has been hypothesized in a publication by Scherzer et al. (2017). It refers to an enhanced CO₂ transport into the phreatic zone by density-induced convective dissolution. The phenomenon is well-known also in CO₂ geologic sequestration and is denoted there typically as solubility trapping. Scherzer et al. (2017) denote this process in caves as nerochytic speleogenesis (from nerochytic = sink in Greek), assuming it has relevance for epigenetic karstification under certain circumstances. This could be relevant in particular in caves where CO₂ concentrations are highly elevated and show strong seasonal fluctuations.

Thomas et al. (2015) have introduced a method to visualize fingering patterns of CO₂ convective dissolution in water with a pH-sensitive color indicator. We have used this approach to produce a set of experimental data in a laboratory flume of dimensions 60 cm x 40 cm x 1 cm. Our aim is to validate a numerical model that we implemented in the simulator DuMu^x (www.dumux.org), which can later on be used for future studies as the basis for investigating the relevance of nerochytic speleogenesis for karstification.

We have applied atmospheres with varying concentrations of carbon dioxid as boundary conditions at the top of the flume and observed the onset times and fingering patterns, in particular we focused on the velocity of the fingers.

The Navier-Stokes model with water density dependent on CO₂ concentration is run in 2D, 3D and pseudo 3D, the latter referring to a 2D approach with a drag term in the momentum balance to account for wall friction at the front and the back plate. Without calibration or fitting of parameters, the results of the comparison between experiment and simulation show reasonable agreement both with respect to the onset of convective fingering and the number of fingers occurring.

References:

H. Scherzer, H. Class, K. Weishaupt, T. Sauerborn, O. Trötschler: Nerochytische Speläogenese: Konvektiver Vertikaltransport von gelöstem CO₂ - ein Antrieb für Verkarstung in der phreatischen Zone im Bedeckten Karst, Laichinger Höhlenfreund 52:29-35, ISSN 0344 6832, 2017.
 

C. Thomas, L. Lemaigre, A. Zalts, A. D'Onofrio, A. De Wit: Experimental study of CO₂ convective dissolution: the effect of color indicators, International Journal of Greenhouse Gas Control 42:525-533,2015.

How to cite: Class, H., Weishaupt, K., Trötschler, O., and Scherzer, H.: Density-induced CO2 dissolution - approaches to test a new hypothesis on a process relevant for epigenetic karstification , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3659, https://doi.org/10.5194/egusphere-egu2020-3659, 2020.

We describe and explore a group of collapse sinkholes located on a
sports field in the village of Münsterdorf close to Hamburg in northern Germany.
The collapse sinkholes develop since 2004 with a rate of one per year, with
sizes between 2-3 m in diameter and 3-5 meter depth, and are aligned to
a narrow east-west trending region..
In 20 m depth, cretaceous chalk is present in the area, topped by peri-glacial
and glacial deposits.
We summarize hydrological, geodetical, and geophysical data collected
and then discuss mechanical concepts for the occurence of the collapse sinkholes,
starting with simple analytical solutions and then expanding to distinct-element modelling.
We conclude that dewatering of the area might be an important aspect for the collapse sinkhole
developement.

How to cite: Kaufmann, G., Romanov, D., Werban, U., and Vienken, T.: The Münsterdorf sinkhole cluster: Void origin and mechanical failure, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6787, https://doi.org/10.5194/egusphere-egu2020-6787, 2020.

The floodplain of the Loire River around Orleans is a sinkhole-prone area due to the highly karstified calcareous Beauce Formation overlying by few meters of weathered materials and alluvial deposits. Such layout makes it necessary to carry out detailed geotechnical and hydrogeological investigations for any important building project.

This paper presents the first results of a study carried out by the French Geological Survey for a bridge project across the Loire River. The geological setting indicates that the study area is near the front door of a supposed-major cave system in which water, coming from the Loire River, not only provides drinking water for an important part of the region, but also supplies the main spring of the Loiret River, located a few kilometers further. The overview of past sinkhole collapses confirms that the study area is regularly concerned by ground collapses of several meters of diameter. Field investigations include microgravimetry (26 hectares), two electrical resistivity profiles (720 m and 470 m long), 149 cone penetration tests (around 15 m deep), a first sequence of 11 drillings (40 m deep) and videos, gamma-ray and sonar logs. A first sinkhole hazard assessment is now quite complete: very weak layers (possibly caves) of several decimeters to a few meters thick need specific mitigation measures to secure the construction project. Other investigations are still being analysed (a second sequence of drillings, dye tracing, injection of salt brine with resistivity profiling) and should help specify the area hydrogeological hazard.

How to cite: Noury, G., Salquèbre, D., Jacob, T., Baltassat, J.-M., Asfirane-Haddadj, F., Buscarlet, E., and Boularas, S.: Sinkhole collapse and hydrogeological hazard assessment in covered karst terrains: Case study of a bridge project across the Loire River (Orleans, France), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11375, https://doi.org/10.5194/egusphere-egu2020-11375, 2020.

The management of karst geohazards requires new and novel strategies to address the complexities inherent in karst systems and the challenges posed by a changing climate. The often rapid and widespread interaction between surface and subsurface hydrology can leave karst terrains uniquely susceptible to flooding from groundwater sources. Quantifying the frequency and magnitude of such flooding is a key step in the management of flood risk. Here, we present a novel interdisciplinary approach developed for predictive groundwater flood hazard mapping in the lowland karst plains of Ireland. This approach ties together direct and earth observation-derived hydrograph data, hydrological modelling, stochastic weather generation and extreme value analysis to generate predictive groundwater flood maps for qualifying sites.

The first step in the approach was the collection of hydrological data for sites susceptible to groundwater flooding. A monitoring network of 50 sites was established in late 2016 to provide baseline data over a 30-month period. Additionally, a methodology for delineating historic flood extents and water elevations from multi-temporal Copernicus Sentinel-1 Synthetic Aperture Radar (SAR) imagery was developed. This allowed hydrograph generation for ungauged sites, whilst also allowing observations of the 2015/2016 extreme flood event at gauged sites which predated monitoring. Next, site-specific hydrological models capable of constructing flood hydrographs from antecedent rainfall and soil moisture conditions were calibrated for 393 sites using a combination of observed and SAR hydrographic data (mean NSE: 0.81). A stochastic weather generator calibrated on 70-year meteorological records was used to generate long-term synthetic rainfall data for each site. These stochastic series, together with long-term average evapotranspiration, were used as input to the site models to produce long-term hydrological series from which annual maxima series were derived. Thereafter, flood frequency analysis was used to estimate predictive flood levels and generate predictive flood maps. This novel applied approach has significantly improved our fundamental scientific understanding of groundwater flooding as a geohazard, whilst also informing regional planning and development to limiting future flood vulnerability.

How to cite: Naughton, O., McCormack, T., and Campanya, J.: Predictive groundwater flood hazard mapping in lowland karst, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20695, https://doi.org/10.5194/egusphere-egu2020-20695, 2020.

Enclosed topographic depressions are characteristic of karst landscapes on Earth. The scale and morphological characteristics of such depressions are variable, but the most common depression type is a sinkhole (doline). Certain karst depressions that are much larger than sinkholes and that display gentler slopes and more complex three-dimensional shapes are known as uvalas. A single uvala typically contains numerous sinkholes within it. The developmental relationship between sinkholes and uvalas has been subject of debate, however, mainly because long developmental timescales impede direct observation in classical limestone karst, where such features are most commonly reported.

Here, we describe the development of five uvalas and numerous associated sinkholes in an evaporite karst setting on the eastern shore of the hypersaline Dead Sea. This karst landscape evolved rapidly over a 25-year period from 1992 to 2017 in response to the anthropogenically-driven decline in the Dead Sea level. Our remote sensing data and field observations show that both the sinkholes and the uvala-like depressions formed through subsidence in a very close spatio-temporal relationship. While many sinkholes developed initially in clusters, the uvalas developed around such clusters as larger-scale and gentler depressions that are structurally distinct both in space and time.

In agreement with inferences for examples in limestone karst settings, the uvalas in this evaporite karst setting do not form by a simple coalescence of sinkholes. Instead, these evaporite-karst uvalas form through subsidence (sagging), interpreted here as in response to distributed subsurface dissolution and physical erosion within a mechanically unstable subsurface volume (e.g. a groundwater conduit network). Sinkholes, on the other hand, are interpreted as discrete subsidence responses within that volume to smaller-scale zones of highly localised material removal (e.g. individual groundwater conduits). Our observations and interpretations are consistent with numerical modelling of subsidence produced by the development of multiple void spaces at progressively deepening levels. Morphometrically, our results also agree well in several respects with a recent re-evaluation of uvalas in some classical limestone karst areas. Consequently, this study helps to clarify the nature, occurrence and genesis of uvalas in karst systems generally.

How to cite: Watson, R., Holohan, E., Al-Halbouni, D., Alrshdan, H., Closson, D., and Dahm, T.: Uvalas and their relationship to sinkholes (dolines) in an evaporite karst setting, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15849, https://doi.org/10.5194/egusphere-egu2020-15849, 2020.

We use numerical simulations to investigate the role of initial aperture heterogeneity under varied in-situ stress loadings in the early-time karstification in an anisotropic natural fracture network. We found that the importance of the stress-dependent initial aperture effect on karstification depends on the relative relationship between the flow direction and structural hierarchy/anisotropy of the fracture network. When the flow occurs in the direction of the dominant fracture set with more through-going discontinuities, karst conduits only develop locally along a few large fractures with a preferential orientation for frictional sliding under the differential stress due to enhanced transmissivity caused by the important shear-induced dilation. In contrast, when flow is in the direction transverse to the dominant fracture set, the far-field stress loading has a negligible impact on the emergent dissolution pattern while only somewhat impact on the onset time of breakthrough. In this case, the developed conduits are much more tortuous with numerous branches. In both cases, the presence of initial aperture variability enhances the stress effects and significantly changes the dissolution pattern and delays the breakthrough time. Our results demonstrate that the flow heterogeneity induced by geometrical complexities and in-situ stress conditions seems to play an essential role in the karstification processes in fractured rocks.

The proposed reactive transport model based on realistic fracture networks may be used to investigate the spatial relationship between tectonic structures and karst cavities. Our results demonstrate that the heterogeneity induced by geometrical complexities and in-situ stress conditions may play a decisive role in the karstification processes in fractured rocks. Thus, they must be properly considered in reactive transport simulations to make reliable designs for practical engineering applications.

Keywords: discrete fracture network, karst, network topology, reactive flow, in-situ stress

How to cite: Wang, X., Aliouache, M., Lei, Q., and Jourde, H.: Influence of initial aperture field under varied in-situ stress conditions on incipient karst formation in carbonate rocks , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21544, https://doi.org/10.5194/egusphere-egu2020-21544, 2020.

Rock masses are typically anisotropic and heterogeneous, due to presence of sin-sedimentary discontinuities as bedding planes and of post-depositional features such as joints and faults. When compared to soil mechanics, therefore, a greater complexity of the simulation models for rock masses derives, which is further increased when dealing with carbonate rock masses. Beside the aforementioned types of discontinuities, other features are produced by karst processes; these latter are able to create highly complex networks of voids and conduits, with passages of variable size, which may reach dimensions enterable by man. These features definitely represent the larger discontinuity families within carbonate rock masses, in terms of size, frequency, and pervasiveness, and significantly control the flow of water.

The peculiar characteristics of karst require dedicated approaches to take into the due account the presence of its typical landforms (voids, conduit/caves of variable size, swallow holes, etc.), and their variable functionality as well. Ignoring karst features in the analysis and characterization of carbonate rock masses, any approach followed, or model implemented, will inevitably result in too great uncertainties (if not errors), and in incorrect information to the engineers.

What stated above is true also as regards the study of water flow in fractured carbonate rock masses, that cannot be initiated without considering the stratigraphic and structural discontinuity families. To implement flow models scholars typically start from traditional structural-geological surveys, characterization of rock masses through the classical geomechanical approaches, and elaboration of outcrop pictures elaborated by means of image process softwares. The statistical outcomes are then used as parameters in mathematical models, where also the hydrogeological boundary conditions need to be defined.

A significant step forward in this approach is the use of data directly collected underground, through surveys within the cave systems. The data so collected can be used with a two-fold goal: first, to add a view from the inside of the karst underground landscapes to what is generally observed only at the surface; second, to validate the models, when these are initially implemented only with surface data.

Plan maps of explored cave systems may be useful to determine in first approximation the main direction of development of the karst processes. Reliable maps of caves are nowadays available, that can be used to extract the main direction of karstification, as well as the average size of the explored karst conduits and passages. All these informations are precious underground-truth data that are definitely worth to be included in hydrogeological models aimed at improving them and their reliability as well.

How to cite: Parise, M. and Liso, I. S.: Using cave data for improving the reliability of karst groundwater flow models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10356, https://doi.org/10.5194/egusphere-egu2020-10356, 2020.

Qualitative vulnerability assessment methods applied in karst aquifers rely on key factors in the hydrological compartments usually assigned different weights according to their estimated impact on groundwater vulnerability. Based on an integrated numerical groundwater model on a snow-governed karst catchment area (Assal Spring- Lebanon), the aim of this work is to quantify the importance of the most influential parameters on recharge and spring discharge and outline potential parameters that are not accounted for in standard methods, when in fact they do play a role in the intrinsic vulnerability of a system. The assessment of the model sensitivity and the ranking of parameters are conducted using an automatic calibration tool for local sensitivity analysis in addition to a variance-based local sensitivity assessment of model output time series (recharge and discharge)  for two consecutive years (2016-2017) to various model parameters. The impact of each parameter was normalized to estimate standardized weights for each of the process based key-controlling parameters. Parameters to which model was sensitive were factors related to soil, 2) fast infiltration (bypass function) typical of karst aquifers, 3) climatic parameters (melting temperature and degree day coefficient) and 4) aquifer hydraulic properties that play a major role in groundwater vulnerability inducing a temporal effect and varied recession. Other less important parameters play different roles according to different assigned weights proportional to their ranking. Additionally, the effect of slope/geomorphology (e.g., dolines) was further investigated.  In general, this study shows that the weighting coefficients assigned to key vulnerability factors in the qualitative assessment methods can be reevaluated based on this process-based approach.

How to cite: Doummar, J. and Kassem, A. H.: Sensitivity analysis of an integrated numerical flow model output to key parameters used in common qualitative vulnerability assessment methods, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10512, https://doi.org/10.5194/egusphere-egu2020-10512, 2020.

    The epikarst ecosystem of karst environments plays a key role in biogeochemical cycling and material storage and transport. Many geochemistry and geophysical methods have been used to research the epikarst. Coherence attributes have already showed advantages in many aspects. Although ground penetrating radar (GPR) has been introduced to characterize karst structure long before, GPR coherence attributes are paid less attention in the interpretation of epikarst structure. Coherence attribute can describe the waveform similarity of traces by the classical mutual correlation algorithm and reflect the discontinuities of media. Two typical limestone epikarst profiles which develop shallow and deep fissure soil in Guizhou karst plateau were chosen. We used MALA GPR equipment to acquire data. After the data processed by conventional methods, we then extracted the coherence attribute data from the GPR data by the procedure coded in C Programing Language. The results show that: (i) coherence attribute displays the valid and invalid signals area of the GPR profiles; (ii) the epikarst is corresponding to the valid area and the bedrock below epikarst is corresponding to the invalid area; (iii) the boundary of valid and invalid areas reflects generally the lower boundary of epikarst. Coherence attribute provides additional evidence that the epikarst developed lots of fractures but the bedrock below is complete. The validity of GPR signals is useful to analyze the structure research of epikarst.

How to cite: Gao, Q., Peng, T., Wang, S., and Yang, T.: GPR coherence attribute applied to the structure interpretation of limestone epikarst in Guizhou karst plateau , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1977, https://doi.org/10.5194/egusphere-egu2020-1977, 2020.

Modeling karst spring discharge while considering potential impacts of land use changes in a recharge area is a crucial task for water resource managers worldwide. Generally, such models are based on sophisticated mathematical functions developed and applied by researchers and their complex nature does not allow an intuitive applicability. To overcome this limitation and to make these models applicable for stakeholders, they need to be integrated in an applicable and open source framework that can be used by water managers without losing the original and full modeling functionalities.

In our work, we introduce a user-friendly modeling environment by integrating the recently proposed LuKARS (Land use change modeling in KARSt systems) model into FREEWAT (FREE and Open Source Software Tools for WATer Resource Management). LuKARS is a lumped, rainfall-discharge model for karst systems that considers impacts of land use changes by changing the area of a so-called hydrotope, representing a landscape unit with homogeneous soil and land use properties. FREEWAT provides an open source toolkit for water resource management that is implemented as a plugin in QGIS. The integration of LuKARS benefits from QGIS’ mapping, visualization and geospatial manipulation capabilities. The plugin of FREEWAT provides a modular concept of pre- and post-processing tools that facilitate the setup, calibration, analysis, storage and sharing of a LuKARS model.

How to cite: Bittner, D., Rychlik, A., Klöffel, T., Leuteritz, A., Disse, M., and Chiogna, G.: Towards making karst hydrologic models more user-friendly – The integration of the LuKARS model into FREEWAT, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6207, https://doi.org/10.5194/egusphere-egu2020-6207, 2020.

Artificial tracer tests constitute one of the most powerful tools to investigate solute transport in conduit-dominated karst aquifers. One can retrieve information about the internal structure of the aquifer directly by a careful analysis of the residence time distribution (RTD). Moreover, recent studies have shown the strong dependence of solute transport in karst aquifers from boundary conditions. Then, results from artificial tracer tests contain information about the internal structure of the aquifers and about the effect of the boundary conditions (mainly high or low water level). So, a multi-tracer test calibration of the model appears to be more consistent in order to identify the effect of change in boundary conditions and to take into consideration their effects on solute transport. In this study, we propose to run the inverse problems based on artificial tracer tests with a numerical procedure composed by the following three main steps: [1] conduit geometry is simulated using a pseudo-genetic algorithm, [2] hypothesis about boundary conditions are imposed in the simulated conduit network and [3] flow and solute transport are simulated. Then, using a trial-and-error procedure, the simulated RTD is compared to the observed RTD on a large range of simulations allowing to identify the conduit geometry and boundary conditions that better reproduce the field data. This constitutes a new approach in order to better constrain inverse problem using a multi-tracer tests calibration including transient flow.

How to cite: Sivelle, V., Renard, P., and Labat, D.: Coupling SKS and SWMM to solve the inverse problem based on artificial tracer tests data in karst aquifers, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10318, https://doi.org/10.5194/egusphere-egu2020-10318, 2020.

Model assessment is a crucial part of hydrological modelling studies. The traditional intuitive approach is to judge model performance to explore its effectiveness and informativeness about the system reality. However, this approach does not necessarily guarantee that one captures system hydrological functioning from the model output. Here, we proposed a novel model assessment strategy that provides a direction to constrain model output space with the concept of model functionality. In the study, we used StorAge Selection (SAS) function approach as a process diagnostic tool to explore the model output space which is simultaneously informative on the system hydrological behaviour and functioning. To do that, the SAS model was fed by a karst-dedicated hydrological model (VarKarst) simulations to model the karst aquifer d¹⁸O transport and young water fraction of system discharge, F_yw. Model functionality was assessed by a new model verification metric, named Exceedance Probability Ranked Score, EPRS. The eligible clusters from the model output were then served in order to examine the model parameter space. Our findings provide direction to indicate that using young water fraction, F_yw as a process-diagnostic metric leads to an improvement in model realism while carrying a physically realistic model parameter set throughout the model output space.

How to cite: Ozdemir Calli, K., Melsen, L., and Hartmann, A.: Model Assessment Strategy in a Karst Hydrological Model Using a Process-based Diagnostic Tool, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20141, https://doi.org/10.5194/egusphere-egu2020-20141, 2020.

TRANSKARST is an interdisciplinary research project with scientists and regional water resource administrators. This 3 years project (2019-2022) is set up on Arcier's karstic watershed used for the drinking water supply of 60 000 inhabitants of Besançon, France. Using this instrumental basin, as a part of Jurassic Karst French observation system, the project aims at defining pathways of mineral, organic and microbiological contamination in karstic system.

The methodology used in the project, combines field expertise with the implementation of analytical tools related to conventional - dissolved phase chemistry (major ions, miners, traces, organic carbon), isotopes (oxygen-18, deuterium, carbon-13) – and prospective analysis - emerging pollutants such as pharmaceuticals and ETM, dissolved organic matter, suspended matter and microbiology (bacteria, fungal species) with a special focus on antibiotic resistance. The TRANSKARST project thus brings together a consortium of researchers from various disciplines: hydrogeology, chemistry, sedimentology, microbiology, geology and geophysic. The project is also highly associated to water managers as Arcier’s spring is used for drinking water supply.

The three main under-objectives following by TRANSKARST could be summarized as follows. First, through geological and geophysical investigation, a conceptual and numerical geological model will be established under geomodeler and will be further used to constrain the pathways of karstic groundwater. The second point focus on the evaluation of karst media contamination: chemical (including emerging molecules) and microbiological. Then we expect to discriminate, by the use of ETM, dissolved, colloidal and particulate pathways of contamination. All the observations will be linked together to identify actions and feedback of different parameters and contaminants within karst hydrosystems.

How to cite: Boudon, A., Celle-Jeanton, H., Bertrand, X., Albaric, J., Amiotte-Suchet, P., Choulet, F., Crini, N., Hocquet, D., Huneau, F., Le Coustumer, P., Loup, C., Mathieu, O., Miege, C., Stefani, V., Steinmann, M., Sue, C., Thevenot, M., and Trap, P.: Presentation and methodology of TRANSKARST project, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21532, https://doi.org/10.5194/egusphere-egu2020-21532, 2020.

Environmental changes, such as alterations in precipitation and evapotranspiration regimes, changes in vegetation type, etc. are triggering direct impact on hydrological cycle through modified amounts and patterns of recharge conditions, as well as occurrence of more frequent and severe hydrometeorological events. Karst aquifers are particularly vulnerable to these effects due to highly dynamic hydrological processes. In this study, we were interested in studying the possibilities to observe changed hydrological behaviour of karst springs on a human timescale. Therefore, we focused on two examples in Slovenia, both regionally important for freshwater supply, agriculture and hydropower. The Unica spring mostly drains areas under moderate continental climate. Its catchment has been repeatedly and severely hit by natural disasters (e.g., ice break, bark beetle attack, windthrow) after 2014 causing large-scale forest disturbances. The catchment of Rižana spring, on the other hand, belongs to the moderate Submediterranean climate. There these types of disturbance did not occur in recent years (excluding some wildfires), but the catchment has been liable to substantial land use changes in the past six decades. For assessment of vegetation cover changes and large-scale disturbances in forests, historical digital orthophotos of the Surveying and Mapping Authority of the Republic of Slovenia since 1957 have been compared with the recent land use data provided by Ministry of Agriculture, Economy and Food and forest state database of Slovenian Forest Service. At the same time, hydrological data of the Unica (Hasberg gauging station) in the period 1962-2018 and Rižana springs (Kubed gauging station) in the period 1966-2018 and precipitation data from Postojna (period 1962-2018) and Podgrad (period 1966-2018) meteorological stations have been processed. Individual flood pulse events over the 57 years for Unica and 53 years for Rižana have been separated. For each flood pulse various information about precipitation amount and intensity, duration of discharge increase, its intensity and amplitude have been specified. We compared these findings with the calculated trends of meteorological and hydrological variables and also changes in land use. The impact of particular environmental change on discharge values of both springs has been evaluated, showing that both, climate and land-use changes, have considerable impact on hydrological regime of studied karst springs. In particular, altered duration of flood pulses increase, their amplitude and intensity have been observed, meaning that the most important issues of water availability that are crucial for water-dependant economic sectors are under threat.

How to cite: Ravbar, N., Kovačič, G., and Petrič, M.: Changes in hydrological behaviour: case studies of the Unica and Rižana karst springs, Slovenia, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12961, https://doi.org/10.5194/egusphere-egu2020-12961, 2020.

Single-well hydrogeophysical approaches have previously been applied to several fractured aquifers in the US and the UK, including karstic carbonate systems, in order to characterise solute transport. These approaches typically use single well hydraulic or tracer tests coupled with image or calliper logs to identify and characterise flowing features.  They have variously been used to estimate fracture/conduit aperture and porosity, permeability and/or groundwater velocities, in order to determine groundwater vulnerability or delineate wellhead protection areas.  Here, we outline a new workflow for application & analysis of single-well dilution tests for characterisation of fractured and karstic aquifers, and apply this to the Cretaceous Chalk aquifer, Yorkshire, UK.

Chalk aquifers typically have transmissivity that derives essentially from a well-developed network of fractures with solutionally-enhanced apertures and small conduits. Such features can lead to high groundwater velocities and high impacts of contamination on water quality. Knowledge of their solute transport properties is therefore important for delineating source protection areas, characterising contaminant fate and transport, determination of the effectiveness of aquifer remediation, and parameter estimation for models. In this work, single well dilution test data were used to characterise flow patterns in wells and infer properties such as the kinematic fracture porosity, and groundwater velocities. The single-well dilution technique relies on the interpretation of specific electrical conductance (SEC) contrasts between aquifer formation fluid and well fluid column following introduction of saline tracer in the well. Our workflow used both uniform injection (tracer introduced throughout the water column) and point injection (specific depth) tests in open wells under ambient flow conditions.  This workflow allowed sections of well showing horizontal versus vertical flow to be distinguished, and the magnitude of such flows and exchanges with the aquifer to be determined.  Flow within wells are then used to characterise aquifer properties as follows i) presence and direction of vertical hydraulic gradients ii) relative permeability and depth distribution of flowing features iii) in combination with hydraulic test data (e.g. overall well transmissivity) and geophysical logs, the porosity and permeability of the flowing features at each depth iv) in combination with local hydraulic head measurements in nearby wells, an estimate of groundwater velocities in the surrounding aquifer. We tested predicted fracture porosities and groundwater velocities against those measured in previous studies via large scale pumping tests and ambient flow well-to-well tracer tests.  The comparison suggests that the open-well dilution approach can provide reliable flowing porosities and groundwater velocities in fractured aquifer systems.

How to cite: West, L. J., Agbotui, P. Y., and Bottrell, S. H.: Characterizing solute transport properties for a fractured carbonate aquifer using open-well dilution tests , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8575, https://doi.org/10.5194/egusphere-egu2020-8575, 2020.

An important phenomenon often encountered when interpreting tracer tests in karst aquifers is the occurrence of dual-peaked breakthrough curves (BTCs). The dual-peaked BTCs are usually attributed to tracer transport through a conduit system consisting of a dual-conduit structure: an auxiliary conduit that deviates from the main conduit at the upstream and converges back at the downstream. In order to understand how the geometric configuration of the dual-conduit structure influences the BTCs, laboratory experiments utilizing plastic tubes were conducted. The physical models were constructed by varying: 1) the total length of the conduits, while fixing the length ratio; 2) length ratio between the two conduits, while fixing the length of the main conduit; and 3) conduits connection angle. The tracer experiments are then fitted by a Multi-Region Advection Dispersion model and a Transfer Function model to derive effective transport parameters. This allows us to quantitatively compare the experimental results, and thus to analyse the conduit geometry effects on solute transport and to compare the performance of the two models.

Results show that the dual-conduit structure causes the double peaks of BTCs. Keeping the length ratio of the two conduits and increasing their total length leads to a larger separation of the two peaks of the BTCs. Keeping the length of main conduit while increasing the length of the secondary conduit causes similar effects. As (θ₁-θ₂) increases, the first peak concentration value decreases, the second peak concentration value increases.

Keywords: karst, lab experiment, dual-peaked BTCs, modelling

How to cite: Wang, C., Wang, X., Majdalani, S., Guinot, V., and Jourde, H.: Solute transport in dual conduit structure: experiment and modelling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21513, https://doi.org/10.5194/egusphere-egu2020-21513, 2020.

The Narbonne-Sigean sedimentary basin is composed by Oligocene sediment, alluvions from the Aude River and the Fontfroide-Montredon limestone massif. The significant tectonics during the Oligocene rifting brought all of these formations to the surface in the Malvesi area. A normal fault affects the area and causes the Jurassic unit to rise in the form of a horst: the Montlaures massif. The Oeillal spring is located on the south border of this massif. The karst spring outflows at 4 pools with different physico-chemical signature. Each of these pools is equipped with a CTD probe. Moreover, the area is monitored with piezometric and temperature measurements that allow characterizing each of the main geological formations near the Oeillal spring. Though measurements started more than ten years ago (2007), continuous monitoring is available on one hydrological cycle, only. Indeed, only sparse data are available over the period 2007-2018, which required proposing a methodology to allow the optimal use of the available data in the modelling workflow.  The present study thus focuses on this methodology and on the use of numerical tools such as time series analysis (auto and cross-correlation analysis, spectral analysis) to determine a suitable modelling approach (lumped or distributed model) adapted to the hydrodynamic modelling of karst springs with sparse data.

How to cite: Sivelle, V. and Jourde, H.: From time series analysis to hydrodynamic modelling in a complex hydrosystem: applications for the hydrodynamic characterization and modelling of a karst aquifer with sparse data (Oeillal spring, France). , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14909, https://doi.org/10.5194/egusphere-egu2020-14909, 2020.