Hervé Jourde

Seasonal variations of CO 2 and 222 Rn in a mediterranean sinkhole - spring (Causse d’Aumelas, SE France)

INTRODUCTION Carbon dioxide dynamics and transfer from soil to the deepest parts of an aquifer underlie the main karstification processes. Cavity ventilation plays an important role in speleothem formation and chemical equilibrium of water in the unsaturated zone of the aquifer. Meteorological parameters (barometric pressure and temperature), cave geometry and fracture networks control exchanges between subterranean and outside atmosphere (De Freitas et al., 1982; De Freitas & Littlejohn, 1987; Smithson, 1991; Christoforou, 1996; Buecher, 1999; Fernandez-Cortes, 2005). Gases currently studied in caves are CO2 and 222Rn. Their concentrations and variations can be explained by different parameters. Carbon dioxide concentrations in cavities vary from 0.03 % to more than 6 % and may have different sources : Batiot-Guilhe C., Seidel J.L., Jourde H., Hébrard O., Bailly-Comte V. 2007. Seasonal variations of CO2 and 222Rn in a Mediterranean sinkhole spring (Causse d’Aumelas, SE France). International Journal of Speleology, 36 (1), 51-56. Bologna (Italy). ISSN 0392-6672.

Flash flood mitigation as a positive consequence of anthropogenic forcing on the groundwater resource in a karst catchment

The Mediterranean coastal region is prone to high-intensity rainfall events that are frequently associated with devastating flash floods. This paper discusses the role of a karst aquifer system in the flash floods of a Mediterranean river, the Lez river. Most of the Lez river watershed is located on karst terrains where interactions between surface water and groundwater take place. During extreme rainfall events, the presence of fractures and well-developed karst features in carbonate terrains enhances the infiltration processes and involves the concentration of the recharge into highly organized and permeable flow paths. The groundwater, therefore, quickly moves towards the natural outlets of the karst system. The influence of the Lez karst aquifer system on the associated river floods dynamics is analysed while considering the spatially distributed rainfall, as well as the time series of the groundwater level within the aquifer and of the Lez river discharge measured at various gauging stations. Special attention is given to the relative importance of the surface and underground processes involved in flash flood genesis. It is shown that the karst groundwater contributes to flash floods under certain conditions, while high-rate pumping within the karst aquifer, which generates significant drawdown, may mitigate flash floods under other conditions.

Origin of fractional flow dimension to a partially penetrating well in stratified fractured reservoirs. New results based on the study of synthetic fracture networks

To determine the origin of fractional flow dimensions identified from the results of pumping tests in fractured reservoir, the geometric specificities of synthetic networks are compared to flow dimensions obtained from simulated pumping tests. The modeled reservoirs studied in this paper constitute a system of orthogonal fractures generated by using a pseudo-random process controlled by simple mechanical rules. Flow induced by pumping is simulated while considering the intersections between discontinuities as pipes with various apertures. In many cases, the simulations result in fractional flow dimension although the flow-path network is not self-similar at different scales. We also show that the range of pipe conductance can affect early-time flow behavior although late-time flow behavior appears to be insensitive to the latter. Finally we propose an explanation for the fractional dimension of flow while considering the spatial distribution of interconnections between flow paths.

Investigation of Groundwater Dynamics in a Mediterranean Karst System by Using Multiple Hydrogeochemical Tracers

The Lez karst aquifer, located in southern France, supplies the Montpellier metropolitan area with potable water and has a maximum exploitation capacity of about 1.700 L/s. The objective of this work is to improve the comprehension of groundwater dynamics in a particular Mediterranean karst system, using hydrogeochemical tracers. The different types of groundwater with similar chemical characteristics (major and trace elements) are identified. During rainy season periods, this multitracer characterization shows that deep mineralized waters emerge at the springs just before rapid infiltration waters. This phenomenon shows that hydrodynamic conditions imply water circuits, with participation of specific compartments or creation of short cuts for superficial percolating waters. The data also highlight the vulnerability of the system to natural or anthropogenic contamination. Key words: karst, hydrochemistry, natural tracing, hydrodynamics.

Application of Large‐Scale Inversion Algorithms to Hydraulic Tomography in an Alluvial Aquifer

Large-scale inversion methods have been recently developed and permitted now to considerably reduce the computation time and memory needed for inversions of models with a large amount of parameters and data. In this work, we have applied a deterministic geostatistical inversion algorithm to a hydraulic tomography investigation conducted in an experimental field site situated within an alluvial aquifer in Southern France. This application aims to achieve a 2-D large-scale modeling of the spatial transmissivity distribution of the site. The inversion algorithm uses a quasi-Newton iterative process based on a Bayesian approach. We compared the results obtained by using three different methodologies for sensitivity analysis: an adjoint-state method, a finite-difference method, and a principal component geostatistical approach (PCGA). The PCGA is a large-scale adapted method which was developed for inversions with a large number of parameters by using an approximation of the covariance matrix, and by avoiding the calculation of the full Jacobian sensitivity matrix. We reconstructed high-resolution transmissivity fields (composed of up to 25,600 cells) which generated good correlations between the measured and computed hydraulic heads. In particular, we show that, by combining the PCGA inversion method and the hydraulic tomography method, we are able to substantially reduce the computation time of the inversions, while still producing high-quality inversion results as those obtained from the other sensitivity analysis methodologies.

Karstmod - A generic modular reservoir model dedicated to spring discharge modeling and hydrodynamic analysis in karst

On the basis of the characterization of the different karst subsystems (Soil/Epikarst—Unsaturated Zone—Saturated Zone) and mathematical models developed on specific sites, we propose an adjustable modeling platform of karst for both the simulation of spring discharge at outlets and the analysis of the hydrodynamics of the compartments considered in the model. This platform was developed within the framework of the KARST observatory network initiative from the INSU/CNRS, which aims to strengthen knowledge-sharing and promote cross-disciplinary research on karst systems at the national scale.

The MEDYCYSS observatory, a Multi scalE observatory of flooD dYnamiCs and hYdrodynamicS in karSt (Mediterranean border Southern France)

In karst hydrosystems, the heterogeneity and the complexity of the transfer processes within the different compartments (soil, epikarst, unsaturated zone, saturated zone) control the recharge-discharge relationship. The MEDYCYSS observatory (Multi scalE observatory of flooD dYnamiCs and hYdrodynamicS in karSt) has been set up to better understand these transfers in a Mediterranean context. This observatory consists in a large karst hydrosystem located between the Herault River, the Vidourle River and the Mediterranean Sea (Southern France, north and west from Montpellier). It comprises two main hydrogeological sites and three main hydrological sites where karst/river interactions occur. The challenges of the MEDYCYSS observatory are i) to better characterize the karst/river interactions for the risk assessment of floods and water resource contamination; ii) to discriminate and quantify flow distribution (rapid and slow transfers) between the recharge catchment and the outlets of the hydrosystem; iii) to understand the role of the unsaturated zone on the long term storage of the aquifer and quantify the water storage variation with time; iv) to characterize groundwater hydrodynamics in karst watersheds under climatic and anthropogenic forcing at nested observation scales; and v) to quantify the recharge, which is a key variable for the assessment and management of groundwater resources in karsts. Accordingly, a continuous monitoring of the hydrochemistry and hydrodynamics (wells, karst network, permanent and temporary springs), as well as of the river and spring discharges has been constructed to characterize transfer processes and parameterize local and regional scale models. Meteorological variables are also monitored at various points where measurements of soil humidity and evapotranspiration recently started.

Assessment of groundwater recharge processes through karst vadose zone by cave percolation monitoring

Recharge processes of karst aquifers are difficult to assess given their strong heterogeneity and the poorly known effect of vadose zone on infiltration. However, recharge assessment is crucial for the evaluation of groundwater resources. Moreover, the vulnerability of karst aquifers depends on vadose zone behaviour because it is the place where most contamination takes place. In this work, an in situ experimental approach was performed to identify and quantify flow and storage processes occurring in karst vadose zone. Cave percolation monitoring and dye tracing were used to investigate unsaturated zone hydrological processes. Two flow components (diffuse and quick) were identified and, respectively, account for 66% and 34% of the recharge. Quickflow was found to be the result of bypass phenomenon in vadose zone related to water saturation. We identify the role of epikarst as a shunting area, most of the storage in the vadose zone occurring via the diffuse flow component in low permeability zones. Relationship between rainfall intensity and transit velocity was demonstrated, with 5 times higher velocities for the quick recharge mode than the diffuse mode. Modelling approach with KarstMod software allowed to simulate the hybrid recharge through vadose zone and shows promising chances to properly assess the recharge processes in karst aquifer based on simple physical models.

Stable and reactive centering in conduits for karstic exploration

Karst groundwaters provide fresh water resources to supply cities. In the case of deep groundwater resources around the Mediterranean basin there is a need to explore karst aquifers and gather data about their structure. Robots are well adapted for this task especially when exploring karst conduits at depths greater than 200m. However, as karst aquifers environment is very complex, robots need to exhibit a collection of abilities to ensure a safe exploration. Among them, being able to “center” safely within the karst conduits and avoid collisions is a key point to ensure the robot safety during its mission. We propose a new reactive way to do so, based on the Deformable Virtual Zone. This allows us to model the interaction of the robot with its environment as a physics inspired equation. We also prove the stability of this controller.

Identifying Flow Networks in a Karstified Aquifer by Application of the Cellular Automata-Based Deterministic Inversion Method (Lez Aquifer, France): CADI KARSTIC FLOW MODEL

The distributed modeling of flow paths within karstic and fractured fields remains a complex task because of the high dependence of the hydraulic responses to the relative locations between observational boreholes and interconnected fractures and karstic conduits that control the main flow of the hydrosystem. The inverse problem in a distributed model is one alternative approach to interpret the hydraulic test data by mapping the karstic networks and fractured areas. In this work, we developed a Bayesian inversion approach, the Cellular Automata‐based Deterministic Inversion (CADI) algorithm to infer the spatial distribution of hydraulic properties in a structurally constrained model. This method distributes hydraulic properties along linear structures (i.e., flow conduits) and iteratively modifies the structural geometry of this conduit network to progressively match the observed hydraulic data to the modeled ones. As a result, this method produces a conductivity model that is composed of a discrete conduit network embedded in the background matrix, capable of producing the same flow behavior as the investigated hydrologic system. The method is applied to invert a set of multiborehole hydraulic tests collected from a hydraulic tomography experiment conducted at the Terrieu field site in the Lez aquifer, Southern France. The emergent model shows a high consistency to field observation of hydraulic connections between boreholes. Furthermore, it provides a geologically realistic pattern of flow conduits. This method is therefore of considerable value toward an enhanced distributed modeling of the fractured and karstified aquifers.