Mathematical modeling of karstogenesis: an approach based on fracturing and hydrogeological processes

The karstogenesis of a synthetic aquifer is analyzed as a function of the matrix permeability and the fracture density affecting the carbonate reservoir. The synthetic carbonate aquifer generation results from numerical simulations based on Discrete Fracture Network (DFN) and groundwater flow simulations. The aim is to simulate karstogenesis processes in an aquifer characterized by a fracture network, while matching field reality and respecting geometrical properties as closely as possible. DFN are simulated with the soft REZO3D that allows the generation of 3-D realistic fracture networks, especially regarding the relative position of fractures that control the overall network connectivity. These generated DFN are then meshed and considered to perform groundwater flow simulation with the model GroundWater (GW). At each time step and for each fracture element, flow velocity and the mean groundwater age are extracted and used in an analogical dissolution equation that computes the aperture evolution. This equation relies on empirical parameters calibrated with former speleogenesis studies. In this paper, karstogenesis simulations are performed using fixed-head hydraulic boundary conditions within a single stratum as a function of two fracture density settings. The results are interpreted in terms of head fields, mean groundwater age distributions; while total flow rate and average aperture are analyzed as a function of time. The effect of fracture density and rock matrix permeability are then assessed

Hydrologic measurements in wells in the Aigion area (Corinth Gulf, Greece): Preliminary results

International audienceTwo wells have been equipped in 2002, in order to provide complementary hydrological information within the framework of the European project devoted to the Corinth Rift Laboratory. In this distensive tectonic domain, temporal series of flow (Neratzes well) and of piezometric fluid level (Trapeza well) have been recorded in 2002. We present here a first analysis of the data. As far as Trapeza is concerned, we present the first results of the calibration of the well, using the tidal and barometric analyses. The strain sensitivity of the well, as well as the aquifer hydraulic conductivity, is derived from tides, whereas the specific storativity of the aquifer and its confinement degree are estimated from the barometric response. These preliminary results show that the aquifer has got properties that make possible the observation of coupled tectonic/hydrological processes. In addition, the time series let appear some fast and permanent variations of level. We analyse these phenomena and detail the various assumptions, which could explain these processes. As far as Neratzes is concerned, we present the first data measured from the flowmeter. Contrary to the piezometric data, the flow measurements are not conventional. At the end of this first year of measurement, we prove that we are able to record the tides effect using the flowmeter. A prospective analysis of all these results will be drawn

Large-scale spatial reconstitution of pressure and tracer tests responses in a karst aquifer (Lez aquifer, France)

International audienc

Large-scale spatial reconstitution of pressure and tracer tests responses in a karst aquifer (Lez aquifer, France)

International audienc

Large-scale spatial reconstitution of pressure and tracer tests responses in a karst aquifer (Lez aquifer, France)

International audienc

Hydraulic characterization and identification of flow-bearing structures based on multi-scale investigations applied to the Lez karst aquifer

International audienceStudy Region: The Lez aquifer is a Mediterranean karst system located in southern France, which supplies groundwater to the Montpellier urban area. Study Focus: Multi-scale hydrodynamic investigations were carried out in a fractured and karstic aquifer in order to identify the flow-bearing structures and evaluate their hydraulic properties. The study is based on an extensive dataset developed from several hydraulic tests, performed at different spatial and temporal scales. The scales ranged spatially from a few meters to more than 15 km and temporally from a few minutes to a few months. New Hydrological Insights for the Region: The data analysis shows that the hydraulic connectivity at both local and regional scales is mainly due to sub-horizontal flow-bearing structures, in which a conduit network has developed. This structure appears mainly to be located at the interface between two stratigraphic units, at the transition between Jurassic and Cretaceous limestones (Kimmeridgian-Berriasian interface). At the regional scale, this flow-bearing structure plays a major role in large-scale connectivity since a compartmentalization of the Lez aquifer appears where the continuity of this structure disappears. The hydraulic properties estimated appear to be strongly dependent on the investigated geological structures and on the different hydrogeological methods used for the borehole, local and regional scale of investigations

Multi-scale Assessment of Hydrodynamic Properties in a Karst Aquifer (Lez, France)

International audienceThe present study focuses on the hydrodynamic characterization of the Lez karst aquifer (Southern France) on the basis of hydraulic field tests performed at different scales of space and under distinct hydrological conditions. Depending on the water level conditions, the organization of the flow paths linked to the geological structure of the reservoir changes and a compartmentalization of the system due the hierarchization of hydraulic connections to the main flow paths was assessed. For the same parameter characterized at borehole scale and at regional scale, a difference of 10\textendash105 has been quantified. This quantification of hydrodynamic parameters provides important constraints on multiscale modeling and the characterization of main flow paths in such a karst system

Quantification of the specific yield in a two-layer hard-rock aquifer model

International audienceHard Rock Aquifers (HRA) have long been considered to be two-layer systems, with a mostly capacitive layer just below the surface, the saprolite layer and a mainly transmissive layer underneath, the fractured layer. Although this hydrogeological conceptual model is widely accepted today within the scientific community, it is difficult to quantify the respective storage properties of each layer with an equivalent porous medium model. Based on an HRA field site, this paper attempts to quantify in a distinct manner the respective values of the specific yield (Sy) in the saprolite and the fractured layer, with the help of a deterministic hydrogeological model. The study site is the Plancoët migmatitic aquifer located in north-western Brittany, France, with piezometric data from 36 observation wells surveyed every two weeks for eight years. Whereas most of the piezometers (26) are located where the water table lies within the saprolite, thus representing the specific yield of the unconfined layer (Sy1), 10 of them are representative of the unconfined fractured layer (Sy2), due to their position where the saprolite is eroded or unsaturated. The two-layer model, based on field observations of the layer geometry, runs with the MODFLOW code. 81 values of the Sy1/Sy2 parameter sets were tested manually, as an inverse calibration was not able to calibrate these parameters. In order to calibrate the storage properties, a new quality-of-fit criterion called “AdVar” was also developed, equal to the mean squared deviation of the seasonal piezometric amplitude variation. Contrary to the variance, AdVar is able to select the best values for the specific yield in each layer. It is demonstrated that the saprolite layer is about 2.5 times more capacitive than the fractured layer, with Sy1=10% (7%<Sy1<15%) against Sy2=4% (3%<Sy2<5%), in this particular example

Specific yield comparison between the two layers of a hard-rock aquifer model with Advar, a new quality-of-fit criterion

International audienceHard-rock aquifers (HRA) have long been considered to be two-layer systems, with a mostlycapacitive layer just below the surface, the saprolite layer, and a mainly transmissive layerunderneath, the fissured layer. Although this hydrogeological conceptual model now attracts a wideconsensus within the scientific community, it is difficult to prove that it can be efficient in terms ofdeterministic modelling, especially with an equivalent porous medium model, w hich would not be thefirst choice for “fractured aquifers”. In order to investigate the storage capacities of both layers and therecharge, a deterministic hydrogeological model was used on a HRA to calibrate the specific yield ofeach layer. The application site is the Plancoët migmatitic aquifer located in north-western Brittany,France, with piezometric data from 40 observation wells surveyed every two weeks for eight years.When the variance, the most common quality-of-fit criterion used in inverse calibration methods, wasfound to be inefficient for calibrating the specific yield and the recharge together, Advar , a newquality-of-fit criterion based on the seasonal piezometric amplitude variation, was developed andcalculated for a set of 512 simulation results. Contrary to the variance, it is able to define the bestvalues for the recharge and the specific yield in each layer. Adding to the conclusion that this two-layermodel is well suited to such a HRA, it is demonstrated that the saprolite is about 2.5 times morecapacitive than the fissured layer