La Loire souterraine : circulations karstiques dans le Val d'Orléans

Lors de son passage dans le Val d'Orléans, la Loire perd une fraction de son débit. Une partie des eaux perdues réapparaît dans le Loiret, court affluent de la Loire qui donne pourtant son nom à tout un département. Les débits de pertes et les circulations souterraines de ces flux sont difficiles à estimer. Leur détermination est pourtant essentielle non seulement pour la caractérisation du système, mais aussi pour la prévention des pollutions et des catastrophes naturelles

Coupling between hydrogeology and deformation of mountainous rock slopes: Insights from La Clapière area (southern Alps, France)

International audienceMeteoric infiltration influence on large mountainous rock slopes stability is investigated by comparing hydrogeologic and gravitational structures from detailed mapping of the 'La Clapière' slope. The slope infiltrated waters are trapped in a perched aquifer that is contained in deposits inside tensile cracks of the upper part of the slope. Flow rates of 0.4 to 0.8 l s−1 from the perched aquifer to the landslide cause landslide accelerations. Numerical modeling shows that a 0.75 l s−1 infiltration yield increases conditions for toppling with failure through tilting of large rock volumes from the perched aquifer bottom down to the foot of the slope

A water-table dependent reservoir model to investigate the effect of drought and vascular plant invasion on peatland hydrology.

International audienceThis paper investigates the water table dynamics in a peatland showing a wide range of water table fluctuations. A reservoir model of water table fluctuations in a double-porosity peat is proposed, by calculating the stored water in effective porosity of the peat from precipitation and evapotranspiration datasets. Calculations conceptualize vascular plant consumption through a crop coefficient. Changes in water storage, located in the effective porosity of the peat, are described through a maximum infiltration rate and a maximum storage capacity. Water discharges take place in runoff and percolation reservoirs. The runoff coefficient is considered to be water table dependent. This model was tested on a peatland that has experienced strong water table fluctuations caused by summer drought and/or by vascular plant water consumption. A water table dependent runoff model appeared to be adequate to describe the water table fluctuations in peatland. From this model, vascular plants were found to increase the crop coefficient and to limit percolation through the peat. The high water table depth in winter was found to change with the year and is related to an equilibrium between slow infiltration in peat versus percolation plus evapotranspiration. In this disturbed peatland, even if overland flows occurred after a drought, the re-saturation of effective porosity was slow with about 30% of air trapped in the porosity 6 months after the drought period. The effects of drought on peat saturation were observed over more than a single hydrological cycle. This can affect the biogeochemical processes controlling the C cycle in peatland

Unstable rock slope hydrogeology: insights from the large-scale study of western Argentera-Mercantour hillslopes (South-East France)

International audienceInventory of unstable hillslopes, hydrogeological mapping and hydrochemical characteristics of natural spring waters were used to determine the long-term relationships between groundwater and gravitational instabilities in the Upper Tinée Valley (South-East French Alps). Water chemistry and flow records allow to propose a conceptual model of water flow within unstable rocky slopes and to back-calculate the volume of infiltrated water and the flow velocity in the aquifers for different deformation states of the slopes. An increase in infiltrated yield, flow velocity and porosity is observed and linked to collapsed and toppled structures in the upper parts of the hillslopes. In these areas, perched aquifers take place in the reworked media. When a large landslide occurs, it modifies the geometry of the slope and bypasses the perched flows down to the foot of the slope. With long-term continuous slope deformation, the associated effect between water flows and slope destabilization changes. In the fractured rock, the coupled effect corresponds to rising water pressures with limited volumes of infiltrated water; in the more fractured and permeable collapsed and toppled areas, the volumes of infiltrated water increase with a lower variation of water pressures

Hydraulic interactions between fractures and bedding planes in a carbonate aquifer studied by means of experimentally induced water-table fluctuations (Coaraze experimental site, southeastern France)

International audienceIn aquifers with variable permeability, the water exchanges between high and low permeability regions are controlled by the hydraulic head gradient. Past studies have addressed this problem mainly considering steadystate hydraulic conditions. To study such exchanges during water-table fluctuations, a spring was equipped with a water-gate that creates 10-meter artificial fluctuations of the water table. The water exchanges are discussed with respect to hydrochemical and pressure measurements in the groundwater. With successive water-table fluctuations the mineralization and pH decrease, but the bicarbonate content increases in response to carbon dioxide dissolution . At this scale of single fractures and surroundings, the hydrochemistry allows water flows from low or high permeability discontinuities to be discriminated. During hydraulic head fluctuations, the waters from low and high permeability discontinuities become mixed. During water-table rise, the low-permeability matrix contributed to the refilling of the permeable faults and to the mixing of the waters. Dynamic flows in the opposite direction to the hydraulic gradient can expel mineralized water, pushing it towards permeable discontinuities. This mechanism could be the cause of the peak in the mineralisation recorded in some karst springs at the start of flood events

Characterization of an internal slope movement structure by hydrogeophysical surveying

International audienceA hydrogeophysical study was carried out by a water controlled injection within a landslide situated on an active part of the La Clapière landslide foot (Alpes Maritimes, France). Coupling of both real-time geophysical and hydrological follow ups allowed the representation and quantification of the surface water drainage in space and time within the slipped mass. Thus, 30% of the injected water is quickly drained by a complex slipping surface meanly situated at 10-m depth. The transit time between injection and outflow of the water allowed an overloading of about 10 m3 (i.e. 10 tons) comparable with classical rain events in the area. This weight and the associated interstitial pressures increase have not led to any movements asking for the origin of the water volumes which could induce destabilizations. This experiment enabled an accurate redefinition of the internal slope structure and the understanding of the dynamics of the slipped mass with a surface hydraulic request

Localisation of a Reactive Transport Zone in a Saturated Karstic Conduit Deduced from Natural and Artificial Tracer Tests

International audienceFor groundwater resources managers, flow modeling is a useful tool to investigate sustainable scenarios of water use. However, in karstic aquifers, the quality of scenarios is limited by the difficulties of locating and describing the position, geometry and possible time evolution of conduits. The location of conduits in the karstic aquifer of the "Val d'Orléans" (France) were defined using 200 boreholes, surface collapses and 24 artificial tracer tests, which facilitated the development of a simplified conceptual model of flow in the saturated conduits and the surrounding rocks. 68 logs present voids > 50 cm and locate a highly porous zone around 80 m a.s.l. with voids that average 3.5 meter in diameter. In this saturated conduit, 1D quantitative interpretation of artificial tracer tests validate the proposed conceptual model of a saturated conduit under pressure, with an efficient section about 10 m2 an input flow about 3.1 m3/s with 2.9 m3/s flowing from the conduit toward the surrounding rock before arriving at the Loiret Spring. The conceptual model of flow and the previous water chemical analysis show that the transported elements in the groundwater react and dissolve carbonate rocks, mainly inside the conduit, and that this may increase the diameter of the conduit zone by an estimated 40 cm in 100 year

Estimation of quantitative descriptors of northeastern Mediterranean karst behavior: multiparametric study and local validation of the Siou-Blanc massif (Toulon, France)

International audienceKey parameters controlling the recharge and behavior of Mediterranean karsts were selected in order to make a quantitative description of northeastern Mediterranean karsts on a regional scale. The methodology was applied to an actual karstic aquifer on the Siou-Blanc Plateau (France). For the recharge study, it was observed that the average yearly rainfall value and δ18O measurements in springs can be considered as good descriptors of climatic variations observed in the Mediterranean area. They can be used to estimate the intake area and the infiltration coefficient. A comparison with a numerical (double permeability) flow model (MODFLOW) on the Siou-Blanc karst improves these exponential relations between effective rainfall and δ18O measurements. Infiltrated water, which flows though different rock types, induces contrasts in the water chemistry. An instantaneous physical and chemical analysis of all the springs of the Siou-Blanc aquifer displays the same expected functioning and variations as had been forecast using the conceptual scheme. Thus, it can be applied to wide areas associated with a northeastern Mediterranean climate for a first approach of a karst study; such a model enables a useful estimation of recharge and behavior with few simple data

Use of continuous measurements of dissolved organic matter fluorescence in groundwater to characterize fast infiltration through an unstable fractured hillslope (Valabres rockfall, French Alps)

International audienceContinuous measurements of natural fluorescence of dissolved organic matter (DOM) in groundwater have been used to characterize infiltration through an unstable fractured hillslope at the event time scale. Within the gneissic Valabres rockfall area in the southern French Alps, two sites, in the unstable massif and in the collapse area, were continuously monitored for electrical conductivity and for DOM natural fluorescence using a field fluorometer, and analyzed weekly for hydrochemical data. Two main results were found. From a methodological point of view, DOM natural fluorescence was a relevant tracer of fast infiltration in fissured media, knowing that continuous measurements improved the study of infiltration processes at the event time scale. From a hydrogeological point of view, the unstable fractured massif showed delayed dilution phenomena and the collapse areas showed fast and slow infiltration by piston-type flows via more or less open fractures. Consequently, from this conceptual model one may suppose that, unlike the collapse zones, the fractured hillslope threatening the valley is not submitted to strong pore-water pressure variations. Des mesures en continu de la fluorescence naturelle de la matière organique dissoute (DOM) dans les eaux souterraines ont été utilisées pour caractériser l'infiltration à travers un versant fracturé instable. Dans la région gneissique de l'éboulement de Valabres dans les Alpes françaises du Sud, deux sites, dans le massif instable et dans la zone de glissement, sont suivis en continu pour des mesures de conductivité électrique et de fluorescence naturelle avec un fluorimètre de terrain, et sont l'objet de mesures hydrochimiques hebdomadaires. D'un point de vue méthodologique, la fluorescence naturelle DOM est un traceur pertinent de l'infiltration rapide en milieu fissuré, sachant que des mesures continues améliorent l'étude du processus d'infiltration à l'échelle de l'évènement. D'un point de vue hydrogéologique, le massif fracturé instable montre des phénomènes de dilution retardée et la zone de glissement montre une infiltration rapide et lente par écoulements de type piston via des fractures plus ou moins ouvertes. En conséquence, à partir de ce modèle conceptuel, on peut supposer que, à la différence des zones de glissement, la porosté de fissure dans le versant fracturé menaçant la vallée n'est pas soumise à de fortes variations de pression d'eau

Geophysical survey to estimate the 3D sliding surface and the 4D evolution of the water pressure on part of a deep seated landslide

International audienceGeophysical surveys were conducted on the very unstable front part of the La Clapière landslide in the French Alps (Alpes Maritimes). The electrical resistivity survey was carried out to obtain, for the first time on this deep-seated landslide, 3D information on the slipping surface and the vertical drained faults. Moreover, we planned to follow within time (6 months) the evolution of the saturated zones (presence of gravitational water) and their percolation into the shearing zones. Our 4D results showed the importance of the complex water channelization within the slope and relation to geological discontinuities