Hydrogéologie | EPFL Graph Search

vignette|droite|redresse=1.2|Bloc-diagramme d'un karst recoupé par une rivière. Lhydrogéologie (de hydro-, eau et géologie, étude de la terre), également nommée hydrologie souterraine et plus rarement géohydrologie, est la science qui étudie l'eau souterraine. Son domaine d'étude repose essentiellement sur deux branches des sciences de la Terre, la géologie et l'hydrologie, mais aussi sur de nombreuses autres branches comme la géostatistique, la physique, la chimie, la biologie, la géochimie, l'hydrochimie, la géophysique, l'hydrodynamique, l'hydraulique souterraine, l'analyse numérique ainsi que des techniques de modélisation. À ce titre, l'hydrogéologie est par excellence une science interdisciplinaire. L'hydrogéologie s'occupe de la distribution et de la circulation de l'eau souterraine dans le sol et les roches, en tenant compte de leurs interactions avec les conditions géologiques et l'eau de surface. L'hydrogéologie par rapport à d'autres branches vignette|Peinture d

Geological heterogeneity in landslides

Boris Matti

Significant progress has been made these last decades in the development of hydrogeological numerical flow modelling for describing the hydrodynamic behaviour of landslides. However, these new sophisticated methods are still very seldom used in the problems of slope instability in particular because of the hydrogeological complexity which characterizes them; thin aquifers, discontinuous media, succession of saturated and unsaturated zones, low permeabilities, high hydraulic gradients, lithological heterogeneity, strong contrasts of permeabilities and heterogeneous infiltration. Predictive models of flow in the subsurface, which are often based on homogeneous porous media types of representation, are badly adapted to natural systems that are characterized by highly heterogeneous media such as landslides. These models are good and reliable on a landslide scale (regional scale), but their quality may be affected on a local scale by strong geological heterogeneities. Geological heterogeneities of the subsurface take part in determining the hydrodynamical and geomechanical behaviour of landslides. However, their spatial distribution is partially unknown. Thus, the principal objectives of this PhD thesis are: (i) To carry out an integrated multidisciplinary characterization study on the internal structure of landslides in flysch and Quaternary environments, in order to clarify the organisation of the geological heterogeneities and to identify the hydrodynamic implications. (ii) To propose a conceptual model representing the geological architecture and the hydrogeological functioning. (iii) To examine the effects of heterogeneity and anisotropy on flow systems. (iv) To better understand the influence of geological heterogeneities on the mechanical behaviour of large landslides by performing numerical sensitivity analyses, by means of different heterogeneity scenarios on the field parameters. (v) Finally, to test the incidences on slope stabilization techniques; evaluation of the efficiency of a drainage gallery work. The main test site of la Frasse landslide (VD, Switzerland) was chosen, and completed with additional landslide cases. The main results are the following: In most of the case studies, the landslide mass is composed of an old prehistoric stabilized mass, pinched between the active sliding mass and the bedrock, and playing an important hydrologic role. The stabilized mass and the bedrock form the substratum of the landslide. Landslides occurring in these types of media are defined by an organized heterogeneous environment with "fracture" flows and discontinuity porosity. The overall hydraulic conductivity is low, and locally high permeable zones exist. Regional groundwater circulations are limited and form local interconnected aquicludes organised in thin aquifers, and presenting saturated and unsaturated zones. The hydrogeological analyses showed that the system presents a bimodal permeability; (i) Low hydraulic conductivities characterizing the global matrix and defining the capacitive fraction, and (ii) high permeable features, with high hydraulic conductivities defining the conductive fraction, and favouring strong channelling effects. Besides, the observation shows that the aquifer system is generally very reactive with important magnitudes. Often, there is a straight correlation between water level variation and climatic conditions (rainy events). Landslides are characterized by two important inflows namely effective infiltration from the surface and lateral inflows from the neighbouring units. Water transfer between the stabilized mass and the active mass may be important and thus have to be considered. The existence of water transfer between the bedrock and the landslide mass (stabilized and active) is not well established. The bedrock and the landslide mass present a hydrological behavioural independence. Theoretical two- and three-dimensional flow models are used to investigate the effects of the spatial variability of the hydraulic conductivity on the underground flows. The role of the connectivity in generating flow channelling is examined thanks to the observation of close relations between the permeability and the hydraulic pressures. The sensitivity analysis shows clearly that the relation between local permeability and hydraulic pressures is not straight, and that the organization of the flows depends on the heterogeneity of the hydraulic properties and their spatial correlation. Strong channelling effects are observed in highly heterogeneous porous media. The development of flow channelling as a function of the variance of the natural log permeability values and the correlation lengths is demonstrated. The integrated multi-disciplinary geological characterization at the La Frasse test site combined with the hydrogeological and lithological data of several additional case studies led to the proposal of a global conceptual model. The following assumptions are considered to enable a subsequent quantification of flow components: The flow occurs under confined to leaky conditions, with leakage varying in space; The flow framework is controlled by a complex multi-layer system, isolated lenses or perched aquifer; The aquifer system is divided into interconnected hydrological zones presenting various degrees of saturation; Each hydrological zone may function individually from the others; Horizontally and vertically, the flow direction in the porous matrix is affected by prevailing structural patterns generating channeling effects; The flow is multidirectional, free and channelized, and is affected by temporal and spatial changes; The aquifer is under an unsteady flow regime due to seasonal variation of natural gradients; A conceptual model based on a simple reservoir approach is proposed. It allows the representation of most of the field observations and the main characteristics, namely the organized heterogeneity and the duality of the aquifers. The system is represented by various reservoirs more or less connected and saturated. Complex storage capacities and plug-flow effects may record past events and reactive sliding processes several months after the last important rainy event. The analysis shows that function of the capacity and the degree of saturation of the system, an important hydrological event is not necessarily associated to a reactivation. And, according to the degree of complexity of the system (saturation, connectivity...) a localized geological modification (variation of permeability, reservoir burst...) may produce a chain reaction, and generate failures in unexpected places. The conductive fraction favours the drainage of the system, whereas the capacitive fraction controls the distribution of the hydraulic heads. The role of the phreatic nappe, through the conductive fraction, is to drain and control the hydrologic equilibrium of the system. Therefore landslide remediation with the help of a deep drainage gallery is obviously the most valuable method for this type of landslide. It supports and enhances the natural effects of the conductive fraction in draining the system. Finally, in this context the efficiency of civil engineering works was evaluated according to the heterogeneity of the medium. This study describes transient hydrogeological and geomechanical models realized jointly in 2006 by the EPFL and GeoMod SA within the framework of the stabilization work of the La Frasse landslide. These models evaluate the impact of a deep drainage gallery with subvertical pipes towards the surface in terms of reduction of the deformation velocities and increase of the factor of safety of the landslide. Three variants consisting of different inter pipe spacings are tested. Considering the local heterogeneities, the results show that a mean spacing between the pipes of the order of 10 m is able to control the temporal head fluctuations between the wells within a range of some meters. Moreover, this solution induces a strong diminution of the predicted displacements during a specific crisis, from 101cm for the model without drainage to around 14 cm for the drained model, and a significant gain of security (from 1.05 to 1.30).

EPFL2008

Méthodes d'analyse du fonctionnement hydrogéologique des versants instables

Pierre Tullen

Significant progress was made these last decades in the development of hydrogeological methods. However, these new methods are still very little used in the problematics of unstable slopes, mainly because of the hydrogeological complexity, which characterizes them (thin aquifers, discontinuous media, succession of saturated and unsaturated zones, lower permeability, higher hydraulic gradients, evolution of the permeability in time in relation to slope movements, etc.). Consequently, the approach usually developed to study slope instabilities is based on the setting-up of relations between hydroclimatologic parameters and slope movements. This approach exclusively concentrates on causes and effects, without developing the processes induced by these causes and responsible for these effects. This "black-box" type of relation is in some cases satisfactory, but in others, the analysis of the induced hydrogeological processes becomes essential to understand the functioning of the instability phenomenon and to predict the movements. Our project aims at introducing this missing link to allow a better understanding of the hydrogeological processes caused by particular hydroclimatologic conditions. It is a matter of determining the relation between hydroclimatologic phenomena and a critical hydrogeological state, which can lead a phenomenon of instability to reactivation or to a state of rupture. Unfortunately, this relation cannot be approached in the same way for all instabilities throughout the world. Owing to the specificity of each slope, the relation must be established individually, which may lead to a hydrogeological typology of unstable slopes. Accordingly, we propose a methodology comprising a succession of stages that are useful for the understanding of the hydrogeological processes underlying slopes. This approach aims at establishing a hydrogeological assessment on the scale of the slope or, if necessary, on a regional scale, in order to determine groundwater infiltration zones, trajectory and velocity of groundwater run-off as well as outlet areas. The usual ways of doing this are generally the measurement of the river flow rate, a pluviometric follow-up or direct hydrogeological measurements via piezometers. These traditional approaches generally remain limited by problems of space representativeness or life span; on top of this come problems of cost due to the often chaotic morphology of the slope instabilities and the fact that they are not easily accessible. To make up for these various problems, we propose a complete methodological approach to improve the understanding of the groundwater run-off systems for the unstable slopes: A characterization of the geological domain based on geophysical measurements in addition to the borehole data. This approach, original in the specific context of unstable slopes, makes it possible to obtain a three-dimensional representation of the heterogeneity which composes the subsurface grounds in a fast, efficient and economic way; A spatialization of the climatologic observations in order to take into account the distribution of the precipitations while considering the various states (rain and/or snow) that constitute the water contribution to a slope in relation to altitude; A hydrogeochemical characterization of the outlets, which makes it possible to assess the general groundwater circulation system within the slope, and to come to a decision about one or several possible trajectories of groundwater run-off; An isotopic characterization of the outlets, which allows estimating the altitude of infiltration of the groundwater. This characterization can also give some information on the infiltration zones. These methods also make it possible to characterize the general groundwater run-off system. In some cases, it is also possible to come to a conclusion, quantitatively speaking, concerning the mixing between several reservoirs and to estimate the time of residence of the water in the aquiferous system; A qualitative validation of the assumptions on the basis of artificial tracing tests which will attest the existence of hydrogeological connections between a catchment area and an outlet. This approach will also inform about the time of residence of water; A validation of the conceptual model on the basis of a bi- or three-dimensional numerical simulation of the groundwater run-off in order to validate the hydrological and hydrogeological relations. The calibration of these numerical models is based on a set of hydrogeological data, in particular the outlet discharge, the isotopic composition of water and/or their hydrochemical composition. These strictly hydrogeological numerical simulations can then be translated into a mechanical state to evaluate the stability of the slope. The whole of the points suggested by this methodological approach is not always essential to determine the hydrogeological processes that govern each slope. According to the characteristics of each study site and the means at disposal, some could be treated only partly, or not at all, depending on the established objectives. In the end, a hydrogeological typology of the unstable slopes should make it possible to propose a standard methodological approach and specific tools for analysis in direct relationship to the hydrogeological context which characterizes each slope. The methodological approach suggested in this PhD thesis was tested and validated on three representative sites through Switzerland and the alpine arc that present quite distinct characteristics of hydrogeological functioning. They are the slopes of Hohberg in the canton of Fribourg, Triesenberg in the Principality of Liechtenstein and Peney in the canton of Geneva. This validation phase allowed to assess the hydrogeological functioning of the three studied slopes and to evaluate the relation between the hydrogeological processes and the phenomena of instability. An approach such as that presented in this study opens up prospects to carry out means of sustainable stabilization with an aim of solving problems of slope instabilities.

EPFL2002

On-site biostimulation evaluation of a remediated perchloroethene-contaminated acquifer located in Jablunkov, Czech Republic

Isabelle Fäh

Chlorinated ethenes solvents (CEs) are one of the most common contaminants identified in groundwater ecosystems. Long used as a degreaser in industry, perchloroethene (PCE) and trichloroethene (TCE) are particularly persistent in the environment as recalcitrant to oxic degradation, and classified as unhealthy and dangerous for the environment. Under anaerobic conditions, CEs can be degraded naturally in the environment by organohalide respiring bacteria (OHRB) through dehalogenase enzymes reducing them into ethene, a non-hazardous end product. However, toxicity of the intermediate daughter compounds of the degradation (TCE, DCE and VC) is higher than PCE. As this natural attenuation is a slow process, intermediate compounds accumulate in the environment and biostimulation is often used to enhance the complete degradation of CEs. Substrates are added to the contaminated aquifer and used by fermentative bacteria releasing by-product such as H2 that is essential for the dehalorespiration of CEs. Biostimulation has been studied in a PCE contaminated-aquifer located in Jablunkov (Czech Republic). Cheese whey (CW) has been injected into the groundwater in November 2012 and June 2013. The objective of the current project is to evaluate the efficiency of the CW applications by analysing microbiologic and physico-chemical parameters describing the aquifer at six sampling dates from September 2012 to December 2013 within six wells allocated over the aquifer. Global bacterial community structures were analysed in laboratory using Polymerase Chain Reaction (PCR) and T-RFLP (Terminal Restriction Fragment Length Polymorphism). OHRB were analysed as well by specific PCR targeting 16S rRNA and dehalogenase genes of bacteria known to degrade completely the CEs. Then, contaminants and ethene concentrations as well as others Terminal Electron Acceptors implicated in the respiratory processes (Terminal Electron Accepting Processes – TEAPs) were measured to give information about the redox conditions within the aquifer. Finally, others hydrogeological parameters such as pH and temperature were measured. All data were analysed using Multiple Factor Analysis (MFA) in the R software. Interpretations provided a global model indicating the characteristics and functioning of the aquifer. The successive CW applied on-site have a gradual impact on the environmental conditions and on the structures of bacterial communities. After the two CW applications, the aquifer has partly changed to conditions favourable for complete dechlorination. It was the case for three wells over six. Two CW injections have been necessary to allow the production of ethene for two wells whereas only one was needed for the other one. Biostimulation were efficient but not sufficient to completely degrade the CEs on the whole aquifer. In one well, the accumulation of DCE were more important than the production of ethene. In two other wells, ethene was not produced but the statistical analysis showed that they tend toward complete dechlorination conditions. New CW applications would be necessary to completely remediate this aquifer.

2014