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Allegre V.,University of Strasbourg | Jouniaux L.,University of Strasbourg | Lehmann F.,CNRS Hydrology and Geochemistry Laboratory of Strasbourg | Sailhac P.,University of Strasbourg
Geophysical Journal International | Year: 2010

The electrokinetic potential results from the coupling between the water flow and the electrical current because of the presence of ions within water. The electrokinetic coefficient is well described in fluid-saturated media, however its behaviour under unsaturated flow conditions is still discussed. We propose here an experimental approach to investigate streaming potential variations in sand at unsaturated conditions. We present for the first time continuous records of the electrokinetic coefficient as a function of water content. Two drainage experiments have been performed within a column filled with a clean sand. Streaming potential measurements are combined with water pressure and water content measurements every 10 cm along the column. In order to model hydrodymanics during the experiments, we solve Richards equation coupled with an inverse problem to estimate the hydraulic parameters of the constitutive relations between hydraulic conductivity, water pressure and water content. The electrokinetic coefficient C shows a more complex behaviour for unsaturated conditions than it was previously reported and cannot be fitted by the existing models. The normalized electrokinetic coefficient increases first when water saturation decreases from 100 to about 65-80 per cent, and then decreases as the water saturation decreases, whereas all previous works described a monotone decrease of the normalized electrokinetic coupling as water saturation decreases. We delimited two water saturation domains, and deduced two different empirical laws describing the evolution of the electrokinetic coefficient for unsaturated conditions. Moreover, we introduce the concept of the electrokinetic residual saturation, Sr,ek w, which allows us to propose a new model derived from the approach of the relative permeability used in hydrodynamics. © 2010 The Authors Journal compilation © 2010 RAS.

Ackerer P.,CNRS Hydrology and Geochemistry Laboratory of Strasbourg | Delay F.,University of Poitiers
Journal of Hydrology | Year: 2010

This work inverts interference hydraulic tests from a karstic aquifer modeled as a single continuum with heterogeneous hydraulic properties (hydraulic conductivity K and specific storage capacity Ss). An automatic technique of parameterization is proposed to solve the inverse problem. Sought parameters are located at the nodes of a parameter grid made of triangular elements and independently superimposed onto the calculation grid of the flow problem. The parameter grid can be refined within the process of inversion so that the solution is improved locally because increasing the number of parameters better depicts spatial heterogeneity. Applied to hydraulic tests carried out at the Hydrogeological Experimental Site (HES) of Poitiers-France, this Adaptative Multi-scale Triangulation (AMT) method of parameterization reveals that hydraulic conductivity K and hydraulic diffusion K/. Ss are structured in space as correlated stochastic continuums. This structure is not imposed a priori but is found as the mere consequence of inverting drawdown data. The sought stochastic continuums are discussed and compared to previous results obtained by processing the same data with various techniques. Even though interference data show evidences of local karstic features, the hydraulic bulk behavior of the aquifer is that of a diffusive continuous medium. This reinforces the idea that at mid to large scales, flow in fractured rocks can also be handled with relevance by means of a continuous heterogeneous approach. This result would extend to fractured aquifers with local draining in open conduits as is the case in this study. © 2010 Elsevier B.V.

Younes A.,CNRS Hydrology and Geochemistry Laboratory of Strasbourg | Ackerer P.,CNRS Hydrology and Geochemistry Laboratory of Strasbourg | Delay F.,University of Poitiers
Reviews of Geophysics | Year: 2010

Mixed finite elements are a numerical method becoming more and more popular in geosciences. This method is well suited for solving elliptic and parabolic partial differential equations, which are the mathematical representation of many problems, for instance, flow in porous media, diffusion/ dispersion of solutes, and heat transfer, among others. Mixed finite elements combine the advantages of finite elements by handling complex geometry domains with unstructured meshes and full tensor coefficients and advantages of finite volumes by ensuring mass conservation at the element level. In this work, a physically based presentation of mixed finite elements is given, and the main approximations or reformulations made to improve the efficiency of the method are detailed. These approximations or reformulations exhibit links with other numerical methods (nonconforming finite elements, finite differences, finite volumes, and multipoint flux methods). Some improvements of the mixed finite element method are suggested, especially to avoid oscillations for transient simulations and distorted quadrangular grids. Copyright 2010 by the American Geophysical Union.

Payraudeau S.,CNRS Hydrology and Geochemistry Laboratory of Strasbourg | Gregoire C.,CNRS Hydrology and Geochemistry Laboratory of Strasbourg
Agronomy for Sustainable Development | Year: 2012

The demand for operational tools at a catchment scale is growing to assess both the sustainability of agricultural practices and the efficiency of mitigation measures on pesticide transfer to surface water. Here a literature review of 286 investigations highlights the large number of indicators and hydrochemical models developed at the catchment scale. Given this large number of indicators and models, the choice is difficult for potential users. Therefore, this article proposes a multi-criteria analysis applied to ten existing tools including physically based and conceptual models, indicators and multi-agent systems. We found the following major points: (1) Indicators and conceptual models are the most popular approaches to assess the transfer of pesticides to surface water at the catchment scale due to a trade-off between environmental relevance and adaptation to user's needs. (2) The latest indicators developed are inferred from the results of conceptual or physically based models to combine the strengths of each approach. (3) Only a handful of physically based models have addressed both flow and pesticide transport at the catchment as affected by the internal heterogeneity of the system. However, it is only physically based models that can simulate the impact of changes to the catchment. Physically based models integrate feedbacks between hydrological and chemical processes not possible from conceptual models or indicators alone. (4) The ability of models to assess the pesticide loads both in the dissolved and particulate phases is a key issue not properly addressed by many indicators or models. A key way forward is the integration of erosion processes with the fate of pesticide adsorbed to these particles. (5) At the catchment, the hydrological connectivity is perhaps the primary hydrological variable required to correctly assess rapid flow processes as surface runoff and associated pesticide transfer. This in turn implies using tools that explicitly represent the connectedness of surface and/or sub-surface water pathways including mitigation measures to correctly assess the risk of pesticide transfer. © INRA and Springer Science+Business Media B.V. 2011.

Clauer N.,CNRS Hydrology and Geochemistry Laboratory of Strasbourg
Clay Minerals | Year: 2011

Micrometric and nanometric illite-rich size fractions of claystones, bentonites and shales were exchanged with alkylammonium cations that have the specificity of stoichiometrically replacing K in trioctahedral mica interlayers. The purpose of the study was a separate evaluation of the K-Ar ages of potentially dioctahedral residual particles not affected by alkylammonium leaching and of potentially trioctahedral illites that were exchanged by the organic molecules. The K-Ar ages of micrometric size fractions from an Estonian Blue Clay sample collected next to another studied previously show that, even if identical in their mineralogical characteristics, the fractions contain variable amounts of trioctahedral particles that are of different origins. The alkylammonium treatment modifies slightly, within analytical uncertainty, the K-Ar ages of the <0.1 and 0.1-0.4 m m size fractions from, respectively, 464±13 and 530±14 Ma before organic exchange to 480±11 and 546±12 Ma after. The K-Ar ages of alkylammonium exchanged nanometric size fractions from the rim and centre of a thick Upper Cretaceous bentonite bed in Montana suggest that trioctahedral illite-rich particles mineralogically and chemically homogeneous and about 30 Ma old precipitated next to older dioctahedral particles of ∼60-65 Ma. The untreated mixtures consist of two generations of authigenic illite having apparently different di/trioctahedral layerings. The same type of authigenic di/trioctahedral illite layering could be demonstrated for nanometric illite particles of a bentonite bed from the East Slovak Basin, one size fraction appearing to even consist of a pure trioctahedral illite as the alkylammonium exchange emptied completely the illite interlayers. The nearby shale level consisted of detrital illite particles that were found to be of different ages, di/trioctahedral layerings and therefore varied origin. K-Ar ages of alkylammonium exchanged micrometric to nanometric illite and illite-smectite mixed layers, either increasing or decreasing, appear to outline variable di/trioctahedral layering assemblages or independent particle mixtures resulting from a more complex smectite illitization process than the conventionally assumed homogeneous reaction. It could record changing chemical compositions of the interacting pore fluid during crystallization, even when illitization progressed slowly. Similar ages before and after alkylammonium exchange suggest a constant chemical composition and therefore an homogeneous dioctahedral crystal structure. Alternatively, a changing chemical composition of the fluids during illitization is potentially recorded by variable K-Ar ages of the alkylammonium-leached illite resulting from differentiated ion exchanges. © 2011 Mineralogical Society.

Imfeld G.,CNRS Hydrology and Geochemistry Laboratory of Strasbourg | Lefrancq M.,CNRS Hydrology and Geochemistry Laboratory of Strasbourg | Maillard E.,CNRS Hydrology and Geochemistry Laboratory of Strasbourg | Payraudeau S.,CNRS Hydrology and Geochemistry Laboratory of Strasbourg
Chemosphere | Year: 2013

Glyphosate is an herbicide used widely and increasingly since the early 1990s in production of many crops and in urban areas. However, knowledge on the transport of glyphosate and its degradation to aminomethylphosphonic acid (AMPA) in ecosystems receiving urban or agricultural runoff is lacking. Here we show that transport and attenuation of runoff-associated glyphosate and AMPA in a stormwater wetland differ and largely vary over time. Dissolved concentrations and loads of glyphosate and AMPA in a wetland receiving runoff from a vineyard catchment were assessed during three consecutive seasons of glyphosate use (March to June 2009, 2010 and 2011). The load removal of glyphosate and AMPA by the wetland gradually varied yearly from 75% to 99%. However, glyphosate and AMPA were not detected in the wetland sediment, which emphasises that sorption on the wetland vegetation, which increased over time, and biodegradation were prevailing attenuation processes. The relative load of AMPA as a percentage of total glyphosate increased in the wetland and ranged from 0% to 100%, which indicates the variability of glyphosate degradation via the AMPA pathway. Our results demonstrate that transport and degradation of glyphosate in stormwater wetlands can largely change over time, mainly depending on the characteristics of the runoff event and the wetland vegetation. We anticipate our results to be a starting point for considering degradation products of runoff-associated pesticides during their transfer in wetlands, in particular when using stormwater wetlands as a management practice targeting pesticide attenuation. © 2012 Elsevier Ltd.

Younes A.,CNRS Hydrology and Geochemistry Laboratory of Strasbourg | Ackerer P.,CNRS Hydrology and Geochemistry Laboratory of Strasbourg
Water Resources Research | Year: 2010

Modeling density-driven flow in porous media may require very long computational time due to the nonlinear coupling between flow and transport equations. Time stepping schemes are often used to adapt the time step size in order to reduce the computational cost of the simulation. In this work, the empirical time stepping scheme which adapts the time step size according to the performance of the iterative nonlinear solver is compared to an adaptive time stepping scheme where the time step length is controlled by the temporal truncation error. Results of the simulations of the Elder problem show that (1) the empirical time stepping scheme can lead to inaccurate results even with a small convergence criterion, (2) accurate results are obtained when the time step size selection is based on the truncation error control, (3) a non iterative scheme with proper time step management can be faster and leads to more accurate solution than the standard iterative procedure with the empirical time stepping and (4) the temporal truncation error can have a significant effect on the results and can be considered as one of the reasons for the differences observed in the Elder numerical results. © 2010 by the American Geophysical Union.

Destandau F.,IRSTEA | Imfeld G.,CNRS Hydrology and Geochemistry Laboratory of Strasbourg | Rozan A.,IRSTEA
Ecological Engineering | Year: 2013

The economic crisis and increasingly stringent water quality requirements demand integrative approaches to reduce pesticide pollution in aquatic ecosystems and limit water treatment. In this study, environmental economists and wetland scientists joined forces to analyze the combination of different abatement measures to reduce pollution with more efficiency. Pesticide reduction directly at source (i.e., reduction of pesticide use), and combining pesticide source reduction with mitigation using a stormwater wetland to treat pesticide runoff are compared. The capacity of the buffer zone to reduce additional diffuse pollution with a given total abatement cost is evaluated, by placing emphasis on how the contribution of a buffer zone evolves according to the total cost. Fungicides were used as a representative class of synthetic pesticides widely used in vine growing, and more largely in conventional agriculture. Our results show that coupling reduction of pesticide source with the use of buffer zones collecting pesticide runoff can be economically advantageous. For a given total cost, the reduction of fungicide runoff is 90% greater when pesticide reduction at source is combined with pesticide mitigation by a stormwater wetland compared to the case of pesticide reduction at source only. However, the higher the total cost is, the more it is necessary to reduce pesticides at source and thus reduce pesticide mass transfer into aquatic systems. The results of this study is anticipated to be a starting point for considering cost and efficiency when combining different measures targeting pesticide mitigation in surface water, and in particular when using stormwater wetlands as a management practice. © 2013 Elsevier B.V.

Clauer N.,CNRS Hydrology and Geochemistry Laboratory of Strasbourg
Chemical Geology | Year: 2013

The strengths and weaknesses of the two Ar isotopic methods (K-Ar and 40Ar/39Ar) were evaluated on the basis of respective recent applications mainly on low-temperature K-bearing illite-type clay minerals. This review includes a presentation of basic, analytical and technical aspects for both methods, as well as a discussion of varied claims on the two methods and of requests about sample preparation and characterization. Whenever possible, the advantages and weaknesses of each method were compared on coeval results obtained by both methods on the same mineral separates. The comparative review examines stratigraphic dating of glauconites, indirect dating of low-temperature ore deposits, dating of burial-related illitization, and dating of polyphased tectono-thermal activity, more specifically of fault gouges. Some pending questions such as the necessary encapsulation due to 39Ar recoil and its restoration into step-heating patterns are also raised, together with the new potential of Ar-dating of nanometric illite crystals.Weakness of the K-Ar method is in its pioneering status that makes many believe that it is no longer accurate, because of its traditional analytical aspects, and of the K determinations leading to somewhat large uncertainties. However, precise evaluation of varied applications points to a K-Ar method having probably larger applicability in sedimentary to diagenetic environments than the 40Ar/39Ar method. The drawbacks become less important if the method is applied to nanometer-sized clay minerals in diagenetic to low-grade metamorphic environments. In this instance, the extracted size fractions are generally homogeneous and the relative uncertainty given by the age calculations, if mathematically justified, can be reduced by duplicate analyses. Weakness of the 40Ar/39Ar method is in its basics such as the 39Ar recoil, the necessary encapsulation, the reintegration of the 39Ar into the step-heating patterns, and the meaning of the step-heating patterns that are more suggestive of variable 39Ar "reservoirs" created among the clay particles by irradiation than of meaningful geologic ages. If the K-Ar method is the preferred method for dating diagenetic clay processes such as glauconitization, illite crystal nucleation and growth, or low-temperature hydrothermal activities, then the 40Ar/39Ar method has more potential in dating low-temperature tectono-thermal activities, and in detailing mixtures of multi-generation illite. © 2013 Elsevier B.V.

In the small Ringelbach research catchment, where studies on the water cycle components in a granitic mountainous environment have been conducted since 1976, the water-saturated areas that are hydraulically connected to the outlet play a major role in the streamflow generation, as it is here that complex interactions between atmosphere, surface and ground waters take place. During baseflow recession periods, which may last several months between two groundwater recharge events, the atmospheric inputs of water and energy on these contributing areas only explain the streamflow fluctuations observed around the master recession curve, which defines the groundwater contribution: fluctuating above it in the case of precipitation input on these areas, below it in the case of evaporation output from these areas. Streamflow may therefore largely deviate from the master recession curve in the case of long, hot, dry spells. Detailed mapping has shown that their variable extent is well related to baseflow by a loglinear curve. On the other hand, a synthetic master recession curve, well fitted by a second-order hyperbolic function, has been obtained from numerous pure recession periods. Both based on these two curves, a simple procedure and a simple model have been used to (i) validate the hypothesis that the connected saturated areas are the only permanent variable contributing areas and (ii) simulate the daily streamflow volumes over long baseflow recession periods by a water balance of the aquifer below these areas only. The storm runoff ratio for small to moderate rainfall events is indeed corresponding to the catchment saturated fraction at that time. The volume of daily streamflow oscillations is indeed corresponding to the evaporation at the potential rate from the saturated areas only. In both cases, streamflow naturally tends towards the master recession curve after the end of any atmospheric perturbation. Introducing these findings into TOPMODEL led to significantly improved simulation results during baseflow recession periods. The master recession curve may therefore be considered as a dynamic equilibrium curve. Together with the relationship between saturated extent and baseflow, it provides the main characteristics necessary to understand and model the interactions at this complex interface and the resulting daily streamflow variations during baseflow recession periods in this type of catchment. Copyright © 2016 John Wiley & Sons, Ltd. Copyright © 2016 John Wiley & Sons, Ltd.

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