Quantification of nitrate removal by a flooded alluvial zone in the Ill floodplain (Eastern France)

The nitrate reducing capacity of a flooded system in the Ill floodplain (Eastern France) was investigated for a period of 2 years. The methodology used consisted of a spatio-temporal monitoring of stream flow and nitrate concentrations in the groundwater and surface water, calculation of input and output fluxes and modelling of groundwater fluxes and nitrate transfer through the alluvial area. A comparison of chloride flux (used as hydrological tracer) and nitrate flux was done to determine a floodplain effect on the retention of nitrate. We show that up to 95% of the nitrate load in the groundwater is retained by the system, whereas the retention in the stream network is very low. Ammonium fluxes increased from inputs to outputs in the stream and in the groundwater. The chloride input in the groundwater is higher than the output, whereas in the surface water the output is higher than the input, the amount evacuated in streams corresponding to the losses from groundwater. The nitrate removal rate calculated for the whole modelized surface area (40 km2) represented 559 t N yr-1 or 1397.7 kg N ha-1 yr-1. The ammonium fluxes exported by the system represented 102 kg N ha-1 yr-1 A part of nitrate is reduced and exported by the groundwater and stream network in the form of ammonium. These results can be explained by the duration of floods which controls the equilibrium between the various forms of nitrogen. Thus, long watering periods favour nitrogen removal (denitrification and plant uptake) and limit nitrate production which compensates elimination during the dry period

Numerical Reliability and CPU Time for the Mixed Methods applied to Flow Problems in Porous Media

This work is devoted to the numerical reliability and time requirements of the Mixed Finite Element (MFE) and Mixed-Hybrid Finite Element (MHFE) methods. The behavior of these methods is investigated under the influence of two factors: the mesh discretization and the medium heterogeneity. We show that, unlike the MFE, the MHFE "suffers" with the presence of flatted triangular elements. A numerical reliability analyzing software (Aquarels) is used to detect the instability of the matrix-inversion code generated by MAPLE which is used in the MHFE code. We also show that the spectral condition number of the algebraic systems furnished by both methods in heterogeneous media grows up linearly according to the smoothness of the hydraulic conductivity. Furthermore, it is found that the MHFE could accumulate numerical errors if the conductivity varies abruptly in space. Finally, we compare running-times for both algorithms by giving various numerical experiments

ANN BASED APPROACH TO SOLVE GROUNDWATER POLLUTION INVERSE PROBLEM

International audienc

Identification of aquifer heterogeneity through inverse methods

The paper underlines the contributions of Ghislain de Marsily (GdM) to the identification of aquifers heterogeneity using inverse methods mainly for modeling subsurface flow. Inverse methods require an objective function to express the goodness of fit of the chosen model, a parameterization to describe the spatial distribution of model parameters, and a minimization algorithm. The resulting inverse problem, which consists in seeking model parameters’ values that render model outputs close to the observations, is usually unstable. GdM developed seminal ideas for the two key inversion issues that are: to stabilize the inverse problem through regularization, and to parameterize it to reproduce the natural heterogeneity of the subsurface with a limited number of parameters. GdM conducted pioneering works that are the basis of current parameterization methods relying upon adaptive zonation and/or interpolation based on pilot points. We take here the opportunity to highlight the GdM’s contributions inspiring currently used techniques

Identification of aquifer heterogeneity through inverse methods

The paper underlines the contributions of Ghislain de Marsily (GdM) to the identification of aquifers heterogeneity using inverse methods mainly for modeling subsurface flow. Inverse methods require an objective function to express the goodness of fit of the chosen model, a parameterization to describe the spatial distribution of model parameters, and a minimization algorithm. The resulting inverse problem, which consists in seeking model parameters’ values that render model outputs close to the observations, is usually unstable. GdM developed seminal ideas for the two key inversion issues that are: to stabilize the inverse problem through regularization, and to parameterize it to reproduce the natural heterogeneity of the subsurface with a limited number of parameters. GdM conducted pioneering works that are the basis of current parameterization methods relying upon adaptive zonation and/or interpolation based on pilot points. We take here the opportunity to highlight the GdM’s contributions inspiring currently used techniques

New two-dimensional slope limiters for discontinuous Galerkin methods on arbitrary meshes

In this paper we introduce an extension of Van Leer's slope limiter for two-dimensional Discontinuous Galerkin (DG) methods on arbitrary unstructured quadrangular or triangular grids. The aim is to construct a non-oscillatory shock capturing DG method for the approximation of hyperbolic conservative laws without adding excessive numerical dispersion. Unlike some splitting techniques that are limited to linear approximations on rectangular grids, in this work, the solution is approximated by means of piecewise quadratic functions. The main idea of this new reconstructing and limiting technique follows a well-known approach where local maximum principle regions are defined by enforcing some constraints on the reconstruction of the solution. Numerical comparisons with some existing slope limiters on structured as well as on unstructured meshes show a superior accuracy of the proposed slope limiters

On the Finite Volume Reformulation of the Mixed Finite Elements Method on Triangles

We analyse the finite volume reformulation of the triangular mixed finite element approximation for the porous flow equation, as proposed in [10] [9]. We show that the finite volumes are obtained by aggregation of finite elements (usually one, sometimes two or more), that the matrix of the finite volume equations is regular, but generally not symmetrical, and that the finite volume formulation is algebraically equivalent to the mixed approximation. The finite volume matrix becomes symmetrical in the stationary case, and positive definite when the triangulation satisfies the Delaunay condition

Couplage par composants logiciels de codes d'hydrogéologie

National audienceNotre objectif est d'étudier la modélisation de phénomènes couplés et leur mise en oeuvre sur une grille de calcul, en développant quatre applications. Les deux premiers couplages sont de type physico-chimique et physico-physique (couplage algébrique d'équations). Ils sont appliqués respectivement à la contamination d'aquifères et à l'intrusion d'eau salée. Les deux autres couplages sont géométriques de type multidomaines ou multi-échelles et sont appliqués au stockage profond de déchets radioactifs, d'une part dans un milieu peu fracturé, d'autre part dans un réseau de fractures.Nous choisissons une approche par composants logiciels, qui permet d'encapsuler chaque code modélisant un phénomène physique. Les interfaces des composants permettent d'eectuer les échanges de données nécessaires au couplage numérique.L'exécutif PadicoTM garantit un calcul à haute performances sur une grille de calcul avec différents types de réseaux, grâce notamment à un modèle de composants parallèles, qui permet de passer à l'échelle