Emmanuel Ledoux

Spatially Distributed Modeling: Conceptual Approach, Coupling Surface Water And Groundwater

This chapter deals with the joint modeling of surface and groundwater flows by presenting and describing the MC model. The purpose of this deterministic physically-based model is to simulated the behavior of available water resources for one or several watershed. The model integrates surface flow, streamflow, flow in the unsaturated zone, groundwater flow and the interactions between rivers and water tables. Its formulation and its structure, especially its nested square meshes of variable sizes, give a great deal of flexibility to the model; this facilitates adaptation to variable modeling scales and to a wide range of geological geographical and climatological conditions. An application of the MC model on the Caramy watershed (France) is presented.

Regional modelling of groundwater flow and salt and environmental tracer transport in deep aquifers in the Paris Basin

A hydrodynamic model which takes into account the aquitard storage effect was developed for the deep multilayered system including the Dogger aquifer and its surroundings in the Paris Basin. It provides a good explanation for a series of observations in the Dogger concerning, for instance, the hydraulic head, the salinity and the transmissivity. The calibration of the model also makes it possible to estimate some unmeasured parameters such as the aquifer and aquitard storage coefficients. Finally, the results of simulations of the transport of 4He and 14C strengthen the representativeness of the model. The Darcy average horizontal velocity in the Dogger, obtained with the model, is of the order of 0.33m year−1.

Diffusion as the main process for mass transport in very low water content argillites: 1. Chloride as a natural tracer for mass transport—Diffusion coefficient and concentration measurements in interstitial water

[1]xa0Argillites are one of the rock types studied by French authorities for their confining properties for the isolation of radioactive wastes. One of the main objectives of such study is the better understanding of water transport through rocks with very low water content and hydraulic conductivity, using modeling of tracer profiles. This article presents the protocol developed and applied for acquiring data on chloride in interstitial water of the Toarcian argillites in Tournemire (southern France). This protocol is based on laboratory experiments involving diffusion process and on modeling. Experimental data obtained during transient and steady parts of diffusion allow for the assessment of the diffusion coefficient and initial concentration in pore water, respectively. Profiles for both have been acquired along the geological sequence; they are used in part 2 of this article for proposing a hydrogeological model where diffusion appears to be the main process for mass transport through the argillites and for comparing deuterium and chloride transport.

Modelling of large multilayered aquifer systems: Theory and applications

Abstract The paper presents flow mechanisms in large sedimentary basins with a distinction between the behaviour of aquifers and aquitards. Taking advantage of this difference, a discretisation method is presented, with nesting square meshes of variable size in quasi three dimensions. A finite-differences technique gives a method for solving the flow equations on this discretised flow domain. An application of the method in the northern Sahara is presented, with special emphasis on the use of the model to optimise the exploitation of groundwater resources. Also problems of mass transport in such systems are outlined.

Diffusion as the main process for mass transport in very low water content argillites: 2. Fluid flow and mass transport modeling

[1] On the basis of chloride concentrations of pore water in the Tournemire massif (part 1), a conceptual model for mass transport in argillites by diffusion is proposed. From this conceptual model and current knowledge of the geological history of the massif, one-dimensional numerical simulations are formulated for chloride transport in Tournemire massif over the past 53 Ma. Good agreement between experimental data and calculated values for both diffusion coefficients and concentrations of chloride confirms that diffusion is the main process for mass transport in the massif. This model is also tested using deuterium contents of pore water, applying variable concentrations to meteoric water (circulating in system boundary layers) based on the thermal dependency of its isotopic composition. These simulations reveal the likely important role of lithologic heterogeneities, such as fractures, in the horizontal distribution of tracer concentrations.

Preliminary studies of groundwater flow and migration of uranium isotopes around the Oklo natural reactors (Gabon)

In specific zones of the Oklo uranium deposit, criticality was naturally reached 2 Ga ago. This site thus provides a unique opportunity to show whether the stable nuclear reaction end-products have remained or not in the vicinity of the reactor zones after the termination of nuclear reactions. In addition, the evaluation of the stability of the uraninite matrix over very long periods of time provides information on the possible long-term stability of waste forms such as spent fuel. The Commission of the European Communities initiated in 1991 the Oklo Natural Analogue Programme, a part of which is devoted to present-day migration studies. The Swedish Nuclear Fuel and Waste Management Company (SKB) supports this programme, with special interest in the Bangombe reactor, a shallow reaction zone possibly affected by surficial alteration processes. The Oklo study comprises hydrogeology, groundwater chemistry, isotopic analyses (environmental isotopes, U series, 235 U/ 238 U), and modelling. Two sites are being thoroughly investigated : the less perturbed OK84 reactor zone in Okelobondo (200 m south from Oklo) and the Bangombe reactor zone, 30 km south of Oklo. We focus our study on uranium migration from these reactor zones, using tracers such as the 235 U/ 238 U isotope ratio. After preliminary field campaigns, a conceptual model was constructed, both for Okelobondo and Bangombe. For this purpose, groundwaters have been characterised for three years in different areas around Oklo : Okelobondo groundwaters in mines and boreholes and surface waters, and Bangombe, both in boreholes and surface waters. Detailed investigations were then conducted in order to validate our conceptual models, and finally to enabling us to model U migration from the reaction zones, and to evaluate the performance assessment of deep geological disposal of radioactive wastes. After the presentation of regional and local geology and hydrogeology, we give a complete description and interpretation of groundwater chemical evolution both in Okelobondo and Bangombe. It concerns major-element behaviour, redox control and uranium behaviour. Preliminary conclusions for the conceptual model drawn from hydrogeological studies and from groundwater chemistry agree closely. In addition, we have successfully tested the use of 235 U/ 238 U isotope ratios in groundwaters to evaluate the migration of uranium from reactor zones.

Geohydrological research in relation to radioactive waste disposal in an argillaceous formation

Abstract Since 1974, the Belgian Nuclear Research Establishement (SCK/CEN) at Mol has been studying the suitability of the Boom clay formation as a potential host formation for wastes arising from the Belgian nuclear power production. This paper presents the method developed to evaluate, from the point of view of the regional hydrology, the capacity of the Boom clays for confining radioactive waste products. This method consist of four successive steps: (1) a reconnaissance of the geological and hydrogeological structure; (2) the development of a numerical model to interpret the field observations; (3) the construction of an appropriate migration model that uses the results obtained by the hydrodynamic model; and finally (4) the simulation of the consequences of possible changes in the hydrogeological system due to natural geological evolution. The analysis also stresses the role of confining layers in multilayered aquifer systems and the importance of leakage flow on a regional scale.