The continuous demand on new residential and economic areas of the modern society has to face up with problems posed by polluted sites related to former industrial activities, typically located in suburbs areas. These sites, known as brownfields, are often located nearby navigable rivers to facilitate transport operations of industrial manufacturing, which increase their potential environmental threat due to the possible migration of pollutants in groundwater to surface water bodies through groundwater discharge. In this context, the objective of this research, performed in the scope of the FP6-IP AquaTerra project, was to contribute to a better assessment of the risk of groundwater contaminant dispersion for a brownfield located next to the Meuse River (Belgium), in a context where strong groundwater - surface water interactions prevail. The brownfield of interest corresponds to the site of the former coke factory of Flémalle. Resulting from industrial activities, soil and groundwater located in the alluvial aquifer are heavily contaminated with various types of organic (BTEX, PAHs, mineral oils...) and inorganic (As, Zn, Cd...) pollutants. To do so, detailed characterisation campaign was performed, consisting of, on the one hand, classical field experiments such as pumping tests, injection tests and tracer experiments; on the other hand, advanced and original field experiments such as detailed monitoring of groundwater - surface water interaction and dynamics, and the development and application of an innovative tracer technique, the Finite Volume Point Dilution Method (FVPDM), used to quantify and monitor groundwater fluxes. Monitoring and field works data was subsequently used to develop and calibrate a groundwater flow model using the finite difference code MODFLOW, with an automatic parameter estimation approach based on an original combined regional scale (zonation) and local scale (pilot points) approach. A transport model was also developed using MT3DMS and calibrated using tracer experiments performed in the brownfield. This groundwater flow and transport model was used to better quantify the dynamics of groundwater - surface water interactions and to model various scenarios of contaminant dispersion through the aquifer - river system. For these scenarios, benzene was considered because it is one of the main pollutants encountered in the site, its large solubility and mobility in groundwater and its acute toxicity. These scenarios were established considering various groundwater flow conditions (steady state vs. transient) and various hydrodispersive processes possibly affecting the mobility of benzene in groundwater, namely advection, hydrodynamic dispersion, sorption - desorption and, as evidenced by the research results of the University of Neuchâtel (Switzerland), benzene degradation under sulphate reducing conditions. These simulations indicate that benzene attenuation is mainly controlled by ongoing benzene degradation processes, aquifer heterogeneity and river stage fluctuations. Based on this analysis, the risk of benzene dispersion is low, and monitored natural attenuation (MNA) is a valuable option with (1) monitoring benzene at control planes downstream from the sources; (2) further investigation on risk of sulphate depletion in the alluvial aquifer; and (3) further investigation on mobilisation/immobilisation of heavy metals related to dynamics of organic pollutant plumes.

Cette thèse est avant tout consacrée à létude des circulations karstiques dans les calcaires carbonifères du Sud de la Belgique. Son objectif est double : dune part, approfondir la connaissance de laquifère à partir de létude dune région limitée située dans le Condroz à lextrémité Est du synclinorium de Dinant (bassins dAnthisnes et du Néblon), dautre part mettre au point des méthodes de mise en uvre et dinterprétation des traçages permettant de caractériser le comportement de laquifère lié à sa « double perméabilité ». Dans ce but, plusieurs aquifères fissurés ou karstiques sont comparés. Lintroduction générale présente une synthèse des conceptions actuellement utilisées en hydrogéologie karstique et définit le calcaire carbonifère en tant quaquifère (intérêt économique, particularités du fonctionnement) dans le contexte plus général des aquifères carbonatés du sud de la Belgique. La première partie est consacrée à létude de la structure (approche structurelle) et du fonctionnement (approche systémique) de deux systèmes karstiques drainant la même nappe dans les calcaires du Tournaisien et du Viséen : le bassin dAnthisnes, dont lexutoire est la résurgence de Comblain-au-Pont, et le bassin du Néblon drainé par les sources du Néblon. Les structures de ces deux systèmes sont comparées à laide de diverses méthodes géophysiques (électriques, électromagnétiques, diagraphiques) et à partir des observations en surface (fissuration, linéaments). Les différences de comportement sont liées au développement de la karstification. Elles apparaissent de manière très nette en considérant les réponses naturelles (hydrodynamique, hydrogéochimie) et provoquées (traçages). Dans la seconde partie, une série de tests en écoulement naturel et en écoulement radial convergent permettent détudier les propriétés de transport des traceurs au sein de différents aquifères fissurés et karstiques et de dégager des tendances liées à la structure du milieu. Trois types daquifère sont envisagés : les calcaires dévoniens et carbonifères (aquifère principalement à porosité dinterstice). Un examen critique des lois découlement et de transport utilisées, ainsi que des différents types de traçage et types de traceur, est effectué au préalable. Plusieurs solutions analytiques sont envisagées pour déterminer les paramètres hydrodispersifs des aquifères. Les dispersivités longitudinales des calcaires sont comprises entre 10 et 100m ; celles des craies entre 2 et 14m et celles des tuffeaux entre 2 et 5m. ABSTRACT. The actual scope of this research is to study the karstic groundwaters in the carboniferous limestones south of Belgium. The study deals with two different approaches, one in a local framework, the other thematic. On the one hand, two karstic systems are described in the synclinorium of Dinant in the Condroz region (Anthisnes and Neblon basins). On the other hand, a methodology of tracer tests, and their interpretation as well, are developed in order to understand groundwater flows related to the dual permeability of the aquifer. As a comparison, the tracer study is enlarged to other karstic and fissured aquifers. The introduction deals with some general features about karst hydrogeology. The carboniferous limestones and their economic interest are presented in the regional framework of all the carbonated rocks in south Belgium. The two karstic systems described in the first part belong to the same water table in the Tournaisian and Visean beds. Their structure is studied by mean of geophysical prospecting (electric, electromagnetic, well logging) and fissuration measurements. The mechanisms of groundwater circulations are then studied by using undirect investigations such as hydrodynamical, hydrogeochemical or tracing investigations. These methods are applied to the main springs of each system: the spring of Comblain-au-Pont (Anthisnes basin) and the springs of the Neblon (Neblon basin). The behaviour of each system is related to its stage of karstification. The second part deals with tracer tests in natural and radially convergent flow pattern. The aim is to distinguish among the features of the tracer curves those due particularly to the structure of the fissured network of the aquifer. Three different kinds of aquifer are studied: the Devonian and carboniferous limestones (both fissured and karstic), the cretaceous chalks (mainly fissured), and the landenian tuffaceous sands (mainly porous). After describing the theory of groundwater flow and transport in fissured media, we justify the use of different tracers and tracing methods. Afterwards different mathematical models are used to calibrate the dispersion parameters. The longitudinal dispersivities are ranging between 10 and 100m in the limestones, between 2 and 14m in the chalk and between 2 and 5m in the landenian sands.