L’objectif de cette thèse est d’arriver à mieux contraindre les paramètres qui contrôlent les processus d’altération à l’échelle d’un petit bassin versant, en se focalisant plus précisément sur le rôle de l’hydrologie. Pour ce faire, les variations temporelles et spatiales des signatures chimiques et isotopiques (U, Sr) des eaux du bassin versant du Ringelbach (Vosges, France) sont étudiées. Plusieurs sources de ce bassin versant, essentiellement composé d’un granite plus ou moins fracturé, ont été échantillonnées mensuellement sur une période de deux ans, le long d’un profil altitudinal. Ce bassin versant présente également l’avantage d’être équipé de trois forages profonds (allant jusqu’à 150 mètres de profondeur) qui ont permis de prélever à la fois les roches et les eaux profondes. Sur la base de l’interprétation géochimique des eaux, la connectivité des différents compartiments hydrologiques a été évaluée et un schéma de fonctionnement hydrogéochimique est proposé s’appuyant sur une bonne connaissance du contexte géologique. Enfin, un taux d’altération et un flux d’eau sont estimés à partir d’une modélisation de l’évolution du rapport isotopique de l’uranium (234U/238U) le long d’un trajet d’eau. L’originalité de cette étude est également de coupler cette approche de traçage géochimique avec une approche modélisatrice en utilisant le logiciel KIRMAT (Kinetic Reactions and Mass transport) qui intègre les équations des réactions géochimiques (dissolution/précipitation) et les équations de transport (1D). Il est ainsi possible de simuler le transport réactif d’une eau traversant la roche le long d’un certain trajet d’eau. Cette modélisation s’appuie sur la caractérisation minéralogique, des propriétés physiques des échantillons de roches prélevés le long des forages et sur les interprétations géochimiques des eaux. Ainsi, la modélisation de la composition chimique des eaux de sources et des eaux de forages a permis d’affiner la compréhension des processus d’altération, notamment le rôle des phases secondaires précipitées. Celle-ci permet également une meilleure compréhension des phénomènes de couplage entre les différents paramètres qui contrôlent la signature chimique des eaux à l’échelle du bassin versant. / The main goal of the present thesis is to better constrain the parameters that control weathering processes at a small catchment scale, focusing specifically on the role of hydrology. For this purpose, temporal and spatial variations of the chemical and isotopic (U, Sr) water composition of the Ringelbach catchment are studied. Several springs of this catchment whose basement is mainly composed of granite more or less fractured located along an altitudinal profile were monthly sampled over a period of two years. The additional interest of this site is that three deep boreholes (up to 150 meters deep) allow the sampling of both deep rocks and waters. The connectivity of the different hydrological compartments is evaluated based on the geochemical interpretation of water samples. A schematic hydrological functioning is proposed based on a good knowledge of the geological context. Finally, a weathering rate and a water flux are estimated from the modeling of the uranium activity ratio along a water path. The originality of this study lays also in combination of a geochemical and modeling approaches using the software KIRMAT (Kinetic Reactions and Mass transport) that integrates geochemical reactions (dissolution/precipitation) and 1D mass transport equations. It allow to simulate the reactive transport of a fluid through a rock along a given water pathway. This modeling is based on the characterization of the mineralogical and physical properties of the rock, sampled along the boreholes and the geochemical interpretation of the waters. Thus, the modeling of the chemical composition of the spring waters and borehole waters permit to improve the understanding of weathering processes including the role of precipitated secondary phases. It also allows to better understand the interplay of parameters that that control the water chemical signature at the catchment scale.

Submitted by Biblioteca de Pós-Graduação em Geoquímica BGQ (bgq@ndc.uff.br) on 2016-04-19T16:19:47Z No. of bitstreams: 1 DISSERTJULIANAGRODRIGUES.pdf: 4883356 bytes, checksum: 5706faa288cd5541e1bfe8cc446e591b (MD5) / Made available in DSpace on 2016-04-19T16:19:47Z (GMT). No. of bitstreams: 1 DISSERTJULIANAGRODRIGUES.pdf: 4883356 bytes, checksum: 5706faa288cd5541e1bfe8cc446e591b (MD5) / Universidade Federal Fluminense. Instituto de Química. Programa de Pós-Graduação em Geociências-Geoquímica. Niterói, RJ / Em janeiro de 2011, a região serrana do Estado do Rio de Janeiro foi atingida por inúmeros movimentos de massa em um evento denominado de “Megadesastre”. A ocorrência de movimentos de massa depende da disponibilidade de material a ser mobilizado, e essa disponibilidade está relacionada com o intemperismo. Dessa forma, essa dissertação buscou avaliar as transformações geoquímicas e mineralógicas em dois perfis de intemperismo desenvolvidos sobre rocha granítica, na bacia do Córrego do Príncipe (Teresópolis/RJ), e a relação com a ocorrência de movimentos de massa. Nas análises químicas empregaram-se os métodos de ativação neutrônica (AAN) e fluorescência de raios-X (FRX). Já na mineralogia utilizou-se petrografia, observação em lupa e difração de raios-X (DRX), e na granulometria, peneiramento e difratometria a laser. A análise estatística multivariada indicou a separação de quatro grupos: G1 - apresenta o maior grau de dissimilaridade e é formado por material ferro-manganoso; G2 - amostras com menor grau de alteração, onde foi verificada a presença de plagioclásio e a escassez de minerais secundários, além da presença de material rochoso (corestones) menos alterado e material inconsolidado com textura grossa; G3 e G4 - apesar de apresentarem fortes similaridades, estão em estágios de alteração diferentes, sendo a principal diferença a mineralogia das frações finas (silte/argila): em G3, há plagioclásio, caulinita e gibbsita, enquanto que em G4 verificou-se a ausência de feldspatos e presença desses minerais secundários, indicando que as amostras em G3 estão em menor estágio de alteração do que as de G4. Quanto ao comportamento dos elementos químicos, houve um empobrecimento em K2O, Na2O, CaO, Rb, Ba e ETR, relacionado principalmente com a alteração dos felspatos, e enriquecimento em SiO2 e Al2O3 (relacionado aos minerais detríticos e neoformados), Fe2O3, TiO2 e MgO, referente a alteração da biotita. O estudo permitiu elaborar um modelo de transformação geoquímica, mineralógica e textural, a partir do qual se pode concluir que a formação de corestones juntamente com a mobilização e reconcentração de elementos e argilominerais, atuaram como heterogeneidades dentro desses perfis de alteração. Estes processos tornaram os perfis susceptíveis a movimentos de massa, mostrando assim uma inter-relação entre o estudo da evolução geoquímica e mineralógica, e a estabilidade mecânica do material / On January 2011, the mountainous region of Rio de Janeiro state was affected by a great number of landslides. This event was called "Megadisaster". The occurrence of landslide depends on the presence of material to be mobilized due to weathering processes. This work aims to assess the geochemical and mineralogical changes in two granite weathering profiles in Córrego do Príncipe watershed (Teresópolis / RJ) related to landslides. Instrumental Neutron Activation Analysis (INAA) and X-ray fluorescence (XRF) were used to quantify the chemical compositions of weathering profile samples. Mineralogy was done by X-ray diffraction analysis (XRD) and by petrography. Particle size analysis was done by sieving and laser diffraction. A multivariate statistical analysis of the data showed four groups: G1 is quite different from the other groups and is characterized by an iron-manganous material; G2 represents the material with the lower degree of weathering. The presence of plagioclase, the lack of secondary minerals, the corestones and the coarse grain size of the unconsolidated samples explain this group; G3 and G4 are fairly homogeneous. They are the most weathered samples of both profiles consisting of secondary minerals and little or no plagioclase present. The main difference between these groups is silt and clay mineralogy. The elements mobility during weathering process showed a depletion of K2O, Na2O, CaO, Rb, Ba and REE (mainly due to feldspar weathering), while SiO2, Al2O3 (due to detrital and secondary mineral growth) Fe2O3, TiO2 and MgO (due to biotite weathering) increase. This study allowed the elaboration of a geochemical, mineralogical and particle size weathering model. The production of corestones, the mobilization and concentration of elements and clay minerals acted as heterogeneities within these profiles, leaving them susceptible to landslides