Evolution geochimique de l'eau interstitielle d'une argile de la Mer de Champlain.

摘要

This thesis belongs to a big characterisation project of the geotechnical, hydrogeological and hydrogeochemical conditions of the Champlain sea clays which represent most of the surficial sediment deposits of the Saint-Lawrence Lowland.;The main objectives of this project were to identify the most important processes controlling the pore water chemistry and its evolution through the time, and to develop a simple model of this hydrogeochemical evolution.;Nine boreholes were drilled in order to take clay core samples for laboratory investigations.;Firstly, a physico-chemical characterization of the clay was conducted: mineralogical analysis with X-ray diffraction, determination of the CEC, the specific area and the loss on ignition. The geotechnical properties were already available.;This work consists in a geochemical study of the aquitard ground water major ions.;Water extractions were conducted (74 in total) for each site on the whole thickness of the clay deposit. The water was analysed for the following major elements: Na, K, Ca, Mg, Cl, HCO3 and SO4. For some sample, the pH was also measured and trace elements Al and Si were analysed.;A high pressure extraction method was developed in order to evaluate the influence of the extraction pressure on the extracted water chemistry.;This extraction procedure showed a global increase of the concentration of the ions Na, K, Ca, Mg and HCO3. The concentration of Cl and the pH stayed approximately constant while sulphate concentration didn't show any particular trend.;A laboratory test method was designed for the simulation of the in situ leaching of the clay, on a much smaller time scale. The collected water was analysed for the same parameters. This experiment allowed to better understand the processes controlling the pore water geochemical evolution and underlined the important production of bicarbonate, which are probably due to microbial activity and in particular to the bacterial reduction of sulphate.;The low pressure extraction method was validated with some quality control tests and provided the porewater geochemical profiles as a function of depth. The analysis of these data integrated with a geochemical modelling of the clay leaching in laboratory and in situ show a control of the water chemistry by the advection/diffusion phenomena coupled with the production of bicarbonate probably due to sulphate reduction and carbonates dissolution when the water becomes sufficiently diluted. Ionic exchanges between divalent and monovalent cations also seem to be important while the dissolution/precipitation of silicates does not seem to have any significant influence.