Mouvement des eaux souterraines et des ions majeurs dans une argile Champlain depuis sa formation

The geotechnical, hydraulic and geochemical properties of the Champlain clay in the region of Lachenaie (Quebec) were investigated in order to understand better the movements of groundwater and some major ions in the clay and the underlying bedrock.;The results of the chemical analysis of the water samples show that the clay pore water and the water flowing in the underlying bedrock are extremely salty and can't be used as drinking water or for agriculture. The 14C analysis indicates that these waters are very old and that their age dates back to the deposition of the Champlain clays in a marine environment. The isotopic analysis with 18O and 2H show that waters present in the clay and the rock are a mixture of waters with 3 different origins: meteoric modern waters, sea waters and waters originating from melting of glaciers.;The mechanical and hydraulic laboratory and in situ tests show that groundwater flows vertically and very slowly in the clay formation. The low hydraulic conductivity (1x10-9 m/s) of this Champlain clay was determined in the laboratory using triaxial and oedometric tests and in situ variable head tests. The author noticed the presence of a scale effect when conducting the permeability tests: the value of k is underestimated for samples smaller than 5 cm. Pumping tests gave on estimate of the hydraulic conductivity of the bedrock (1x10-6 m/s). The mechanical and geotechnical properties of the clay were obtained using consolidation tests in oedometric cells. The porosity of the clay sampled from the nine sites varies between 0,45 and 0,60. The preconsolidation pressures vary between 180 kPa and 580 kPa. The liquidity limits are typical of the limits found in other Champlain clays in Quebec and Ontario and vary between 40% and 77%.;The numerical modeling shows the evolution of hydrogeological conditions in the region during the last 10000 years. The change in the topography caused by surface erosion of the clay is mostly responsible for this evolution. Despite the low hydraulic gradients in the clay, water infiltrating from the surface of the clay reached the discharge area (Milles-Iles River) after going through the entire clay, the till and the fractured rock. After the erosion occurred, zones of resurgence and watersheds appeared in the studied region, creating subregions with different geotechnical and hydrogeological properties.;Two pore water extractors were then developed in the Hydrogeology laboratory at the Ecole Polytechnique de Montreal: A low pressure and a high pressure device. The low pressure device made it possible to obtain representative pore water samples without affecting the major ions' concentrations and the pH for pressures lower than 800 kPa. The second device uses pressures up to 30 MPa. Large variations in the concentrations of the ions and in the pH were observed when the pressure of extraction exceeded 1 MPa. Also, as a result of the author's experiments, it is recommended that ground water in the bedrock be sampled using the low-flow purging and sampling method.;Regarding the migration of Cl- ions, it was observed that this process was not only controlled by the molecular diffusion but also by advection. The concentration profiles obtained with the chemical analysis with the results obtained with the numerical modeling. Cl- diffuses in the clay and is also displaced with the movement of the infiltrated rain and water of melting snow.;The migration of the ions in the clay was influenced by the presence during 3000 years of the Lampsilis freshwater Lake at the surface of the clay. This presence drove the process of diffusion of the salts toward the surface. After the Lake disappeared, both diffusion and advection took place. Dispersion was, during these 7000 years, influenced by the change in the hydrogeological regimes in the region. (Abstract shortened by UMI.)...