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

The Champlain clay that covers most of the Saint-Laurent Lowlands, and in particular the region of Lachenaie near Montreal, was deposited during the marine episode of the Champlain Sea 10000 years ago. The high concentrations of Na+ and Cl- in the clay pore water and in the underlying till and rock, which sometimes reach 40% to 50% of the concentration of sea water, prevent one from using the water for human consumption or agricultural irrigation. The presence of these important contents of salts in the clay and the rock raised several questions about the nature of the movements of ground water in the Champlain clays since their formation as well as on the migration of the major ions, and especially the chlorides, during these last 10000 years. 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. A network composed of nine piezometers in the rock and eighteen in the clay was installed in order to monitor the chemistry of water and to do hydraulic tests. The clay was sampled continuously using Shelby tubes to minimize its reworking. These samples were extracted in the laboratory and used in the chemical and isotopic analysis and in the mechanical and geotechnical tests. Two pore water extractors were then developed in the Hydrogeology laboratory at the École Polytechnique de Montréal: 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. 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.