HYDROGEOCHEMICAL INVESTIGATIONS AT THE

In November 1999 Andra began building an Underground Research Laboratory (URL) in eastern France. The geological formation selected for this laboratory is a 130-meter thick argillaceous rock level. This clay rich layer is located at a 400 to 600 meter depth. To characterize the confining properties of the clay, pore water composition had to be studied. For this purpose an innovative device was designed for gas equilibration and direct sampling of the pore water. The experimental device consists of a vertical ascending borehole with a 5 meter long test interval at its far end in which a gas circulation is established. After a few weeks, due to the hydraulic gradient between the test interval and the rock formation, the water flows freely at a rate of 0.5 to 1.3 litters per month in the borehole and it is sampled. The chemical composition of this water is compared with a theoretical composition deduced from core analyses and thermodynamic modelling