Geochemical modelling of processes inside a packed-off interval in the Opalinus Clay formation (Switzerland, Mont Terri Project): a kinetic approach

摘要

Geochemical modelling techniques, according to a kinetic point of view, have been applied in order to explain the differences between the pore water chemical composition of two seep pore waters (A2-18 and A2-37) of borehole BWS-A2. These samples were obtained in two different sampling campaigns belonging to phase 3 (1998) and phase 4 (1999) of the Mont Terri Project in Switzerland, respectively. In the proposed model, the atmospheric O_2 intake caused a kinetic oxidation of the pyrite present in the Opalinus Clay inside the packed-off interval. As a consequence, the calcite of the Opalinus Clay kinetically dissolve and buffer the acidification caused by the oxidation process. The released calcium from the calcite dissolution modified the cationic population in the exchanger and, hence, the pore water chemistry. Some kinetic relationships have been used for the dissolution/precipitation of calcite, for the dissolution of pyrite in the presence of dissolved O_2 and ferric iron and for the oxidation of Fe~(2+) by dissolved 02. Three specific surface areas for pyrite were tested, 0.5, 0.1 and 0.05 m~2 g~(-1) 1~(-1). The sulphate production by pyrite oxidation, calculated in the model, mainly depends on the specific surface area of the pyrite.