O 2 Consumption in a Granitic Environment

The fate of O 2 in a granitic repository has been addressed by an international project: The redox experiment in detailed scale (REX) . The emphasis of the project was on a field experiment involving groundwater in contact with a fracture surface. To this aim a borehole, ≍20 cm in diameter, was drilled at 380 m depth in the tunnel of the Aspo Hard Rock Laboratory, Sweden. Injection pulses of molecular oxygen were performed at in situ temperature and pressure. Several microbial and chemical parameters were studied as a function of time: microbial counts, pH, O 2 -concentration, Eh (redox potential), etc . The field study has been supported by laboratory experiments to determine O 2 reaction rates and mechanisms. These laboratory studies have been performed with Aspo samples (both for inorganic and microbially mediated processes). A replica experiment has also been completed at CEA, France, with the other half of the fracture surface obtained in the drilling procedure of the field experiment. The aim of the replica experiment has been to duplicate as far as possible the conditions of the REX in situ experiment, for example by using groundwater sampled at the REX site in Sweden, shipped in special containers to France. The data that has been collected from the O 2 injection pulses in the REX field and replica experiments have been compared with the rates of O 2 uptake determined in the laboratory experiments. These data allow an estimate of the life-times for oxygen uptake in fractures in granitoids, which is of consequence for performance assessment calculations.