Diffusion experiments at Mont Terri (Switzerland): overview and results

Several diffusion experiments have been performed at the field scale in the underground rock laboratory of Mont Terri (Switzerland) to verify the reliability of diffusion parameters obtained at lab scale. The principle of in situ diffusion experiments is based on the injection, in a packer-off section of a borehole, of a tracer cocktail diffusing into the rock and whose its concentration decrease is monitored by means of a circulation circuit located at the surface. Subsequently, the interval section is over-cored and analysed for the tracer profiles. Overall, the obtained tracer data confirmed that diffusion is the dominant transport process for solutes in Opalinus Clay. The diffusivity and diffusion porosity of anions was found to be lower than tritiated water, indicating anion exclusion effect. Concerning the cations, one observed that when the interaction of these species with clay surface was stronger, their decrease relative to HTO in interval was faster and their penetration depth was lower (max 3 cm after 10 months for Cs+). Finally, in situ data were found to be consistent with small-scale lab diffusion experiments performed parallel to the bedding plane, suggesting that the up-scaling effects for diffusion are small. (author)