Diffusion of HTO,Br, I,Cs, Sr and Co in a clay formation: Results and modelling from an in situ experiment in Opalinus Clay

The migration of radioactive and chemical contaminants in clay materials and argillaceous host rocks is characterised by diffusion and retention processes. Valuable information on such processes can be gained by combining diffusion studies at laboratory scale with field migration tests. In this work, the outcome of a multi-tracer in situ migration test performed in the Opalinus Clay formation in the Mont Terri underground rock laboratory (Switzerland) is presented. Thus, 1.16 × 10⁵ Bq/L of HTO, 3.96 × 10³ Bq/L of ⁸⁵Sr, 6.29 × 10² Bq/L of ⁶⁰Co, 2.01 × 10⁻³ mol/L Cs, 9.10 × 10⁻⁴ mol/L I and 1.04 × 10⁻³ mol/L Br were injected into the borehole. The decrease of the radioisotope concentrations in the borehole was monitored using in situ γ-spectrometry. The other tracers were analyzed with state-of-the-art laboratory procedures after sampling of small water aliquots from the reservoir. The diffusion experiment was carried out over a period of one year after which the interval section was overcored and analyzed. Based on the experimental data from the tracer evolution in the borehole and the tracer profiles in the rock, the diffusion of tracers was modelled with the numerical code CRUNCH.