NON-CONSERVATIVE BEHAVIOUR OF IODIDE TRACER IN ARGILLACEOUS LIMESTONE INDICATED BY X-RAY RADIOGRAPHY

Iodide is commonly used as a tracer as it behaves conservatively in many geologic environments. However, it is known that iodide has an affinity for organic matter and in this work we provide evidence for non-conservative behaviour of iodide during measurement of the diffusive properties of the Cobourg Formation, an Ordovician argillaceous limestone from the Michigan Basin currently under investigation as a potential host rock for a deep geological repository (DGR) at the Bruce nuclear site in Ontario, Canada. The Cobourg Formation is of interest for storage of low- and intermediate-level radioactive waste due to its low hydraulic conductivity (<10^-14 m/s) and porosity (1-2 %), such that diffusion is the principal mechanism of solute transport. In our experiments, we used a non-destructive X-ray radiography technique to monitor the diffusion of iodide tracer. One-dimensional profiles of iodide mass distribution are consistent with a diffusion transport mechanism, but there are zones of anomalous iodide accumulation that correspond to organic-rich regions in the rock. The association between iodide and organic matter was corroborated by investigations with scanning electron microscopy and electron microprobe. These results demonstrate that it is necessary to understand the organic content of rocks before considering the use of iodide as a conservative tracer for transport experiments. However, where iodide behaves non-conservatively, the iodide mass distribution profiles generated by X-ray radiography provide an opportunity to investigate the potential for retardation of ¹²⁹I - a long-lived and mobile radionuclide in radioactive waste.