Colloid-facilitated radionuclide transport in single fractures: Sensitivities and scoping calculations for tracer tests at Aespoe

We consider colloid-facilitated radionuclide transport (CFRT) through a single fracture between an injection point and a detection/pumping point. The experimental conditions are assumed comparable to those of the Tracer Retention Understanding Experiments (TRUE) tests, with mean water residence time in the range 10-100 h. Two sorbing tracers are considered: moderately sorbing Ba and strongly sorbing Cs; tritium is also considered as a reference tracer. The two controlling parameters for the CFRT experiments are forward sorption rate αf and the colloidal filtration rate ε. A sensitivity analysis is carried out with regard to these two controlling parameters, for different experimental conditions (water residence time and injection modes), and assuming different retention conditions for the tracer in the rock matrix (limited and unlimited diffusion). We find that Cs is preferable to Ba for the CFRT tests, and that 100 h mean water residence time is preferable to 10 h. Diffusion limitations do not seem to affect the early peak of CFRT and hence need not be a concern for the experiments. Reversibility of sorption on colloids seems to have a relatively small impact on CFRT for the assumed experimental conditions. Injection conditions for the tracer do not seem to be important for CFRT tests, hence injection can be carried out in a similar manner as was done for the TRUE-1 tests. Filtration of colloids can strongly reduce the impact of CFRT for ε > 0.01 l/h, but has a relatively modest impact for ε < 0.01 l/h. For ε < 0.01 l/h, CFRT of Cs would be detectable for αf as low as 5x10-6 l/h; for increasing filtration rate, increasingly higher sorption rate is required for CFRT to be detectable. For ε > 0.01 l/h, we established a linear log-log relationship which is simple to use once an estimate of obtained in the laboratory using for instance a batch methodology of the type presented in Lu et al