The Grimsel (Switzerland) migration experiment: integrating field experiments, laboratory investigations and modelling

For several years tracer migration experiments are performed at Nagraʹs Grimsel Test Site in the Swiss Alps as a joint undertaking of Nagra, PNC and PSI. The aim is to develop methods for field experiments at possible sites for nuclear waste repositories and to test radionuclide transport models. A hydraulic dipole field is generated in a well-defined fracture zone in granite. The tracers used are non-sorbing (uranine, 3He, 4He, 82Br−, 123I−), mildly sorbing (22Na+, 24Na+), and more strongly sorbing (85Sr2+, 86Rb+, 134Cs+, 137Cs+). These experiments have been complemented by extensive laboratory investigations on petrography, on water-rock and nuclide-rock interaction as well as by migration experiments with bore cores. The main questions addressed are: What are the relevant geometric factors and mechanisms for transport, how well can breakthrough curves be extrapolated from one dipole arrangement to another, which parameters are scale dependent, is there a difference in sorption values between laboratory and field experiments or between static and dynamic experiments. Evaluating the experimental results for the non-sorbing uranine and the mildly sorbing tracers sorption, Strontium, we show that a consistent picture of tracer transport, and specifically of tracer sorption, is obtained when exploiting all available experimental information and using not too simplistic models.