Tracer and reactive transport modelling of the interaction between high-pH fluid and fractured rock: Field and laboratory experiments

Abstract

In order to understand the interaction of a hyperalkaline solution with a fractured shear zone in granite and its influence on the migration of radionuclides, laboratory and underground field experiments at the Grimsel Test Site (Switzerland) were analysed by means of numerical modelling. Supporting data came from hydrogeological testing, structural and mineralogic characterisation of boreholes and cores and from dye tracer experiments in different dipole geometries within the shear zone. One-dimensional flow and reactive transport models have been applied to reproduce the breakthrough curves measured in a small-scale laboratory experiment (forced infiltration into a drill core containing a fracture with fault gouge). Major ion concentrations and water flow through the system could be fitted using a kinetic approach for mineral dissolution and precipitation. To reproduce the measured reduction of the flow rate over time, a small value for the effective mineral surface area had to be chosen together with an empirical relationship between the hydraulic conductivity of the fracture and the amount of precipitated calcium silicate hydrate. Tracer dipole experiments have been interpreted using different concepts to reconcile transport processes and radionuclide migration within a shear zone at the Grimsel Test Site altered by high-pH fluid. Parameter fits were possible for a multiple fracture-matrix approach, as well as for a two-dimensional heterogeneous medium approach. A discrimination between approaches was not possible, although the extended dipole flow field geometry, the quite dissimilar breakthrough curves measured for experiments with different dipole geometries and the measured lateral spreading of the high-pH plume favoured the heterogeneous porous medium approach. Using a heterogeneous porosity distribution, together with an empirical Kozeny–Carman equation that relates porosity and hydraulic conductivity, a heterogeneous flow field could be calculated for the shear zone. This flow field was used to predict the interaction of the hyperalkaline solution with the shear zone. Calculations were also performed to predict the spreading of Cs, Co and Eu radionuclide tracers within the shear zone altered by high-pH interaction. It has been shown that the calculated Cs, Co and Eu breakthroughs and their concentration distributions depend on the assumptions on sorption behaviour. In addition, the observed decrease in hydraulic conductivity of the system, which is observed both in the field and in small-scale core infiltration experiments, and the related changes in the flow field, which are linked to mineral alteration, will strongly influence the migration of radionuclides.