Interaction of hyperalkaline fluid with fractured rock: Field and laboratory experiments of the HPF project (Grimsel Test Site, Switzerland)

The HPF project (Hyperalkaline Plume in Fractured Rock) at the Grimsel Test Site (Switzerland) comprised an underground long-term field experiment in a water-conducting shear zone, in situ radionuclide transport experiments, two laboratory core infiltration experiments, and supporting studies. The feasibility of longer-term field experiments under difficult geochemical conditions has been demonstrated, accompanied by advances in equipment design, measurement and analysis techniques, and in the integration and interpretation of a large and diverse hydrological, structural and geochemical data set. A hyperalkaline solution (K–Na–Ca–OH, pH = 13.4 at 15 °C) representing an early leachate of Portland cement degradation was used in both the laboratory and field experiment. Results to date indicated a decrease in the overall field transmissivity of the tested shear zone over a duration of 2.5 years accompanied by focussing of flow as evidenced by repeat dipole tracer testing with Na-fluorescein, 82Br, 131I, 24Na, and 85Sr. The associated evolution in fluid chemistry and more direct evidence indicated the in situ formation of Ca–Si-hydrates. A core infiltration experiment was performed with a sample from a Grimsel shear zone where the fault gouge had been preserved in situ. The hydraulic behaviour was characterised with a NaCl tracer breakthrough experiment. This was followed by 9 months of infiltration with hyperalkaline solution at 15 °C. The experiment was carried out at constant and controlled head difference through a rock core under hydrostatic confining pressure. A gradual decrease in flow rate (hydraulic transmissivity) by a factor of 25 was observed over the duration of the experiment. This reduction is attributed to clogging of flow paths by secondary mineral precipitates (Ca–Si-hydrates) as a result of rock–cement leachate interaction. The HPF project has so far demonstrated conclusively the following issues: (1) hyperalkaline fluids are very reactive under ambient conditions and, in this case, cause significant dissolution and precipitation that induce changes to the flow field; (2) the general scenario of the high-pH plume is therefore valid and needs to be addressed in the performance assessment of a deep repository in fractured rock; (3) a trend towards self-sealing of flow-paths is observed in both the field and laboratory experiment; (4) the phenomena associated with the high-pH plume can be effectively addressed by a combination of laboratory and