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 field experiments supported by evidence from natural analogues. Results are encouragingly similar and, hence, transferable between the laboratory and the field scale if boundary conditions and set-up are selected carefully, despite the large heterogeneity present at both scales.