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

摘要

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 deg 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,~(82)Br,~(131)I,~(24)Na,and ~(85)Sr.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 deg 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.