High-pH Plume in a Fractured Granite: Mineral Reaction and Permeability Changes.

Abstract:
One of the objectives of the HPF experiment (Hyperalkaline Plume in Fractured Rock) at the Grimsel Test Site (Switzerland) is to study the alteration of a fractured granite due to the circulation of high-pH solutions derived from the degradation of cement. A K-Na-Ca-rich high-pH solution was injected into a fracture during 3 years. An extraction borehole was located about 80 cm away from the injection borehole. The modeling of dipole tracer tests has indicated that flow in the fracture plane was highly heterogeneous. A small-scale laboratory version of the experiment (1D core infiltration experiment) was performed at the University of Bern. Both laboratory and field experiments showed significant changes in solution chemistry and an important decrease in permeability despite the small amount of secondary mineral precipitation observed in the laboratory experiment (excavation and rock sampling are currently under way in the field at Grimsel). However, dipole tracer tests showed a decrease in first arrival and peak arrival times and an increase in peak concentrations with increased alteration. The amount of mixing with natural groundwater at extraction also decreased. Detailed reactive transport modeling coupled with flow of the laboratory experiment has shown that the evolution of the system is consistent with mineral reactive surface areas smaller than measured specific surface areas by a factor of 10 to 100. However, the initial transient period in solution chemistry seems to indicate the presence of short lived fine particles (large surface areas), which may account for the difference between measured and calculated surface areas.