The HPF project (Hyperalkaline Plume in Fractured rock) at the Grimsel Test Site comprises an underground long-term field experiment in a granitoid shear zone with hyperalkaline fluid (K-Na-Ca-OH, pH=13.4 at 15°C). Results document distinct mineral alteration (including Ca-Si-hydrate) that lead to ever decreasing transmissivities in an induced dipole flow field over a 3-year period. 3-D evolution of fluid chemistry, recorded hydraulic changes (radionuclide tracer tests), and post-mortem mineralogical/geochemical and radionuclide-specific analyses make this a one of a kind reactive transport field experiment.