Water percolating behavior indicated by the water chemistry within a decomposed granite slope, central Japan

Monitoring rainfall, discharge amount, and the chemistry of water from just beneath the groundwater table yielded a conceptual model of water percolation behavior within a decomposed granite slope in central Japan, providing a clue to evaluate a landslide hazard potential by rainstorms. When rainfall is strong enough, more than about 50 mm a day in this case, air pressure in the vadose zone probably increases temporarily because of the downward movement of a wetting front, pushing down the water of the capillary fringe. The capillary water has SiO2 concentrations controlled by the precipitation of cristobalite. The water is rich in Ca, which is supplied from plagioclase with SiO2. After the pushing down of the capillary water, rainwater with soil water in the unsaturated zone reaches the groundwater surface. The cation concentrations in the unsaturated soil water and the capillary water are affected by ion exchange.