Chemical Characteristics of Groundwaters from the Matsunoyama Landslide Area, Niigata Prefecture

Abstract

The Matsunoyama landslides at the southern Niigata Prefecture have long been manifesting a lot of slumps and occasionally earthflows intermittently. The landslides occupy on a dome of Tertiary sediments composed of thick dacitic tuff and mudstone. Various landslide controls, such as drilling of collecting wells, have been made since 1962 areal slumping which destructed many houses, roads and firm yards.Groundwater samples were collected from collecting wells, horizontal and perpendicular drill holes and springs. Some of the groundwaters discharged in the landslide are unusually high in dissolved materials and warm in temperature suggesting them as deep thermal origin . Following five types of ground waters, Ca-HCO3 type, Na-HCO3 type, Ca-SO4 type, Na-SO4 type and Na-Cl type are chemically recognized in the Matsunoyama landslides. Ca-HCO3 type distributes at head to middle of the landslide mass. Na-HCO3 type predominates at foot of the landslide. Ca-SO4 type and Na-SO4 type find at head to middle and foot of the landslide in mudstone area, respectively. Ca is major dissolved in the tuff area. In the mudstone area, Na is major dissolved and SO4 is a characteristic constituent. Ca2+ is exchanged with Na+ from Na-rich clay minerals during migration from head to foot in subsurface of landslides. Na-rich clay minerals dominate in the mudstone rather than tuff. SO4 is supplied from oxidation of pyrite included mudstone. High concentration of Na-HCO3 is produced under at high CO2 partial pressure and reductive conditions. Na-Cl type waters and warm waters are influenced by the deep geopressured hydrothermal system.