Abstract
Na-Cl type groundwaters of high concentration have been often found at landslides in the Higashi-kubiki area, including the Utsunomata landslide, in Niigata Prefecture, Japan. The well loggings were carried out to profile the electric conductivity of groundwater in the landslide mass, and simultaneously groundwater samples were collected for the chemical analysis and the determination of oxygen isotopic ratio. The groundwater profiles in the active landslide mass showed the drastic increase of the electric conductivity around the depth of sliding surface. Such a feature is similar to the chemocline. On the other hand, in the inactive landslide mass, no significant variation was recognized in the profile. Groundwater collected from shallow to deep layer in the landslide mass were chemically classified into following four types in descending order: (1) Ca-HCO3 and Na, Ca-HCO3, (2) Na-SO4, (3) Na-HCO3, and (4) Na-Cl. Considering the relationship between the oxygen isotope ratio and chloride concentration, Na-Cl type groundwater were formed by the mixing the meteoric water with the altered fossil seawater that underwent a similar diagenesis to the the Nishiyama formation. These groundwater suggest that the deep fossil seawater as an abnormally pressured hydrothermal fluid ascend through the fault fractures and inject the landslide mass.
