Hydrochemical evolution process at the Horonobe area, northern part of Japan

Mizuno, Takashi   

The Horonobe URL which is operated by JAEA is one of the generic purpose-built URL in Japan. Reconstruction of hydrogeological and hydrochemical properties around the URL site in geological time-scale is a key components of the R&D program. Previous studies indicates that groundwater in the area is mainly Na-Cl type of groundwater originated from fossil seawater which is trapped during the sedimentation. However, salinity of groundwater is less than seawater. This study aims to reveal the dilution process of groundwater based on existing hydrochemical data (Cl, O and D) obtained from surface-based boreholes drilled at the Horonobe area. We find that degradation of gas hydrate will be one of the possible source of less salinity groundwater. Further study will be needed for understanding the more detail process.