Validation of surface-based hydrochemical investigation and the impact of underground facility construction at Horonobe URL

Abstract

We verified the surface-based hydrochemical investigation for deep underground at Horonobe Underground Research Laboratory (URL) at Horonobe, Hokkaido, Japan, and identified the hydrochemical changes during the URL construction. The evaluation of the relationship between the number of boreholes and understanding of water chemistry suggests that three basic borehole investigations and an additional borehole for high permeable geological zone (fault and fractured zone) are required to illustrate cross-sectional hydrochemical distribution including the uncertainty for kilometers scale survey line. The observation and numerical analysis of hydrochemical variation (salinity, pH, ORP) around the URL indicate that the groundwater pressure and the salinity in the vicinity of high permeable geological zone are varying due to groundwater inflow into the drift. The variation was consistent with the prediction reported previously. These results are considered to be referred to the management during surface-based investigation and construction of underground facility at the other sedimentary rock area.