It’s important to predict in advance the volume and recharge area of discharge water in the tunnel. These are related to the structure of the aquifer, and the degree of connection between groundwater and surface water (well water, river water) is a key for prediction in advance. Previous studies (Kasama et al., 1972), (Ii et al., 1994) have suggested that the changes in the water quality (Electrical Conductivity (EC), pH, redox potential, chemical composition etc.) of the discharge water in the tunnel could be related to the changes in the groundwater flow and geology around the tunnel. This paper discusses about the applicability of the method to evaluate the degree of mixing of surface water in the tunnel, which focuses on the difference in water quality between surface water and groundwater.
To verify this applicability of the evaluation method, chemical composition of the discharge water collected from the advancing boring in the Yamaguchi section of the Central Alps Tunnel, wells and river water around the tunnel was analyzed. In this area, multiple fracture zones of the Atera Fault, which is one of the largest active faults in Japan, are distributed around tunnel and almost orthogonal to the tunnel.
As a result of this survey, discharge water in the tunnel showed pH, EC, and bicarbonate concentration higher than the surface water. This difference means that the discharge water in the tunnel stayed at the aquifer longer than the surface water. Measuring of discharge water from the advancing boring in some areas showed that EC increased with amount of discharge water but in other area, EC decreased. The former is due to the inflow of groundwater with long residence time, and the latter is due to the inflow of surface water.
It was found that analysis of water quality from advancing boring can help assess whether groundwater is connected to surface water or not.
