Recently, the variety of utilization of underground rock cavern requires the increase of problem of underground water in a rock mass. Especially, at the construction of the repository of high level radioactive waste, it is nessesary to exactly evaluate deep and regional groundwater flow in order to keep safety of the rock mass as natural barrier if harmful nuclides are leaked because of some accident. For these needs, one of the suitable methods is geochemical method, which gives information about real groundwater flow recharge area, residence time, source and so on by means of dissolved ions and environmental isotopes.
In this report, the result was shown to apply this geochemical method to groundwater flow in a granitic rock mass at the tunnel site under construction. To evaluate regional groundwater flow in detail, this investigation was focused on these points, (1) principal component analysis for major dissolved ions, (2) altitude effect of stable isotope 18Oin water, (3) dissolved 222Rn gas, (4) stable isotope 13C in dissolved carbonate. From these results, it was concluded that regional groundwater flow in this site can be modeled as a flow in porous medium, and that changes of groundwater flow because of tunnelling can be evaluated by geochemical method.
