Abstract
Over 25 years, approximately 11 million m3 of water inflow in the Matsumoto tunnel at thenorth of Matsumoto city was drained. Water inflow in the tunnel during tunnel construction varied from 1 to8 m3 per minute decreasing to 1 to 0.6 m3 per minute between 1993 and 1995. At present, inflow maintains arate of 0.6 m3 per minute. Currently, it remains at 0.6 T.U. (Tritium Unit) and the age of water in flowing intothe tunnel has been estimated to be over 30 years old from initial 3H concentration for surface water whichvaried from 4 to 10 T.U. As oxygen stable isotope values and HCO3- concentration of water inflow in thetunnel were uniform for 25 years, the source of water inflow in the tunnel is not thought to have changed.Roughly 14 to 39 million m3 water was still stored in rocks above the tunnel in the plateau from 10 %porosity measured by rock sample. When only 3 million m3 groundwater, less than one tenth of porositycoincided with effective porosity (0.48 %) or specific yield (0.6 to 1.2 %) was drained, groundwater leveldecreased widely. Drained water after over 25 years of tunnel construction was derived from unsaturatedsmall pore water excluding main pores or cracks. Therefore, total pore volume was used for slowgroundwater migration (flow velocity is 3 to 7 m per year). Cracks or main pores connecting with manypores were important for fast groundwater migration (flow velocity is over 100 m per year).
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