TUNNEL CONSTRUCTION IMPACT ON GROUNDWATER CHARACTER OVER 25 YEARS OF OBSERVATION AT MATSUMOTO TUNNEL, MATSUMOTO CITY, JAPAN

Abstract

Over 25 years, approximately 11 million m3 of water inflow in the Matsumoto tunnel at the
north of Matsumoto city was drained. Water inflow in the tunnel during tunnel construction varied from 1 to
8 m3 per minute decreasing to 1 to 0.6 m3 per minute between 1993 and 1995. At present, inflow maintains a
rate of 0.6 m3 per minute. Currently, it remains at 0.6 T.U. (Tritium Unit) and the age of water in flowing into
the tunnel has been estimated to be over 30 years old from initial 3
H concentration for surface water which
varied from 4 to 10 T.U. As oxygen stable isotope values and HCO3
- concentration of water inflow in the
tunnel 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 porosity
coincided with effective porosity (0.48 %) or specific yield (0.6 to 1.2 %) was drained, groundwater level
decreased widely. Drained water after over 25 years of tunnel construction was derived from unsaturated
small pore water excluding main pores or cracks. Therefore, total pore volume was used for slow
groundwater migration (flow velocity is 3 to 7 m per year). Cracks or main pores connecting with many
pores were important for fast groundwater migration (flow velocity is over 100 m per year).