Seepage into a mountain tunnel and rain infiltration

Abstract

To clarify the processes governing recharge of rainwater into an unconfined aquifer in a mountain basin, the seepage rate of water into a mountain tunnel was monitored continuously between May 1988 and December 1991 in Yura, Wakayama, Western Japan. The electrical conductivity and ionic composition of the seepage water was also measured regularly from June 1989. The mountain is composed of fractured sedimentary rocks such as sandstones and cherts. Seepage appeared only at specific locations on the roof of the tunnel. Seepage was measured through overlying sandstones of 10 m depth and cherts of 5 m depth. Time variations in the discharge suggested that seepage is formed both by rapid flow and basic flow components which correspond to the so-called fissure flow and matrix flow. This was confirmed by analysis of time variations in the concentration of chemical species in the seepage water. Fissure flow contributes the initial increment of seepage discharge, immediately after the occurrence of rainfall, and its velocity for sandstone can be represented by a function only of water content (θ), αθ n . α is constant and larger in winter than in summer, but n remains constant, at approximately two. Infiltration time in fissures through the base of the cherts is negligible, compared with that through the overlying soil layer. Whereas matrix flow through sandstones persists through the year, for cherts it disappears within a week after rainfall events and its decay is dependent on the seasonal magnitude of evapotranspiration. The behavior of the matrix flow for sandstones can be analyzed through a kinematic wave model, as can the fissure flow.