Observational study on rainwater infiltration into a mountain

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Through continuous monitoring of seepage into a tunnel in a mountain composed of sedimentary fractured rocks and CO_2 gas concentraiton (C_<co2>) within the tunnel, the following is disclosed : (1) Increase in the discharge of seepage water (Q) owing to rainfall was observed to be faster than that in C_<co2>, when Q was large. Comparing a theoretical analysis with the observed result, it was concluded that, when C_<co2> shows a peak value, a slowly infiltrating component (matrix flow) begins to contribute to the seepage discharge. Pore-airs with high concentration of CO_2 gas near the ground surface are transported downwards by movement of the matrix flow. These ascertain the existence of two infiltration components called 'fissure flow' and 'matrix flow' which occur in relatively large and small voids of fractured rocks base, respectively. Overall, information of CO_2 gas concentraiton is useful for understanding the infiltration process. (2) A detected quite good correspondence of variation in the electrical conductivity of seepage water to that in C_<co2> is based on the chemical process that, the concentraiton of HCO_3^- (dominant anion) changes with varying C_<co2> in void-airs via dissolution/degassing.