Using short- and long-term transients in seepage discharge and chemistry in a mountain tunnel to quantify fracture and matrix water fluxes

Infiltration of rain-water into a fractured sedimentary rock mountain is explored through continuous observations of discharge rate, Q, and electrical conductivity, EC, of seepage water into a mountain tunnel. Also concentrations, CCO2, of carbon dioxide gas near the tunnel ceiling, and the chemistry of the seeping water are examined. Earthquake events occurred in the period of the seepage observation and influenced characteristics of the time trends in Q and EC. This provided a mechanism for the identification of rapid flow (fissure flow) and slow flow (matrix flow) in the infiltration components in the fractured rock base. Also, a cycling of discharge water from the matrix via the fissures and back into the matrix was expected to occur. CCO2 increased due to rainfall events, and its response was with a phase-shift to increased Q. For a heavy rainfall event, the increase in Q was mainly caused by the occurrence of fissure flow, and as soon as Q began to decrease moderately after a rapid decrease from a peak value, CCO2 showed a peak value. The CCO2 peak seemed to coincide with increased matrix flow. Wetting in the rock matrix was assumed to behave as a shock wave. For a light rainfall event, where only matrix flow is likely to occur in the fractured rock base, Q increases were delayed in comparison to CCO2 increases. The variations in CCO2 due to rainfall events appeared to relate to the movement of the matrix wetting front, when high moisture contents were apparent. The wetting front was inferred to be pushing void-airs with high concentrations of CO2 gas towards the tunnel. High CO2 concentrations were assumed to be formed near the ground surface via dissolution of organic matter and respiration of plant roots. The chemistry of seepage water observed at two close locations is seen to differ distinctly. Time-variations in EC for one location (A1) are consistent with those for CCO2, while for the other location (A3) this was not the case. The variations are due to dominant anions in the seepage water; HCO3− for A1 and SO42− for A3. These occur via dissolution of CaCO3 and CaSO4 into infiltrating water, and CO2 gas plays an important role in the former process. The time trends and integrated interpretation of the seepage volumes, chemistry of seepage water, and the concentration of CO2 gas are shown to be useful indicators for understanding rainwater-infiltration process in the fractured rock mountain, and for separation of the seepage into fissure flow and matrix flow components.