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 ex plored through continuous observations of discharge rate, Q, and electrical conductivity, EC, of seepage water into a mountain tunnel. Also concentrat ions, C-CO2, of carbon dioxide gas near the tunnel ceiling, and the chemist ry of the seeping water are examined. Earthquake events occurred in the period of the seepage observation and inf luenced characteristics of the time trends in Q and EC, This provided a mec hanism for the identification of rapid flow (fissure flow) and slow flow (m atrix flow) in the infiltration components in the fractured rock base. Also , a cycling of discharge water from the matrix via the fissures and back in to the matrix was expected to occur. C-CO2 increased due to rainfall events, and its response was with a phase-s hift to increased Q. For a heavy rainfall event, the increase in Q was main ly caused by the occurrence of fissure flow, and as soon as Q began to decr ease moderately after a rapid decrease from a peak value, C-CO2 showed a pe ak value. The C-CO2 peak seemed to coincide with increased matrix flow. Wet ting in the rock matrix was assumed to behave as a shock wave. For alight r ainfall event, where only matrix flow is likely to occur in the fractured r ock base, Q increases were delayed in comparison to C-CO2 increases. The va riations in C-CO2 due to rainfall events appeared to relate to the movement of the matrix wetting front, when high moisture contents were apparent. Th e wetting front was inferred to be pushing void-airs with high concentratio ns of CO2 gas towards the tunnel. High CO2 concentrations were assumed to b e formed near the ground surface via dissolution of organic matter and resp iration of plant roots. The chemistry of seepage water observed at two close locations is seen to d iffer distinctly. Time-variations in EC for one location (A1) are consisten t with those for C-CO2, 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 in to infiltrating water, and CO2 gas plays an important role in the former pr ocess. The time trends and integrated interpretation of the seepage volumes, chemi stry of seepage water, and the concentration of CO2 gas are shown to be use ful indicators for understanding rainwater-infiltration process in the frac tured rock mountain, and for separation of the seepage into fissure flow an d matrix flow components. (C) 2000 Elsevier Science B.V. All rights reserve d.