The use of 3H and 18O tracers to characterize water inflows in Alpine tunnels

Water inflows in 9 tunnels and galleries through the Alpine crystalline massifs have been analysed for their ³H activities and δ¹⁸O contents. Tritium provides information on water transit times and the dynamics of deep water circulation, whereas δ¹⁸O contributes to understanding the origin and flow paths of water in such mountainous regions. Owing to ambiguities arising from the irregularity of the historical ³H input function since 1945, a unequivocal and straightforward interpretation of water transit times in Alpine tunnels is not possible. Nevertheless, the ambiguity can be resolved by considering the ³H data in combination with (a) the generalized hydraulic conductivity of the massif obtained from discharge data, and (b) the Na and silica content of the water as an indication of the extent of rock-water interaction. When the data are resolved in this way, the waters that were sampled in the tunnels/galleries can be divided into 3 age groups, i.e. <15, 15–40 and >40 a. In general, water beneath a rock-cover thickness of <500 m is less than 15 a old, which confirms the active circulation of groundwater in a “decompressed zone” (i.e. a zone of unloading fractures that is expected to have a depth of this magnitude). Moreover, tunnel excavation can radically alter the hydrology, as is shown by the ³H content of a water inflow in the Gothard gallery. Oxygen-18 data primarily reflect the recharge altitude, which can be predicted a priori by considering the large-scale geological structures of each massif and the extent to which they control the subvertical paths followed by the groundwater. Anomalous δ¹⁸O data may reflect local or general departures from this interpretation. A general pattern is that downslope flow in the better jointed “decompressed zone”, which parallels the topography, may divert recharge from a higher to lower altitude before it follows the structural pathways into the tunnel. This results in a somewhat lower δ¹⁸O value than would be predicted from structure alone, but tends to confirm the existence and role of the “decompressed zone” indicated by the ³H. More local δ¹⁸O anomalies reflect recharge from rivers or lakes entering the tunnels, and are illustrated by examples in this paper. Results show that environmental isotopes contribute to a better understanding of the hydrogeology of mountain massifs and of the interactions between tunnels/galleries and groundwater. They provide information not given by other tracing methods and are thus a precious tool for tunnelling engineers and geologists.