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

Water inflows in nine tunnels and galleries through the Alpine crystalline massifs have been analysed for their 3H (titrium) activities and  18 O contents. Tritium provides information on water transit times and the dynamics of deep water circulation, whereas  18 O 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 univocal 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 sodium 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 three age groups, i.e. <15, 15-40 and >40 years. In general, water beneath a rock-cover thickness of <500 m is less than 15 years 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 tritium 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  18 O 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  18 O value than would be predicted from structure alone, but tends to confirm the existence and role of the " decompressed zone " 2 indicated by the tritium. More local  18 O anomalies reflect recharge from rivers or lakes entering the tunnels, and are illustrated by examples in this paper. Our study shows that environmental isotopes contribute to a better understanding of the hydrogeology of mountain massifs and of the interactions between tunnels/galleries and …