Chemistry, isotope values (δD, δ18O, δ34SSO4) and temperatures of the water inflows in two Gotthard tunnels, Swiss Alps

Water inflows in the Gotthard Highway Tunnel and in the Gotthard Exploration Tunnel are meteoric waters infiltrating at different elevations, on both sides of an important orographic divide. Limited interaction of meteoric waters with gneissic rocks produces Ca–HCO3 and Na–Ca–HCO3 waters, whereas prolonged interaction of meteoric waters with the same rocks generates Na–HCO3 to Na–SO4 waters. Waters circulating in Triassic carbonate-evaporite rocks have a Ca–SO4 composition. Calcium-Na–SO4 waters are also present. They can be produced through interaction of either Na–HCO3 waters with anhydrite or Ca–SO4 waters with a local gneissic rock, as suggested by reaction path modeling. An analogous simulation indicates that Na–HCO3 waters are generated through interaction of Ca–HCO3 waters with a local gneissic rock. The two main SO4-sources present in the Alps are leaching of upper Triassic sulfate minerals and oxidative dissolution of sulfide minerals of crystalline rocks. Values of δ34SSO4 < not, vert, similar+9‰ are due to oxidative dissolution of sulfide minerals, whereas δ34SSO4 >not, vert, similar+9‰ are controlled either by bacterial SO4 reduction or leaching of upper Triassic sulfate minerals. Most waters have temperatures similar to the expected values for a geothermal gradient of 22°C/km and are close to thermal equilibrium with rocks. However relatively large, descending flows of cold waters and ascending flows of warm waters are present in both tunnels and determine substantial cooling and heating, respectively, of the interacting rocks. The most import upflow zone of warm, Na-rich waters is below Guspisbach, in the Gotthard Highway Tunnel, at 6.2–9.0 km from the southern portal. These warm waters have equilibrium temperatures of 65–75°C and therefore constitute an important low-enthalpy geothermal resource.