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 delta S-34(SO4) < <similar to>+ 9 parts per thousand,
are due to oxidative dissolution of sulfide minerals, whereas delta
S-34(SO4) > similar to+ 9 parts per thousand 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 degreesC/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 degreesC and therefore constitute an important
low-enthalpy geothermal resource. (C) 2001 Elsevier Science Ltd. All
rights reserved.