Environmental Engineering Reference
In-Depth Information
What is remarkable is that the subsidence due to groundwater withdrawal
recorded in the period 1952-1970 had practically stopped by 1993. Comparison of
Figures 5.5 and 5.13 show clearly that subsidence is essentially an elastoplastic
phenomenon: even though piezometric levels are restored, only a very small part of
the aquifer compaction and surface lowering is recovered.
5.1.5.
Second study case: the subsidence of Albano Terme
5.1.5.1.
Background and data
The Euganean hills extend over an area of 100 km
2
and are located to the south-
east of the city of Padua. They are mainly composed of volcanic rocks, which are
surrounded by alluvial plains with clay and silt deposits. Geological and chemical-
isotopic investigations resulted in the conclusion that the Euganean underground
water system is a deep circuit with remote feeding (alpine and prealpine zones).
Figure 5.14 presents a tectonic map of the hydrothermal reservoir with isotherm
lines at a depth of 150 m.
Figure 5.15 shows the geological cross-section of the region: several faults, also
of regional importance, characterize the area and can clearly be seen. In the area of
Abano, the thickness of quaternary cover ranges from some 10 m to a maximum of
about 230 m and increases north-east toward Padua, where it is more than 500 m
thick.
The hydrothermal field occupies an area of about 20 km
2
immediately east of the
Euganean hills and can be classified as a convection system, dominated by the water
phase. The heated fluids, at depths greater than 4,000 m, rise along a fracture system
due to their lighter density. They are replaced by colder water coming from the
recharging zone. Hence a great heat flux is rapidly transferred by convection to the
carbonatic reservoir, which is situated at a relatively low depth and exploited by
about 250 wells.
In deep layers a more relevant vertical permeability with respect to the
horizontal was found. This is due to the presence of fractured rocks. A lateral
expansion takes place when the thermal water reaches the shallow zone of the
reservoir. Some of the water is stored inside the fractures and some moves sideway
or rises again within the alluvial cover to within 10 m of the surface. The
temperature of the thermal water ranges between 80°C and 87°C, when the vertical
flux is more pronounced. This temperature remains practically the same as at the
bottom holes.
