Soil contamination and land subsidence raise concern in the Venice
watershed, Italy
L. Carbognin, F. rizzzetto, L. Tosi
Institute of Marine Sciences, CNR, Venezia, Italy
G. Gambolati, M. Putti, P. Teatini
Dept. Mathematical Methods and Models for Scientific
Applications, University of Padova, Padova, Italy
ABSTRACT
The southern catchment of the Venice watershed is threatened by shallow aquifer
salinization and anthropogenic land subsidence due primarily to microbial
oxidation of organic soils that outcrop in the coastal farmland reclaimed from
the Adriatic Sea over the last century. Recent hydrogeological and geophysical
surveys provide documentary evidence that saltwater intrusion may extend
inshore up to 20 km far from the Adriatic coastline with the contaminant plume
from near ground surface down to 100 m depth in some areas. The actual salt
distribution is the outcome of a number of factors, including the ground
elevation markedly below the mean sea level (down to -4 m locally), the
seawater encroachment along the final 10-15 km of the regional watercourses
(Brenta, Bacchiglione, Adige, Gorzone), and the drainage practices implemented
in the reclaimed area. The fresh-salt water interface is generally between 2
and 30 m deep and exhibits a pronounced seasonal variation. At the same time
an ongoing settlement due to peat oxidation promoted by farming activities is
observed in most of the area south of the Venice Lagoon that was reclaimed from
1892 to 1967 and is rich in organic matter.
Overall land subsidence over the last 70 years ranges between 1.5-2 m and is
still in progress at a rate of 1.5-2 cm/y. As a major result large fraction of
the reclaimed land lies below the mean sea level with an increasing exposure
to flooding during severe winter storms and saltwater intrusion from the
Adriatic Sea, the nearby Venice Lagoon, and the river beds that locally lie
above the surrounding ground surface. To mitigate both hazards the
implementation of a drainage strategy of the reclaimed area intended to
maintain the water table as high as possible would be required. This could
decelerate the peat oxidation (hence the related land subsidence) and oppose
the inland subsurface salt convection and dispersion. Moreover the design of
mobile gates at a few river mouths (e.g. the Brenta river) could create an
effective barrier against the seawater migration upstream the watercourses
in the hottest and driest summers.
