Anthopogenic Venice uplift by seawater pumping into a heterogeneous
aquifer system
P. Teatini M. Ferronato, G. Gambolati, M. Putti
Dept. Mathematical Methods and Models for Scientific
Applications, University of Padova, Padova, Italy
D. Bau'
Dept. Civil and Environmental Engineering, Colorado State University,
CO
ABSTRACT
In recent years, a project of anthropogenic Venice uplift caused by seawater
injection into a 600-850 m deep brackish aquifer underlying the lagoon has
been advanced. While an extensive dataset based on marker measurements from
a number of gas producing wells of the Northern Adriatic is available for a
most realistic evaluation of the geomechanical properties of the injected
geologic formation, permeability data are very scarce and sparse throughout
the area. Previous Finite Elements (FE) predictions relying on a uniform
hydraulic conductivity K as derived from pumping tests suggest that a flat
uplift of the city is produced over 10 years from the inception of injection.
However, it is well known and widely recognized that in natural porous media
very seldom K exhibits an even spatial distribution. In this study, a random
distribution is then assumed to address the influence of a variable K on the
uniformity of the city uplift. To limit the otherwise prohibitive computational
burden, the study is performed relative to the pilot project designed to raise
a reduced area at the margin of the lagoon. Monte Carlo groundwater flow
simulations are performed using a FE discretization of the injected aquifer
system based upon a hydraulic conductivity distribution characterized by a
log-normal, stationary random process. The resulting pore over-pressure is
then implemented into a deterministic FE geomechanical model. A sensitivity
reflected by both the log-K variance σ2 and the correlation
length over the ranges 0.2-1.0 and 20-1000 m, respectively, which are quite
plausible for normally consolidated sedimentary formations such as the Northern
Adriatic basin. The Cumulative Distribution Function (CDF) of the ground
surface uplift, uz, and its horizontal gradient,
ρz, are computed and used to evaluate the probability
for ρz to be larger than a few significant threshold
values as discussed later. It is shown that, even within the most pessimistic
scenario (i.e., σ2=1.0 and λ=1000 m), the maximum
ρz is comparable, namely of the same order, with the one
obtained from the deterministic case and i) two-three times smaller than the
ρz caused in the city by groundwater withdrawal in the
nearby industrial area over the 1960s (10×10-5), ii) one order of
magnitude less than the maximum bound as indicated in the literature for the
safety of multi-floor masonry buildings (50×10-5), and iii) about
20 times smaller than the maximum ρz values that the city
is currently experiencing (100×10-5). The results highlight the
strong effect exerted by the overburden in smoothing the uneven expansion of
the injected heterogeneous formation.
