Anthropogenic Venice uplift by seawater pumping into a heterogeneous aquifer system

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 data set based on marker measurements from a number of gas‐producing wells of the northern Adriatic is available for a realistic evaluation of the geomechanical properties of the injected geologic formation, permeability data are very scarce and sparse throughout the area. Previous finite element (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 lognormal, stationary random process. The resulting pore overpressure is then implemented into a deterministic FE geomechanical model. A sensitivity analysis is performed to account for the uncertainty on the stochastic process, reflected by both the log K variance s2 and the correlation length l 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, rz, are computed and used to evaluate the probability for rz to be larger than a few significant threshold values as discussed later. It is shown that, even within the most pessimistic scenario (i.e., s2 = 1.0 and l = 1000 m), the maximum rz is comparable, namely of the same order, with the one obtained from the deterministic case and (1) 2–3 times smaller than the rz caused in the city by groundwater withdrawal in the nearby industrial area over the 1960s (10 × 10−5), (2) 1 order of magnitude less than the maximum bound as indicated in the literature for the safety of multifloor masonry buildings (50 × 10−5), and (3) about 20 times smaller than the maximum rz 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.