Mathematical simulation of the subsidence of Venice: 2. Results

Land subsidence at Venice is the result of aquitard compaction caused by extensive groundwater withdrawals from the unconsolidated Quaternary aquifer‐aquitard system that underlies the Venetian lagoon. A two‐step mathematical model consisting of a regional hydrologic model and a local subsidence model has been used to analyze the system. Results show that the total subsidence over the period 1930–1973 has been about 15 cm. Geodetic records, which cover only the latter portion of this period, provide documentary evidence for a 10‐cm subsidence during 1952–1969. Simulations of the past used to calibrate the model show that extractions from a single large well in the Tronchetto region of Venice have exerted a small but measurable influence on the subsidence, but the main responsibility for subsidence at Venice must be placed on the heavy industrial pumpage at the mainland port of Marghera. Predictive simulations show that any changes in the withdrawal rate at Marghera will tend to propagate their effects to Venice in a relatively short time, and there will be little delay between the occurrence of head declines at Venice and the resulting subsidence. If withdrawals at Tronchetto and Marghera are kept constant in the future as they have been since 1969, about 3 cm of further subsidence can be expected. Subsidence could be arrested at its present level by a shutdown of the Tronchetto well and a reduction of Marghera pumpage to three quarters of its present value. Complete cessation of all pumping would provide a modest rebound of about 2 cm over the next 25 years. The results of this study are based on a model that was limited by the requirements of radial symmetry and a calibration that was hampered by the sparseness of the available data. The subsidence situation at Venice should remain under continued evaluation as further data become available and more complex models become feasible.