University IUAV of Venice – Faculty of Architecture 20th SWIM Salt Water Intrusion Meeting Salt Water Intrusion in The Shallow Aquifers of Venice F. Zezza, E. Di Sipio Naples, Florida, USA - June 23-27, 2008

to determine shallow aquifers presence and thickness to analyze salt water intrusion and influence of tide variations to characterize the different hydrochemical facies of perched and confined aquifers Aim of this research is to determine the water circulation of the first 50 m depth of the Venice town subsoil

The city of Venice (Po plain, NE Italy) is located in the southern threshold of the Veneto Platform (from Brambati et al., 2003, modified)

the upper Quaternary stratigraphical sequence is characterized by a great deal of variability in both the horizontal and vertical directions lack or fragmentation of geological interpretation in the first 50 m of depth what is the hydrogeological characterization of the subsoil (first 50 m depth) ?

The lithostratigraphic method allows to distinguish the cyclothemic organization, typical of lagoonal area, from the multistorey sandbody, typical of the city centre subsoil Venice sedimentary structure (from Zezza, 2007)

groundwater circulation in the Late Pleistocene – Holocene deposits of Venice historical centre

61 piezometers: > 50 at 6-8 m depth the remaining under 12 m depth Well points  pH  T  EC  water level influence of tidal variations (Punta della Salute marigraph) San Basilio (SB) has been selected as sampling area SGO=San Giacomo dall’Orio, SGB=San Giovanni in Bragora, SM=San Martino, PS=Punta della Salute, PL=Ponte dei Lavraneri (Giudecca), SE=Sant’Eufemia, San Marco, SC=San Cassian, SFV=San Francesco della Vigna, SB=San Basilio

perched aquifers: A in landfill deposits (k= cm/s, thickness of m) widespread all over the city water level ranging between m m.s.l directly connected with the lagoon tide fluctuations determines water level variations: PS1 PS6 PS4 tide  60 cm near the channel bank (PS1)  2-3 cm at a distance of 8-9 m (PS6)

perched aquifers: A the rainfall contribution to recharge factor is observable only in the piezometers located inland (more than 8 m from the channel bank) relationship between water table, tide and rainfall for Punta della Salute (June 21- July )

perched aquifers: B in semipermeable sediments at 6-8 m depth (aquiclude) limited extension and thickness (about 2 m) water table trend: between and m g.l. influenced mainly by low tide San Cassian (June 18- July )

confined aquifers: C1-C2-C3 in Pleistocene sand bodies deposits not influenced by rainfall the influence of tide fluctuations diminishes progressively with depth

physical – chemical analyses San Francesco della Vigna (SFV): May 2005 San Basilio (SB, F): June 2007 EC Lagoon measurement (San Nicolò Channel) : 48,9 mS/cm (June 2007)

The EC/depth relationship for samples collected in the piezometers of San Basilio - Santa Marta area point out the existence of water salinization with depth physical – chemical analyses

geochemical analyses geochemical analysis in mg/l relationship between sea water and Venice water ions ratio San Basilio area

the Venice shallow waters, both of perched and confined aquifers, are chloride- sulphate-alkaline waters geochemical analysis the anion ternary diagram focuses the existence of a mixing process between fresh and salt water related to the increasing of Cl content San Basilio

Isotopic analysis San Basilio the isotopic analysis confirms the existence of a mixing process between fresh and salt water of marine and lagoonal origin

the hydrogeological characterization of the first 50 m of the Venice subsoil has been done tide variations affects the water level in all the identified aquifers the perched and confined aquifers are all interested by salt water intrusion the Venice shallow waters have the same geochemical facies the evolution process of the fresh water towards the composition of seawater is confirmed by the geochemical-isotopic analyses Results

according to lithostratigraphic information, to improve the knowledge of contact zones between the different aquifers recognized to collect more data to confirms the hydrogeological model presented to focus on the soil-water interaction and its effects on buildings Future development