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

Punta della Salute_20/07/2001

-60

-40

-20

0

20

40

60

80

100

19/07/2001 23.00 20/07/2001 3.48 20/07/2001 8.36 20/07/2001 13.24 20/07/2001 18.12 20/07/2001 23.00

wat

er t

able

(cm

)

PS1 PS4 PS6 tide PS

perched aquifers: A in landfill deposits (k=10-1-10-3 cm/s,

thickness of 3.5-4 m) widespread all over the city water level ranging between 0.70-0.05

m m.s.l directly connected with the lagoon

tide fluctuations determines water level variations:PS1

PS6

PS4tide

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 26 2001)

perched aquifers: B

in semipermeable sediments at 6-8 m depth (aquiclude)

limited extension and thickness (about 2 m)

water table trend: between -0.70 and -0.80 m g.l.

influenced mainly by low tideSan Cassian (June 18- July 17 2004)

confined aquifers: C1-C2-C3

in Pleistocene sand bodies deposits

not influenced by rainfall

the influence of tide fluctuations diminishes progressively with depth 0

5

10

15

20

25

10/08/2003 11/08/2003 12/08/2003 13/08/2003 14/08/2003 15/08/2003

rain

fall

(mm

)

San Basilio 11-14/08/2003

-80

-60

-40

-20

0

20

40

60

80

100

10/08/200312.00

11/08/20030.00

11/08/200312.00

12/08/20030.00

12/08/200312.00

13/08/20030.00

13/08/200312.00

14/08/20030.00

14/08/200312.00

15/08/20030.00

15/08/200312.00

wat

er le

vel (

cm)

tide Punta della Salute C3 C2 C1

physical – chemical analyses

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

piez pH T EC (mS/cm)perched aquifer F1 7.05 21.9 28.9

SFV1bis 8 17.4 4.17SFV2bis 7.3 16.5 9.3SFV3bis 7.4 16.3 8.6SFV4bis 8 15.6 2.95

confined aquifer SB3 7.48 17.4 21.2SFV1 7.2 16.9 11.1SFV2 7.4 16.4 12.1SFV3 7.1 17.7 12.4SFV4 7.1 17.2 12.9

C2 SB2 7.8 18.9 3.8C3 SB1 7.8 17.4 27.7

A

C1

-30

-25

-20

-15

-10

-5

0

5

0 10 20 30 40 50 60

EC (mS/cm)d

ep

th (

m m

.s.l.

)

perched aquifer San Nicolò channel confined aquifer C1

confined aquifer C2 confined aquifer C3

C2

C3

C1

F1SFV1-2-3-4

SFV2bis-1bis

SFV3bis-4bis

San Nicolò channel

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

piez Na/Cl (K/Na) (Ca+Mg)/(K+Na) SO4/Cl (Cl-(Na+K))/Clsea water 0.86 0.02 0.27 0.10 0.12

0.08C3 SB1 0.90 0.02 0.27 0.04confined aquifer

C1 SB3 0.91C2 SB2 1.18 -0.27

0.03 0.30 0.00 0.070.07 0.31 0.09

perched aquifer 0.12A F1 0.86 0.02 0.31 0.10

piez HCO3 Cl SO4 Na K Ca Mg

confined aquiferSB1 1714SB2 702

C3C2 912

45684767838114 698 82

6464019086 69

892

77439021810865

5857 254 78

6055perched aquifer 1537

C1 SB3 3630 9949 2

A F1 622

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