Transcript Diapositiva 1 - University of Florida

20th SWIM Salt Water Intrusion Meeting
Naples, Florida, USA - June 23-27, 2008
Salt Water Intrusion in The
Shallow Aquifers of Venice
F. Zezza, E. Di Sipio
University IUAV of Venice – Faculty of Architecture
Aim of this research is to determine the
water circulation of the first 50 m depth of
the Venice town subsoil
 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
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)
?
Venice sedimentary structure
The lithostratigraphic method allows to distinguish the
cyclothemic organization, typical of lagoonal area, from
the multistorey sandbody, typical of the city centre
(from Zezza, 2007)
subsoil
groundwater circulation in the Late
Pleistocene – Holocene deposits of Venice
historical centre
Well points




pH
T
EC
water
level
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
61 piezometers:
> 50 at 6-8 m depth
the remaining under 12 m depth
influence of tidal variations (Punta della Salute marigraph)
San Basilio (SB) has been selected as sampling area
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:
Punta della Salute_20/07/2001
100
PS1
water table (cm)
80
tide
60
PS4
40
20
PS6
0
-20
-40
-60
19/07/2001 23.00
20/07/2001 3.48
20/07/2001 8.36
PS1
PS4
20/07/2001 13.24 20/07/2001 18.12 20/07/2001 23.00
PS6
tide PS
 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 tide
San Cassian (June 18- July 17 2004)
confined aquifers: C1-C2-C3
San Basilio 11-14/08/2003
100
water level (cm)
80
60
40
20
0
-20
-40
-60
-80
tide Punta della Salute
C3
C2
C1
25
20
rainfall (mm)
 in Pleistocene sand
bodies deposits
 not influenced by
rainfall
 the influence of tide
fluctuations
diminishes
progressively with
depth
10/08/2003 11/08/2003 11/08/2003 12/08/2003 12/08/2003 13/08/2003 13/08/2003 14/08/2003 14/08/2003 15/08/2003 15/08/2003
12.00
0.00
12.00
0.00
12.00
0.00
12.00
0.00
12.00
0.00
12.00
15
10
5
0
10/08/2003
11/08/2003
12/08/2003
13/08/2003
14/08/2003
15/08/2003
physical – chemical analyses
piez
pH
T
EC (mS/cm)
perched aquifer
F1
7.05 21.9
28.9
SFV1bis
8 17.4
4.17
A SFV2bis
7.3 16.5
9.3
SFV3bis
7.4 16.3
8.6
SFV4bis
8 15.6
2.95
confined aquifer
SB3
7.48 17.4
21.2
SFV1
7.2 16.9
11.1
C1 SFV2
7.4 16.4
12.1
SFV3
7.1 17.7
12.4
SFV4
7.1 17.2
12.9
7.8 18.9
3.8
C2 SB2
7.8 17.4
27.7
C3 SB1
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)
physical – chemical analyses
EC (mS/cm)
0
10
20
30
40
50
60
5
SFV1-2-3-4
depth (m m.s.l.)
0
F1
San Nicolò
channel
SFV2bis-1bis
SFV3bis-4bis
-5
C1
-10
C2
-15
-20
C3
-25
-30
perched aquifer
San Nicolò channel
confined aquifer C2
confined aquifer C3
confined aquifer C1
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
geochemical analyses
piez
HCO3
Cl
SO4
Na
K
Ca Mg
perched aquifer
A
F1
622
10865 1537 6055 218 390 774
confined aquifer
C1
C2
C3
SB3
SB2
SB1
3630
702
1714
9949
912
7838
2 5857 254 78 892
114 698 86 69 82
476 4568 190 40 646
geochemical analysis in mg/l
piez Na/Cl (K/Na) (Ca+Mg)/(K+Na) SO4/Cl (Cl-(Na+K))/Cl
0.86 0.02
0.27
0.10
0.12
sea water
perched aquifer
A
F1
0.86
0.02
0.31
0.10
0.12
confined aquifer
C1 SB3
C2 SB2
C3 SB1
0.91
1.18
0.90
0.03
0.07
0.02
0.30
0.31
0.27
0.00
0.09
0.04
0.07
-0.27
0.08
relationship between sea water and Venice water ions ratio
San Basilio area
geochemical analysis
San Basilio
the Venice shallow
waters,
both
of
perched and confined
aquifers, are chloridesulphate-alkaline
waters
the anion ternary diagram focuses the existence of a
mixing process between fresh and salt water related to the
increasing of Cl content
Isotopic analysis
San Basilio
the isotopic analysis confirms the existence of a
mixing process between fresh and salt water of
marine and lagoonal origin
Results
 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
geochemical facies
waters
have
the
same
 the evolution process of the fresh water towards the
composition of seawater is confirmed by the
geochemical-isotopic analyses
Future development
 according to lithostratigraphic information, to
improve the knowledge of contact zones
between the different aquifers recognized
 to collect more data to confirms
hydrogeological model presented
the
 to focus on the soil-water interaction and its
effects on buildings