368
R. Cecchi· G. Ghermandi
and generally Cr, are enriched in the surface layer with respect to Cu, Ni and Zn (Zago
et al. 1994).
A more detailed study (Cecchi et al.1996) examined sediment cores (from the sediment-water interface to -34 cm depth) collected in the same sites previously sampled.
These cores were sliced in specimens 3 cm thick. The pore water was extracted by centrifugation and analysed for trace metal, Eh, pH, H 2 S and Carbon determination
from six slices of each core (corresponding to depths of: 0/-3 cm, -3/-6 cm, -6/-9 cm,
-13/-16 cm, -19/-22 cm, -31/-34 cm). Fe, Cr, Cu, V, Mn and Ni concentration shows a
decrease in the values with the increasing depth, while H2S, dissolved Carbon and the
pH values have an opposite behavior. In addition, the vertical profiles from sites with
comparable hydro dynamical characteristics exhibit common features, while other sites
(close to the inner part of the marsh) affected by bioturbation and compaction phenomena provide less significant information.
A further investigation (Iotti 1998) has been preformed recently with core collection in three sites of the Lagoon with different environmental characters. The first site
is in a small channel ("rio") in the center of the city of Venice. The second site is close
to the Giudecca Island, in an area with relevant algae growth. The third one is located
at the Malamocco port entrance. The sediment cores extracted from these sites had
an average length of about 35 cm. They were sliced in order to obtain a continuous
series of specimens 1 cm thick, that all were centrifugated for further PIXE analysis of
pore water. AVS, H2S, FeS2 and SO~- where also determined. The resulting vertical profiles of Fe, AVS, H2S and SO~- and the FeS2 measurements in the same sediment samples
show significantly the transition of iron (abundant in the surface sediment layer) to
the sulfide phase. At the same time the sulfate was substituted by sulfide in the solid
phase. Given that the sulfide is released more slowly than the metals, the metal concentration peaks generally occurred before (at lower depth) with respect to the peaks
of H 2 S, released by the solid phase. These resulting behaviours of metals, SO~-, and
H 2 S (whose concentration in solution is very low) are in agreement with other studies
about pore water (Carignan and Nriagu 1985).
19.7.4
Trace Elements in Surface Water and the Effect of Fresh and Salt Water Mixing
The surface water quality was analysed from 1984 both in the Cona Marsh and in the
Dese River that discharge in the marsh. The seasonal behavior of the pollutant concentration, obtained from total water samples (i.e. soluted plus suspended matter),
shows a maximum in June, probably depending on agricultural wastes.
Given that the particle settling is certainly the fundamental process in accumulation phenomena, we have investigated the metal distribution in suspended particulate, in order to evaluate the element flux from the water column toward the sediment.
The first analyses were performed in 1988 in the test area. The trace metal concentration in the samples passed through 3 flm and 0.4 flm pore size filters were comparable,
and equal to a low fraction of the total metal content. This indicates that the particles
larger than 3 flm are mainly responsible for metal transfer in this ecosystem.
Field measurements of chemicophysical parameters, hydrodynamics and sample
collection along the whole water column were performed in 1989 in two sites (one in
a river and the other at the river mouth in the studied marsh), and completely covered
R. Cecchi· G. Ghermandi
and generally Cr, are enriched in the surface layer with respect to Cu, Ni and Zn (Zago
et al. 1994).
A more detailed study (Cecchi et al.1996) examined sediment cores (from the sediment-water interface to -34 cm depth) collected in the same sites previously sampled.
These cores were sliced in specimens 3 cm thick. The pore water was extracted by centrifugation and analysed for trace metal, Eh, pH, H 2 S and Carbon determination
from six slices of each core (corresponding to depths of: 0/-3 cm, -3/-6 cm, -6/-9 cm,
-13/-16 cm, -19/-22 cm, -31/-34 cm). Fe, Cr, Cu, V, Mn and Ni concentration shows a
decrease in the values with the increasing depth, while H2S, dissolved Carbon and the
pH values have an opposite behavior. In addition, the vertical profiles from sites with
comparable hydro dynamical characteristics exhibit common features, while other sites
(close to the inner part of the marsh) affected by bioturbation and compaction phenomena provide less significant information.
A further investigation (Iotti 1998) has been preformed recently with core collection in three sites of the Lagoon with different environmental characters. The first site
is in a small channel ("rio") in the center of the city of Venice. The second site is close
to the Giudecca Island, in an area with relevant algae growth. The third one is located
at the Malamocco port entrance. The sediment cores extracted from these sites had
an average length of about 35 cm. They were sliced in order to obtain a continuous
series of specimens 1 cm thick, that all were centrifugated for further PIXE analysis of
pore water. AVS, H2S, FeS2 and SO~- where also determined. The resulting vertical profiles of Fe, AVS, H2S and SO~- and the FeS2 measurements in the same sediment samples
show significantly the transition of iron (abundant in the surface sediment layer) to
the sulfide phase. At the same time the sulfate was substituted by sulfide in the solid
phase. Given that the sulfide is released more slowly than the metals, the metal concentration peaks generally occurred before (at lower depth) with respect to the peaks
of H 2 S, released by the solid phase. These resulting behaviours of metals, SO~-, and
H 2 S (whose concentration in solution is very low) are in agreement with other studies
about pore water (Carignan and Nriagu 1985).
19.7.4
Trace Elements in Surface Water and the Effect of Fresh and Salt Water Mixing
The surface water quality was analysed from 1984 both in the Cona Marsh and in the
Dese River that discharge in the marsh. The seasonal behavior of the pollutant concentration, obtained from total water samples (i.e. soluted plus suspended matter),
shows a maximum in June, probably depending on agricultural wastes.
Given that the particle settling is certainly the fundamental process in accumulation phenomena, we have investigated the metal distribution in suspended particulate, in order to evaluate the element flux from the water column toward the sediment.
The first analyses were performed in 1988 in the test area. The trace metal concentration in the samples passed through 3 flm and 0.4 flm pore size filters were comparable,
and equal to a low fraction of the total metal content. This indicates that the particles
larger than 3 flm are mainly responsible for metal transfer in this ecosystem.
Field measurements of chemicophysical parameters, hydrodynamics and sample
collection along the whole water column were performed in 1989 in two sites (one in
a river and the other at the river mouth in the studied marsh), and completely covered