CHAPTER 19 . PIXE Analysis for Trace Elements in Marine Environments
a tide half-cycle. Filtration was performed in the field using 8 Ilm and 0.4 Ilm pore size
filters. The metal distribution in the separated fractions of the suspended matter, obtained for the two sites, shows that in the marsh the metals are apparently bound to
relatively coarse particles, since about one half of the total metal content was stopped
by the 8 Ilm pore size filter, while in the river the metals are mainly carried by grains
smaller than 8 Ilm. It is probable that part of the finer river load forms greater aggregates discharging in the marsh, where the deposition is favoured. In fact the grainsize distribution of the sediment in the northern marsh was centered around diameters a little smaller than 10 Ilm (Bernardi et al.I988). Nevertheless, resuspension phenomena induced by water dynamics (Ghermandi et al.1991) at the river mouth in the
marsh are indicated by the comparison between the element concentration in the water
column and the bottom current velocity along the axis of the mouth.
The processes occurring during salt- and fresh-water mix are already under investigation: these processes concern at first the soluted fraction in surface water. Two station sites along the Dese River (one site upstream characterized by fresh-water flow
at the surface, and one downstream more subjected to the salt wedge) where chosen,
and a third in a channel of the marsh, where a slack of hydrodynamics occurs. Physicochemical parameters measurement and water sample collection (with filtration
operation in field (Zonta et al.I994» were performed at the three sites during a tide
half-cycle. Water dynamics were continously monitored.
The high concentration of suspended matter strongly affects and masks the behavior of the dissolved fraction (even if filtered samples were analysed). In spite of the
difficulty to schematize the relationship between soluted trace metal distribution and
physicochemical variables, the results confirm the significant correlation of salinity
and pH with the trend of the ions (Ghermandi et al.I993). The behaviour of both these
variables strongly depends on the fresh and salt water mixing affecting the concentration of metals in solution. In addition each ion probably follows its particular pattern. On the other hand, given that the salinity gradient may represent a chemical
boundary for dissolved metal circulation, the model of salinity vertical profile along
the river and its branches towards the marsh is now under study.
This study demonstrates that the fate of the trace element load discharged in the
marginal areas of Venice Lagoon is strongly affected by the complex water circulation
and physicochemical processes in the water/sediment column. The main land-lagoon
interface zones, as the tested marsh, operate as a filter for the metal load discharged
from the drainage basin, and here transformation and accumulation of pollutants
occur: the study of the quality of the water/sediment system in this areas is fundamental to the understanding of the future of the Venice Lagoon ecosystem.
References
Aprilesi G, Cecchi R, Ghermandi G, Magnoni G, Santangelo R (1984) Calibration and errors in the
detection of heavy metals in fresh and sea waters by PIXE in the ppb-ppm range. Nucl Instr Meth
231 (B3) 1-3:172-176
Bernardi S, Costa F, Vazzoler S, Cecchi R, Ghermandi G, Magnoni G (1983) Transfer of fresh water and
pollutants into the Lagoon of Venice. In: Proe. VI C.LE.S.M. Workshop on Pollution of the Mediterranean, Cannes 1982, pp 73-78
Bernardi S, Costa F, Vazzoler S, Vincenzi Z, Cecchi R, Ghermandi G (1985) Lagoon of Venice: fresh
water and pollutant transfer. In: Proe. VII C.l.E.S.M. Workshop on Marine Pollution in the Mediterranean Sea, Lucerna 1984, pp 91-97
a tide half-cycle. Filtration was performed in the field using 8 Ilm and 0.4 Ilm pore size
filters. The metal distribution in the separated fractions of the suspended matter, obtained for the two sites, shows that in the marsh the metals are apparently bound to
relatively coarse particles, since about one half of the total metal content was stopped
by the 8 Ilm pore size filter, while in the river the metals are mainly carried by grains
smaller than 8 Ilm. It is probable that part of the finer river load forms greater aggregates discharging in the marsh, where the deposition is favoured. In fact the grainsize distribution of the sediment in the northern marsh was centered around diameters a little smaller than 10 Ilm (Bernardi et al.I988). Nevertheless, resuspension phenomena induced by water dynamics (Ghermandi et al.1991) at the river mouth in the
marsh are indicated by the comparison between the element concentration in the water
column and the bottom current velocity along the axis of the mouth.
The processes occurring during salt- and fresh-water mix are already under investigation: these processes concern at first the soluted fraction in surface water. Two station sites along the Dese River (one site upstream characterized by fresh-water flow
at the surface, and one downstream more subjected to the salt wedge) where chosen,
and a third in a channel of the marsh, where a slack of hydrodynamics occurs. Physicochemical parameters measurement and water sample collection (with filtration
operation in field (Zonta et al.I994» were performed at the three sites during a tide
half-cycle. Water dynamics were continously monitored.
The high concentration of suspended matter strongly affects and masks the behavior of the dissolved fraction (even if filtered samples were analysed). In spite of the
difficulty to schematize the relationship between soluted trace metal distribution and
physicochemical variables, the results confirm the significant correlation of salinity
and pH with the trend of the ions (Ghermandi et al.I993). The behaviour of both these
variables strongly depends on the fresh and salt water mixing affecting the concentration of metals in solution. In addition each ion probably follows its particular pattern. On the other hand, given that the salinity gradient may represent a chemical
boundary for dissolved metal circulation, the model of salinity vertical profile along
the river and its branches towards the marsh is now under study.
This study demonstrates that the fate of the trace element load discharged in the
marginal areas of Venice Lagoon is strongly affected by the complex water circulation
and physicochemical processes in the water/sediment column. The main land-lagoon
interface zones, as the tested marsh, operate as a filter for the metal load discharged
from the drainage basin, and here transformation and accumulation of pollutants
occur: the study of the quality of the water/sediment system in this areas is fundamental to the understanding of the future of the Venice Lagoon ecosystem.
References
Aprilesi G, Cecchi R, Ghermandi G, Magnoni G, Santangelo R (1984) Calibration and errors in the
detection of heavy metals in fresh and sea waters by PIXE in the ppb-ppm range. Nucl Instr Meth
231 (B3) 1-3:172-176
Bernardi S, Costa F, Vazzoler S, Cecchi R, Ghermandi G, Magnoni G (1983) Transfer of fresh water and
pollutants into the Lagoon of Venice. In: Proe. VI C.LE.S.M. Workshop on Pollution of the Mediterranean, Cannes 1982, pp 73-78
Bernardi S, Costa F, Vazzoler S, Vincenzi Z, Cecchi R, Ghermandi G (1985) Lagoon of Venice: fresh
water and pollutant transfer. In: Proe. VII C.l.E.S.M. Workshop on Marine Pollution in the Mediterranean Sea, Lucerna 1984, pp 91-97