CHAPTER 19 • PIXE Analysis for Trace Elements in Marine Environments
yearly inputs in the Lagoon are of 414.1 ton ofFe,4.os tonofNi,s.s ton ofCu and 4.6 ton
of Pb (Bernardi et a1.1983; Bernardi et al.198S).
The evaluation of the mass budget of pollutant substances in the Lagoon requires
also the estimation of the pollutant transfer from the Lagoon to the Adriatic Sea. It is
really difficult, given the much higher water volume moved during each tidal cycle
through the Lagoon inlets with respect to the net fresh-water flux from the drainage
basin towards the sea. In addition, the pollutant distribution in the system is greatly
affected by the complex processes of interaction among the biogeochemical phases in
the Lagoon (surface water, bottom sediment, pore water and also living organisms).
Focusing attention on the distribution processes of heavy metals, a little shallow
water area (Cona Marsh) has been identified in the bordering part of the northern
Lagoon. This marsh constitutes the estuary of one of the eight fresh-water sources
included in the previous study (Dese River). The investigations have been carryied on
for some years in this test area, with systematic monitoring of surface water, bottom
sediment, pore water and inorganic pollutant evaluation by means of PIXE.
19.7.2
Trace Element Distribution in Surface Bottom Sediment
The trace element distribution in surface bottom sediment (S cm depth) of the test
area was investigated because significant exchange processes take place at the sediment-water interface. In addition a part of the pollutant load may be remobilized from
bottom sediments (given external factors that induce turbulence) reaching the water
column again. The map of trace element distribution in surface bottom sediment of
the Cona Marsh showed considerably lower concentrations near and in front of the
mouth of a channel discharging into the marsh, with respect to greater distances or
laterally away from the mouth. It is due to the more intense fresh stream from the channel. At both the sides of the mouth, on the contrary, insufficient currents determine
the trace metal accumulation. The fine-grained fractions (that are the main carriers
of metals among sediment grain sizes) are transferred far from the mouth (Bernardi
et al. 1988).
The calculation of concentration gradients of elements among the upper sediment
layer, pore water and bottom water allows for the evaluation of the fluxes of substances
among these geochemical phases.
19.7.3
The Vertical Profile of Trace Elements in Pore Water
The trace element distribution in pore water, extracted by centrifugation (Iotti 1998)
from sediment samples collected in Cona Marsh, has been investigated with the support of the PIXE technique. Pore water sample preparation has been performed in an
inert N 2 atmosphere. Both the spatial variability of element concentrations and also
their behaviour with depdt have been tested in samples collected in sites that represent different conditions in the dynamics of the marsh waters. Several investigations
were performed. At first pore water was extracted from two main depths alone: the
surface (o-s cm depth) and subsurface (S-10 cm) layer. Two groups of metals having
a different spatial trend are distinguishable at the two sampling depths. Fe and Mn,
yearly inputs in the Lagoon are of 414.1 ton ofFe,4.os tonofNi,s.s ton ofCu and 4.6 ton
of Pb (Bernardi et a1.1983; Bernardi et al.198S).
The evaluation of the mass budget of pollutant substances in the Lagoon requires
also the estimation of the pollutant transfer from the Lagoon to the Adriatic Sea. It is
really difficult, given the much higher water volume moved during each tidal cycle
through the Lagoon inlets with respect to the net fresh-water flux from the drainage
basin towards the sea. In addition, the pollutant distribution in the system is greatly
affected by the complex processes of interaction among the biogeochemical phases in
the Lagoon (surface water, bottom sediment, pore water and also living organisms).
Focusing attention on the distribution processes of heavy metals, a little shallow
water area (Cona Marsh) has been identified in the bordering part of the northern
Lagoon. This marsh constitutes the estuary of one of the eight fresh-water sources
included in the previous study (Dese River). The investigations have been carryied on
for some years in this test area, with systematic monitoring of surface water, bottom
sediment, pore water and inorganic pollutant evaluation by means of PIXE.
19.7.2
Trace Element Distribution in Surface Bottom Sediment
The trace element distribution in surface bottom sediment (S cm depth) of the test
area was investigated because significant exchange processes take place at the sediment-water interface. In addition a part of the pollutant load may be remobilized from
bottom sediments (given external factors that induce turbulence) reaching the water
column again. The map of trace element distribution in surface bottom sediment of
the Cona Marsh showed considerably lower concentrations near and in front of the
mouth of a channel discharging into the marsh, with respect to greater distances or
laterally away from the mouth. It is due to the more intense fresh stream from the channel. At both the sides of the mouth, on the contrary, insufficient currents determine
the trace metal accumulation. The fine-grained fractions (that are the main carriers
of metals among sediment grain sizes) are transferred far from the mouth (Bernardi
et al. 1988).
The calculation of concentration gradients of elements among the upper sediment
layer, pore water and bottom water allows for the evaluation of the fluxes of substances
among these geochemical phases.
19.7.3
The Vertical Profile of Trace Elements in Pore Water
The trace element distribution in pore water, extracted by centrifugation (Iotti 1998)
from sediment samples collected in Cona Marsh, has been investigated with the support of the PIXE technique. Pore water sample preparation has been performed in an
inert N 2 atmosphere. Both the spatial variability of element concentrations and also
their behaviour with depdt have been tested in samples collected in sites that represent different conditions in the dynamics of the marsh waters. Several investigations
were performed. At first pore water was extracted from two main depths alone: the
surface (o-s cm depth) and subsurface (S-10 cm) layer. Two groups of metals having
a different spatial trend are distinguishable at the two sampling depths. Fe and Mn,