Metal fluxes to the sediments of the northern Venice Lagoon

ELSEVIER

Marine Chemistry 58 (1997) 275-292

Metal fluxes to the sediments of the northern Venice Lagoon Mauro Frignani a,*, Luca G. Bellucci a, Leonardo Langone a, Herbert Muntau b ’ Istituto di Geologia Marina de1 CNR, Via Gobetti 101, 40129 Bologna, Italy ’ Enl;ironment Institute, Joint Research Center Ispra, EC, 21020 Ispra (VA), Ital) Accepted 7 September 1996

Abstract Eighteen short cores were analyzed for major and trace metals (Al, Fe, Ca, Mg, Mn, Si, K, Ti, Pb, Zn, Cu, Ni, Cr), ““Pb, “‘Cs, and other sediment characteristics, so as to describe the chronology of pollution and calculate metal concentration factors and fluxes. Substantial evidence was found that trace metal profiles are influenced by anthropogenic sources and by changes in sediment composition. Only Zn presents concentrations (up to 13.1 pmol g - ’ ) and concentration factors (1.3 to 13.2) that can be attributed to heavy contamination. Pb, Cu and Ni, in this order, are less significant. The area1 distribution of concentrations and inventories reflects the importance of direct sources, in particular the industrial area of Port0 Marghera and the Dese river. The inventories of excess metals, above pre-industrial levels, were determined for each core and the three different parts of the study area: the amounts of Zn accumulated in sediments are 11.0 Mmol, 5.1 Mmol and 0.37 Mmol in the Campalto, S. Erasmo, and Palude di Cona areas, respectively. Fluxes were also calculated and compared with those suggested for the atmospheric delivery by Cochran et al. [1995b. Atmospheric fluxes of heavy metal contaminants to the Venice Lagoon, Rdpp. Comm. Int. Mer Midit., 34, 136.1: the atmospheric contribution is predominant or significant in many cases, especially at sites far from the major local inputs. Concentrations and fluxes show a significant increase in the anthropogenic metal supply starting from the second decade of this century, with maximum inputs in the period between the 1930s and the 1970s. At some stations a decrease in heavy metal contamination of surlkial sediments was found and this could be ascribed to a reduced input of pollutants in recent years. 0 1997 Elsevier Science B.V. Keywords: heavy metals; sediments; “‘Pb chronology; sedimentary fluxes; Venice Lagoon

1. Introduction The Venice Lagoon is a complex shallow aquatic ecosystem; its morphology is characterized by a network of channels of various depths, mud flats, tidal marshes and islets. This leads to the formation of different sub-environments with particular hydro-

* Corresponding author. E-mail: [email protected]. Fax: +39-51.6398940.

dynamics as well as physical, chemical, and biological conditions. The lagoon is connected to the Adriatic Sea and exchanges waters and sediments following the tidal cycle. This environment is heavily affected by anthropogenic activities and the uncontrolled discharge of pollutants from both diffuse and point sources. Rivers, streams, land runoff, urban wastes, industrial discharges, and atmospheric wet and dry depositions are all sources of pollutants to the lagoon. In theory, the accumulation of particles and anthropogenic chemicals in bottom sediments

03044203/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PI1 SO304-4203(97)00055-S

276

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depends on a complex set of processes ultimately driven by water circulation, abundance and composition of particulate matter, relative importance of pollutant sources, as well as production and cycling of organic matter. However, in spite of the latest scientific efforts (Basu and Molinaroli, 1994; Martin et al., 1994; Bertolin et al., 1995: Sfriso et al., 19951, the biogeochemical behavior of the system is not fully understood, and quantitative data on the sizes of fluxes and the rates of processes are still missing. The aims of this research are: (1) deciphering the most recent history of environmental changes within the lagoon and its surrounding territory, so as to gain some insight in the system evolution and the relative importance of the various processes; (2) determining the fluxes related to excess heavy metal accumula-

Citemisty

58 (19971275-292

tion and distribution tion with time.

in sediments,

and their evolu-

2. Study area The Venice Lagoon is a water body with surface area of 549 km*. It is connected to the sea through three inlets (Lido, Malamocco and Chioggia). The tidal channels originate from the inlets and then branch out in numerous secondary channels, the section of which decreases towards the mainland. The average water depth is about 0.6 m and the tidal excursion is less then 1 m. The water volumes exchanged with the sea during a semi-tidal cycle vary between 1.6 and 5.2 X lo8 m3, with an average

Fig. 1. Study area and core sampling locations. The intertidal areas are shown in light grey.

M. Frignani et al. /Marine

value of 3.3 X 10’ m3 (Cossu and de Fraja Frangipane, 1985). On the basis of the inlet flows, the basin volume and the recycle of the lagoon waters, the average renewal time of the waters is two days, with a minimum of 1.4 days and a maximum of 3 days. The closer to the mainland, the lower the water dynamics. The study area (Fig. 1) is a large section of the northern basin of the Venice Lagoon. There are 8 main tributaries debauching into the lagoon, but the study area is primarily affected by the inputs from the Dese river as well as the Silone and Osellino channels (Bemardi et al., 1986). The study area can be divided in three subareas: Campalto, S. Erasmo and Palude di Cona. The Campalto area is located between the town of Venice, the Pot-to Marghera industrial area, the Murano island and the mainland. It is crossed by the S. Secondo, Campalto and Tessera channels, 2 m deep, which drain the surrounding shallow zone mainly during the last stages of ebb tide. In recent years these channels were partially silted up, particularly the Campalto channel. The Osellino channel collects civil and industrial sewage effluents and discharges them into the area. In summer frequent dystrophic events lead to the production of macroalgae (in particular Uluu rigida) which are efficiently regenerated. Suboxic and anoxic sediments can be found in areas near the mainland. The S. Erasmo area is defined by the mainland and the S. Erasmo and Burano islets. The remaining part corresponds to the Palude di Cona which is an interface with the mainland and receives the discharge from the Dese river. This is a typical shallow water environment with estuarine characteristics and a complex water circulation pattern, where heavy metals accumulate onto its bottom close to the mainland (Zonta et al., 1994).

3. Sample collection

and analysis

Eighteen short sediment cores (65 cm long, 4 cm in diameter) were collected in February-March 1992 at the locations shown in Fig. 1. The number of sites is large enough to represent the variability of sedimentary deposits. The average water depth at all sampling stations of Campalto and S. Erasmo is

Chemistry 58 (1997) 275-292

277

about 0.5 m. The sampling locations within the Palude di Cona area are as follows: three sites (PC4, PC5 and PC6) along the Dese river; the PC1 and PC13 sites in its northern part; and the PC9 site in the southern part. The average water depth is about 2.2 m at the river sites, 1.8 m in PC1 and about 0.5 m at the other two sites. The cores were extruded in situ and sectioned at intervals of 2 cm, 3 cm and 4 cm, with higher resolution near the top of the sediment column. Sediment slices were dried at 60°C slightly disaggregated and then sieved through a 2-mm mesh nylon net to discard coarse materials, mostly shell fragments. A second set of cores (55-90 cm long, 6 cm in diameter) was taken at each site; these samples were scanned for whole-core magnetic susceptibility. In addition, box cores were collected for a first sediment description. Major and trace metal analyses were performed by wavelength dispersive X-ray fluorescence spectrometry using a Siemens sequential SRS 300. Calibration curves were established on the basis of available Certified Reference Materials from NIST and BCR with aid of the SPECTRA-AT software. Sediment pellets of 3 cm diameter were prepared using a hydraulic press, applying a pressure of 20 ton cm ~‘. Package net intensities where computed from row data correcting for the dead time, background, standard and instrument drift. The major constituents (SiO,, Al,O,, CaO, K,O, Fe,O,, MgO, TiO,, S, and P,O,) and the trace elements (Pb, Zn, Cu, Ni. Mn, and Cr) were determined. In the PC1 core Cu. Pb, Zn, Hg and Cd were analyzed also by AAS and DPASV after acid digestion with HNO, and HClO, (Frignani et al., 1994). The organic matter contents, which probably account for most of the biogeochemical reactivity of the system, were measured as organic carbon (OC) contents with the use of a CHN EA analyzer by Carlo Erba Strumentazioni. The carbonate fraction was eliminated through a HCl treatment in a silver capsule. Alpha counting of “‘PO was used for 2’0Pb determinations, assuming secular equilibrium between the two isotopes. 2’oPo was extracted from the sediment and plated on silver discs, following the method outlined by Frignani and Langone (1991). ‘j7Cs was measured by gamma spectrometry. Mineralogical

h4. Frignani et al. /Marine

278

Chemistr?, 58 (19971275-292

palto and S. Erasmo, in the Palude di Cona the clay fraction becomes important and prevails in two branches of the Dese river. Generally, the amount of fine material increases from the inlet towards the mainland, i.e. where the water dynamics induced by tidal motion is low. Fig. 2 shows the grain size distribution and the ratios between mineral constituents. The main mineral constituents are, in order of quantitative ranking, dolomite, quartz, calcite, feldspar, mica and chlorite. The silicate/carbonate and calcite/dolomite ratios were calculated from XRD analyses. The highest values refer to the Palude di Cona where terrestrial materials are directly supplied from the river, which shows the maximum values. Campalto and S. Erasmo sediments have lower values due to particles coming from the Adriatic Sea through the Lido inlet. The calcite/dolomite ratios are relatively high in the Palude di Cona, where the dolomite contents are low. The quantity of

composition and grain size were determined in a number of selected samples. The first was analyzed through XRD, the second by wet sieving, to separate the sand fraction from the mud, after a pretreatment with H,O, treatment which oxidized organic matter. Mud was then analyzed by means of a X-ray Sedigraph.

4. Results and discussion

4.1. Area1 distributions Grain size and mineralogical composition of surficial sediments follow the same area1 distribution described by Barillari and Rosso (19751, Hieke Merlin et al. (19791, and Menegazzo Vitturi and Molinaroli (1984). Most sediments are clayey silts or silty clays: while the silty component prevails at Cam-

El

E3

E4

E5

E6

E8

a

Cl0

n

Cl1

Sand %

Cl2

q Silt

Cl3 %

Cl4

sediments.

size, mineralogical

composition

(ratios

PC9

PC13

PC4

PC5

PC6

Clay %

Silicate/Carbonate

Fig. 2. Grain

PCI

Calcite/Dolomite

of mineral

constituents),

organic

carbon

and total nitrogen

contents

of surticial

M. Frignani

et al. /Marine

Chemist?

dolomite depends on the importance of sedimentary materials delivered by the Piave river to the Adriatic Sea (Barillari and Rosso, 1975; Hieke Merlin et al., 1979) and then transported into the lagoon by tidal currents. Obviously, the Adriatic influence, with high carbonate material enriched with dolomite, is stronger at S. Erasmo and Campalto which are close to the Lido inlet. Other differences among surficial samples can be observed in their organic material contents. Organic carbon and total nitrogen - the latter can be assumed to be almost entirely organic (Giordani and Angiolini, 1983) - differ considerably in the various zones (Fig. 2). In the Dese river concentrations as high as 2.5% and 0.2% were measured for OC

58 (IS71

275-292

219

and N, respectively. In the Campalto and S. Erasmo areas, sediments show OC concentrations from 0.7% to 1.5%, and N concentrations from 0.06% to 0.2%, respectively. The highest values were found in the Dese river and at some Palude di Cona sampling sites because of the direct organic matter input from the inland polluting sources. This confirms that most algal production in the lagoon is efficiently mineralized and does not accumulate in sediments. The concentrations of major metal (SiO,, Al,O,, Fe,O,, K,O, TiO,, CaO, MgO) in surficial samples are shown in Fig. 3. These concentrations corroborate the indications of mineralogical analyses. Al, Fe, K, and Ti are correlated to SiO,, since they are constituents of silicates. This correlation is weak at

Si02

CaO

507

Fe24

&O

10

10

8

8

i

Pb

Ni 2.0 $

0.8 0.6

~

8

1.5 $6

Fig. 3. Surficial concentration

of major and trace metals.

280

M. Frignani et al. /Marine

site C9, where the sandy fraction consists mainly of quartz. The highest trace metal concentrations (Fig. 3) were found in surficial sediments collected near polluting sources. Zn has by far the greatest concentrations, especially in the sediments which are heavily polluted by the waste effluents from Port0 Marghera. The Dese river is the other main point source of almost all trace metals, particularly of Cu, Ni and Mn. Mn follows rather closely the pattern of silicate constituents. Cr seems to have neither a clear pattern, nor a prevailing input.

Chemistry 58 (1997) 275-292

but slightly in most sediment cores. There is a carbonate content increase in the topmost part of some cores, which means that some locations recorded a greater input of carbonates in recent times. This feature can be seen clearly in the samples taken close to the mainland, since the others have always experienced a prevailing marine influence. Core E8 is the only case with a dramatic change of mineral composition with an abrupt passage from prevailing silicates to predominant carbonates at a depth of about 30 cm. 4.3. Metal profiles

4.2. Core description Fig. 4 summarizes various types of observations in the form of stratigraphic columns for the most interesting cores. Three main characteristics are indicated: sediment lithology, type and concentration of and presence of burrows organogenic residues, caused by bioturbation. Sediment lithology is fairly constant except for core C14, where the grain size shows a coarsening upward trend. In general, some burrows exist in surficial sediments, in particular because of the presence of Polychaeta. Sediments are populated also by Olygochaeta, Crustacea, and larvae of Chironomidae (D. Prevedelli, personal communication). Shells of bivalve and gastropod mollusts can be found. Some cores are rich in mollusc tests throughout the sediment column, while only localized accumulations exist in other samples. Perhaps, these tests were transported from other areas to and accumulated at some sites as a result of water dynamics. Vegetal residues were observed in some cores, particularly at site PC 13. Surficial sediments are oxidized down to a depth of a few mm: oxygen concentration becomes zero within the first l-2 mm, while the redox potential is negative just below the sediment-water interface and does not decrease significantly with depth (Cochran et al., 1995a). Below the thin brown surficial layer the sediment color is gray to black. Generally, volume magnetic susceptibility values are low (< 150 X 1O-6 u.S.1.) and depth-profiles do not show marked trends. Nevertheless, they made it possible to correlate some cores taken from the Palude di Cona area (PC1 vs. PC4, PC5, and PC9). Grain size and mineralogical composition vary

In general, SiO, variations are correlated with metals contained in silicate minerals (Al, Fe, K and Ti), whereas CaO and MgO concentrations have an inverse trend, since they represent the carbonate fraction. Fig. 5 gives the chemical composition of four cores, including E8. The concentration-depth profiles of trace metals detected in cores can be influenced by sediment texture and composition, input from polluting sources and diagenesis. It is important to establish the key process, because the history of pollution can be read in the sediment record if trace metal distributions are mainly controlled by the inputs. Fig. 6 reports the trace metal profiles for cores which are representative of the different areas. Ni, Mn and Cr concentrations throughout the sediment column do not show significant changes. A normalization against Al concentrations (Boust et al., 198 1) was made so as to overcome effects of sediment composition variations. Ti can be used as normalizer (Salomons and Forstner, 1984) but here Al was chosen, because it accounts better for variations in sediment grain size (Boust et al., 1981). In most cases the normalization procedure eliminated fluctuations in concentrations and peaks. Moreover, the vertical distribution of Mn, a metal very sensitive to redox conditions, is here a function of the concentrations of Al (and other silicate-related elements). This means that diagenetic processes do not play a very effective role in influencing metal profiles. Furthermore, Bertolin et al. (1995) studying three sediment cores taken close to Porto Marghera, found that the formation of sulfides does not significantly affect heavy metal distribution in cores.

*, _ _

c

stiff clay

I---_\

-.

silty clay

WI

- _ ^ __

clayey silt

pgg

_

_, ^

_ _ r, ”

__ _

Bittium reticulatum

_ f \ _. _

_

of the most interesting

‘4 w q w

& Q VI 57 Gibbula sp.

Fig. 4. Stratigraphy

Cerastoderma glaucum

/ --.> Tellina sp.

‘~.‘.P,

,-I,

LEGEND

-.-.

cores.

<”

‘1-1 ‘3

,”

-

.

.

-

vegetal remains

shell fragments

_

-

_

I

_

:’

_

-

burrows

90cm

Lo . _

_

_

M. Frignani et al. /Murine

282

Si02 (%)

4

Ot 10 20 30 40 50 60 70

-m

I I

t

CaO (%)

CaO (%) 25

30

35

40

-r--

0

5

10

IS

20

25

CaO (%)

CaO (%) 30

35

40

IS

20

25

30

35

40

10

15

20

25

30

T 0123456

Fig. 5. Major constituent

Fe24 (%)

Fe24 (%)

Fe203 (%)

Fe203 (%) 0123456

SiOz (%)

SO2 (%)

Si02 (%)

18 20 -_

20

Chemistry 58 (1997) 275-29.2

“123456

profiles of the E3, ClO, E8 and PC1 cores

0123456

M. Frignani et al. / Marine Chemist?

58 (1997) 275-292

283

E3

Zn (pmol/g) 00

Ni (pmoUg)

Ni (pmol/g)

00

30

Cr (pmoUg) 04 “8 12 16

60

90

Zn (pmol/g) I20 ,5n

00

05

00

15

20

Z’

Ni (pmol/g)

Ni (pmoUs)

20

1”

Cr (pmoUg) 04 08 12 16

io

00

04

0

2

Cr (pmoUg) 08 12 16

20

Or-‘ot

Mu (pmoUg)

Mn (pmoUp)

r 0

LO 0 10 20 30 40 50 60 70

2

4

6

8

Fig. 6. Trace metal concentration

10

0

2

4

6

Mn (pmoUg) *

IO

profiles of the E3, ClO, C 12 and PC1 cores.

4

6

x

I"

284

M. Frignani et al./Marine

Zn, Pb and Cu profiles present a more or less evident concentration increase from low or background values at a certain depth, followed by fluctuations and surface or subsurface peak values. Variations are more clear in the cores from Campalto and Palude di Cona and not so significant in the samples from S. Erasmo. The observation that the concentration of some metals decreases in the vicinity of the sediment water interface gives rise to an additional problem of interpretation. Some authors maintain that redox conditions have an effect on the release of metals from surficial sediments (Sfriso et al., 199.51, particularly during the mineralization of organic matter originated by the production of Ulna rigida. We do not have direct proof of the importance of this process, but our information from the various metal profiles suggest that this process is not responsible for the concentration pattern directly below the interface. There is a strong similarity between our profiles of polluted lagoon sediments (Fig. 6) and the profiles produced by Cochran et al. (1995b) for a salt marsh near S. Erasmo, where Ag and Cd atmospheric inputs were found to have increased until the present

Table I Maximum concentration metals. The background pm01 g- ’

factors and background values of trace values, in parentheses, are expressed in

Station

Pb

Zn

CU

El E3 E4 E5 E6 ES c9 Cl0 Cl1 Cl2 Cl3 Cl4 PC1 PC9 PC13 PC4 PC5 PC6

1.6 (0.33) 1.8 (0.28) 1.7 (0.33) 1.6 (0.47) 2.4 (0.18) 2.7 (0.30) 1.7 (0.29) 3.2 (0.23) 4.4 (0.36)’ 4.1 (0.31) 4.0 (0.28) 3.7 (0.28) 2.4 (0.34) 1.9 (0.30) 5.2(0.31) 2.6 (0.30) 2.7 (0.31) 1.6 (0.45)

1.4 (0.24) 1.6 (0.21) 1.6 (0.23) 1.7 (0.29) 1.8 (0.23) 1.9 (0.24) 1.5 (0.29) 3.4 (0.26) 4.2 (0.35) 13.2 (0.35) 2.7 (0.23) 9.1 (0.29) 2.1 (0.29) 1.3 (0.23) 2.0(0.23) 3.2 (0.34) ” 3.1 (0.35) a 1.6 (0.44)

1.3 (0.31) 1.4 (0.73) 1.4 (0.83) 1.5 (0.39) 1.4 (0.78) 1.2 (0.88) 1.8 (0.27) 1.5 (0.97) 1.3 (1.10) 1.5 (0.39) 1.4 (0.82) 1.3 (0.92) 1.7 (0.22) 2.0 (0.89) 1.4 (1.00) 1.5 (0.42) 1.5 (0.80) 2.1 (0.90) 1.4 (0.38) 1.5 (0.87) 1.3 (0.98) 1.8 (0.35) 1.3 (0.97) 1.1 (1.10) 1.6 (0.36) 1.3 (1.01) 1.3 (1.13) 2.2 (0.46) 1.2 (0.85) 1.1 (0.96) 1.8 (0.35) 1.4 (0.73) 1.3 (0.83) 2.1 (0.44) 1.6 (1.14) 1.2 (1.29) 2.0 (0.50) 1.3 (0.83) 1.3 (0.94) 1.4 (0.28) 1.6 (0.85) 1.5 (0.96) 2.0(0.33) 1.7cO.65) 1.5 (0.73) 2.6 (0.49) ‘1.5 (1.06) “1.2 (1.19) a 2.3 (0.52) Y.4 (1.11) “1.1 (1.25) a 1.3 (0.46) 1.4 (0.78) 1.3 (0.88)

’ Background Al(Me’)/(Al’).

’ a a

a a

values estimated by See text for more details.

Ni

the

CI

formula

Me =

Chemistry 58 (1997) 275-292

time, while Zn, Pb, Cu and Ni showed significant reductions in recent years. In summary, most profiles seem to be influenced mainly by the heavy metal contributions from diffuse and point sources. The background values of each core are needed for calculating excess metal concentrations, concentration factors and inventories. In most cores two or more levels with constant concentration indicated exactly or nearly their background values. However, in the absence of consistent data, the points of minimum concentration in a profile normalized against Al were chosen. Cores PC4 and PC5 show high concentration levels almost throughout the sediment column and, in order to estimate their background values, the normalized values of nearby cores (PC6, PC11 were applied to the formula: Me=Al$ where Me’ and Al’ are the background concentration levels of the reference cores. Concentration factors were calculated by dividing the peak values of each metal by the relating background level. Concentration factors and background levels are reported in Table 1. Zn is the most important pollutant of all these metals, since its enrichment factor is frequently as high as 13, while the maximum Pb concentration factor is 5, as detected in core PC13 at 18 cm depth. This layer was probably enriched in Pb because of taphocenosis, which is suggested by the presence of shells and sulphur. 4.4. Inventories,

chronology

andfluxes

Inventories were calculated using excess metal concentrations and sediment dry densities. Since inventories are dependent on both concentrations and accumulation rates, they are the most reliable indicators of pollutant accumulation. Trace metals and radionuclide inventories are summarized in Table 2. They substantially confirm the pattern observed in the surficial distribution of pollutants and this could mean that the pollutant accumulation at certain location maintained the same relative importance in time. With reference to the assessment of the relative importance of polluting sources, Basu and Molinaroli

M. Frignani et al. /Marine Table 2 “‘Pb, “‘Cs

and trace metals inventories “OPb (Bq cm-‘)

El E3 E4 E5 E6 E8 Cl0 Cl1 Cl2 Cl3 Cl4 PC1 PC9 PC13 PC4 PC5 PC6 s. marsh s. marsh s. marsh s. marsh

2’op,,/‘37Cs

’ “Cs

Station

’ ’ ’ b

0.20 0.53 0.45 0.22 0.23 0.33 0.28 0.67 0.23 0.40 0.27 0.52 0.55 0.32 0.40 0.22 0.25 0.45 0.30 0.33 0.42

(Bq cm-‘) 0.18 0.17 0.08

Pb ( kmol cm-‘)

1.2 3.2 5.0 4.5 0.8 3.0 4.0 2.0 2.9 2.1 1.1 0.4

0.05 0.42 0.10 0.17 0.12 0.13 0.13 0.45 1.35

Chemistq

> > >

> >

1.1 1.6 1.9 1.4 0.9 2.5 3.7 7.4 6.4 4.2 3.2 2.6 1.4 1.8 2.7 2.6 0.8

of agreement

Campalto A S. Erasmo b Palude di Cona ’ Total

in the sediments

Surface

Cr (pm01 cm-‘)

11.2 22.2 27.5 35.1 8.2 16.1 36.0 74.6 171.5 46.0 89.9 16.6 8.3 10.3 > 48.6 > 44.9 8.8 -

2.1 4.5 8.8 4.8 2.2 5.1 5.8 3.6 7.7 6.9 4.8 5.1 2.5 4.5 > 15.5 > 11.7 1.7

4.2 5.1 6.5 3.9 6.9 5.8 3.5 2.9 3.6 7.4 4.4 2.9 5.4 8.8 > 5.4 > 3.0 2.6

13.8 3.9 4.3 4.2 7.4 13.3 4.2 6.1 3.x 5.3 4.4 6.2 13.4 12.5 > 6.7 > 4.3 3.5

1.2

2.1

12.2

hand, Martin et al. (1994) estimated that metal contributions from the dissolved river input the dissolved phase being dominant - are small compared to the atmospheric input. Atmospheric inputs would account for more than 90% of Fe, Zn, Cd and Pb, 60% of Cu, and 40% of Ni. These considerations are based on rough approximations which, after extrapolation from a limited area, are extended to the

from the northern part of the Venice Lagoon

(km*)

Pb (Mmol)

13.1 25.3 3.1 41.5

0.65 0.40 0.06 1.11

’ Cores ClO, Cll, C12, C13, Cl4 b Cores El, E3, E4, E5, E6, E8. ’ Cores PCI, PC9, PC13.

Ni ( pmol cm-‘)

of the sources

(1994) proposed a model for trace metal distribution based on the assumption that Port0 Marghera is the only source of toxic metals in the lagoon and that their distribution can be described as a function of the distance from this input. Though some model results seem questionable, this work points out the importance of Port0 Marghera as a polluting source and the role of the dispersal system. On the other

Area

Cu ( pm01 cm-‘)

2.2

Table 3 Amount of metals accumulated

Zn ( pm01 cm-‘)

-

0.10 0.12

The ““Pb/“‘Cs ratios are indicators > indicates a minimum estimate. ’ From Battiston et al. (1988). h From Cochran et al. (1995b).

285

58 f 19971275-292

10.97 5.06 0.37 16.40

0.76 1.16 0.13 1.89

Ni (Mmol)

Cr (Mmol)

0.58 1.40 0.17 2.15

0.62 2.04 0.33 2.98

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58 (1997) 275-292

inventories in sediments of the Palude di Cona. Atmosphere supplies to the S. Erasmo area, all Pb but only 43% of Zn while at Campalto the fallout contribution of Pb and Zn accounts only for the 35% and 10% of the inventories, respectively. Cu and Ni

whole lagoon. According to Cochran et al. (1995b,Cochran et al. (submitted), atmospheric fluxes are higher by one or more orders of magnitude than those reported by the previous authors. In particular atmospheric deposition can account for Pb and Zn

Cl1

Cl0 ‘%

(Bq/kg) 10

ws IS

20

(Bqncp) 10

5

0

Cl0

15

20

Cl3 *“‘Pb(B&z)

“‘+b VW&)

E3 “% (Bqlkg) 0

10

20

30

40

50 I

0

10

20

30

‘lo

50

0 10 7.0 30 4l 50 60 70

Fig. 7. Some 13’Cs activity-depth

profiles and the most regular *“Pb profiles.

60

70

M. Frignani et al. /Marine

appear to be derived mostly from local inputs, These views are in agreement with our results, in particular Cu is very concentrated in the sediments of two riverine cores. The inventories were also used to obtain first order estimates of the total amounts of the various polluting metals in sediments. For this purpose the surface areas of the Campalto, S. Erasmo, and Palude di Cona zones were first calculated and came up to 13.1 km’, 25.3 km* and 3.1 km*, respectively. The inventories of each zone were then averaged and multiplied by the relating surface area. The results are reported in Table 3. The amounts of Cu, Ni, and Cr are roughly proportional to the surface areas, while the very high contents of Zn and Pb in the sediments from the Campalto region confirm the importance of the Porto Marghera industrial area as a source of pollution. The resulting inventory for the Palude di Cona area is relatively low, perhaps because two cores out of three do not represent the zones of major pollutant accumulation. Even the inventory of the PC1 site is low because of the slow mass accumulation rate, in spite of the high concentrations of some metals here. The time scale was established by calculating

Table 4 Apparent

sedimentation

rates, mass accumulation

Station

Sedimentation (cm yr’)

El E3 E4 E5 E6 E8 Cl0 Cl1 Cl2 Cl3 Cl4 PC1 PC9 PC13 PC4 PC5 PC6

0.28 0.30 0.47 0.66 0.4 I 0.70 0.45 0.45 0.40 0.58 0.43 0.62 0.48 0.41 0.77 1.16 0.33

rate

CF-CS: Constant Flux-Constant Sedimentation CRS: Constant Rate of Supply of “aPb.

287

accumulation rates from *lOPb and 13’Cs. The *“Pb inventories of salt marsh cores analyzed by Battiston et al. (1988) and Cochran et al. (1995b) vary from 0.30 to 0.45 Bq cm *. Compared to the inventories for subtidal sediments (Table 21, the direct atmospheric input accounts for most *“Pb in sediments. Furthermore, Battiston et al. (1988) found that the ratios of inventories of *“Pb and 13’Cs in the Venice Lagoon are about 3, when both radioisotopes have atmospheric source. They stated that only in this condition a chronology based on radionuclide profiles is reliable. Table 2 shows that some deviations were found, the most important being core PC9, where ‘37Cs activity is rather high. Generally, 13’Cs activity depth profiles are used for the validation of *“Pb chronology (Frignani and Langone, 1991) or the evaluation of the importance of deep mixing (Nittrouer et al., 1983/1984). We would expect two 13’Cs peaks, one corresponding to 1963 (atomic bomb testings) and one to 1986 (the Chernobyl accident). Generally, we found very low activities in the sediments: there is a peak just at the surface in some cases and the 1963 one is difficult to recognize. In other cores two peaks were detected and could be used for dating purposes. The 13’Cs dates for the E4

rates and methods used for their calculation

Mass accumulation (gem-* yr-‘) 0.31 0.32 0.51 0.66 0.54 0.75 0.47 0.64 0.45 0.6 1 0.42 0.44 0.48 0.35 0.41 0.58 0.18

Chemistry 58 (IY97l 275-292

rate

Calculation

methods

Pb profile correlated CF-CS

with the profile of core C 10

137cs

Cu profile correlated CF-CS CF-CS CF-CS

with the profile of core C IO

177(3

CRSM90 CF-CS AlzO, profile correlated with the CF-CS magnetic susceptibility correlated CF-CS magnetic susceptibility correlated magnetic susceptibility correlated CF-CS

profile of core EX with core PCI with core PC I with core PCI

M. Frignani et al. /Marine

288

and Cl 1 cores confirm the 21”Pb dates. Fig. 7 shows some 13’Cs profiles and the most regular *“Pb profiles obtained from lagoon cores. As a single method could not be applied to the whole area, sediment accumulation rates were calculated using different approaches and the most reliable values were chosen based on their consistency with the main variations in trace metal profiles and core correlations (Table 4). Values range between 0.28 cm yr-’ (0.31 g cm-* yr-‘) for core El to 0.77 cm yr-’ (0.75 g cm-’ yr- ‘> for core PC4, with a peak value of 1.16 cm yr- ’ in the Dese river. With reference to sediment mixing, the first observation is that a mixed layer is never clearly visible neither in radionuclide profiles, nor in metal or other profiles. We tried to quantify the mixing rates by using the activity-depth profiles of the short lived nuclide ‘Be (t,,, = 53.4 days) at several stations. Unfortunately, the only site where a ‘Be profile could be found is PCl, because of its input from the river. 7Be concentrations are very low in other cases, and no ‘Be vertical distribution can be obtained. A mixing rate of D, of 3.73 cm’ yr-’ was obtained assuming a negligible effect of accumulation on the ‘Be vertical distribution and applying the formula of Nittrouer et al. (I 983/ 1984): Da = h( z./ln( A,/A;))* where Da is the mixing rate, A the decay constant for 7Be, z the depth in core, A, and AZ the activities on the surface and at depth z, respectively. Using the excess *“Pb activities in to the upper 10 cm and the D, value obtained from ‘Be analyses

Cl1 251 (woW

Chemistry 58 (19971275-292

we were able to calculate the true sediment lation rate by means of the formula: S = AZ/@

&/A,)

- ((Da/z)@

accumu-

Ao/A,))

We found that neglecting biomixing causes an overestimation of around 15% in accumulation rate calculations. This error was considered insignificant compared to the other uncertainties associated to accumulation rate determination. Core chronologies were established mostly using the average accumulation rates (Table 4). The “‘Pb profile of core PC1 appears one of the most regular, and the application of the CRS model, which assumes constant flux of *lOPb (Appleby and Oldfield, 19781, gives reliable results. Because of this it was possible to obtain good estimates of the age at many depths. The previously mentioned carbonate concentration increase, which is related to the construction of the channel between Port0 di Lido and Port0 Marghera, begins about 60 years ago on the basis of these chronologies. Fig. 8 shows the distribution of Zn as a function of time in the three cores closest to the industrial area of Port0 Marghera. Strong increases in Zn, Cu, and Pb concentrations date about 50 years back from core collection and represent the effects of the metallurgical activities which started in the Port0 Marghera industrial area in the mid thirties (Cossu and de Fraja Frangipane, 1985). However, low levels of pollutants in sediments were also found which date back to the beginning of the century. Whether the pollutant contents downcore are really attributable to pollution or to the effects of biomix-

Cl2 Zn (wo@) 15

IO

1990 1970 1950 1930 1910 1890 1870 1850

,990 1970 1950 1930 1910 1890 1870 1850

1990 1970 1950 1930 1910 1890 1870 1850

Date, A. D.

Date, A. D.

Date, A. D.

Fig, 8. Zn concentrations

as a function of time in cores closest to industrial area of Port0 Marghera.

M. Frignani et al. /Marine Pb flux (pm01 cm* yr*)

0 20 0 161

Chemistv

58 (lY971275-292

Zn flux (pm01 cm-2yr1)

IO

_

03,

Date, A. D.

Date, A. D.

D.

Cr flux

Ni flux (pm01 cm-2 yr’)

0.5 -

(pm01 cm* yrI)

05.

04’

0.4 ;

03;

03:

02 I

02. -*-e

01, OOL k, 2000 ,980 I%0

1940

*--= 1920

00I v. -e,w0.1 2MN ,980 I%0

1900

Fig. 9. Metal fluxes calculated

from average accumulation

ing or diffusion remains an open question. Concentrations peaked about 18 years before collection and then decreased. This could represent the consequences of industrial discharge limitations provided for in the water pollution law adopted in 1976. Frignani et al. (1994) described Hg concentration depth profiles at site PC1 for which C. Locatelli (unpublished data) determined also Cd using acid extraction followed by AAS measurements. At this location, Hg increases regularly from the beginning of the century (from 2.0 X 10e3 to 7.5 X lop3 pmol g- ’ 1, whereas Cd shows a very low concentration before 1970 and an abrupt change from 1.8 X lo- 3 to 12.5 X 10m3 pmol gg’ thereafter. This means Mass accumulation (g cm-2 yrI)

rate 0 IO1 008

08?

c‘1940

rates for core Cl2.

Ag. Fluxes can be calculated on the basis of excess metal concentrations in sediment sections and the relative accumulation rates. The results for core Cl2 and PC1 are shown in Figs. 9 and 10, respectively. In the first case metal fluxes were calculated from a single average accumulation rate, whereas the use of a CRS model for core PC1 permitted also the calculation of changes of mass accumulation rates with time. The patterns of these fluxes are similar to those found by Cochran et al. (1995b), with an increase in

Pb flux (Km01 cm-Zyr’)

04 4

-mm-.-j 1960 1940

1980

1920

1900

::’ j’;\ 002 0.00 I -~ __-A_. m0 1980 1960

Date, A. D.

1920

,900

I%0

,940

:

2owl

~_ 1980

0 20

1920

,900

Date, k D.

Fig. 10. Mass accumulation

2wO

l%a

1940

1920

1900

l92C

1900

Date, A. D.

(pm01 cm* yrl)

0 20 T

0 16 1

1980

ci

x-?rL. O2j 1940

Ni flux

(km01 cm-2 yr’)

0161

2(x)(,

-7 fi$

Date, A. D.

cu flux 020,

Zn flux (pm01 cm-z yrl)

05; ox-

02! 04, o.o2ooo

1900

that the input of some metals to the lagoon did not decrease recently and perhaps is still increasing. This is confirmed by Cochran et al. (1995b) for Cd and

061 urm

1920

Date, A. D.

Date, A. D.

10.

r

0‘l:

6 !

Date, A.

cu flux (pm01 cm-Zyrl)

05

8

0121

289

Cr flux (pm01 cm-2yr1)

0.16 +

1980

1960

1940

Date, A. D.

1920

1900

200l

1980

1960

1940

Date, A. D.

rates (from the CRS model) and metal fluxes in core PC1

290

M. Frignani

et d/Marine

Chemistry

58 (19971275-292

Table 5 Metal fluxes to surface sediment ( pmol cm-*

yr- ‘1

Station

Pb

Zll

cu

Ni

Cr

El E3 E4 E5 E6 E8 Cl0 Cl1 Cl2 Cl3 Cl4 PC1 PC9 PC13 PC4 PC5 PC6

0.01 0.01 0.00 0.01 0.01 0.04 0.05 0.10 0.07 0.05 0.06 0.03 0.02 0.02 0.03 0.05 0.01

0.01 0.16 0.00 0.03 0.02 0.73 0.79 1.86 2.75 0.92 2.34 0.41 0.04 0.12 0.56 0.85 0.08

0.01 0.04 0.00 0.03 0.01 0.11 0.08 0.13 0.13 0.12 0.13 0.09 0.02 0.07 0.17 0.24 0.02

0.03 0.02 0.00 0.03 0.07 0.08 0.01 0.04 0.03 0.03 0.03 0.03 0.07 0.01 0.06 0.09 0.02

0.03 0.03 0.08 0.18 0.06 0.07 0.03 0.10 0.03 0.13 0.00 0.04 0.23 0.01 0.12 0.05 0.02

the thirties and a decrease starting from about the mid seventies. This decrease is clear-cut in the sediments directly influenced by the Port0 Marghera industrial area, such as those of core C12, and not so marked in the PC1 core, where perhaps even the very recently delivered sediment preserve some memory of the maximum pollution because of what is stored in the drainage basin of the Dese river. Table 5 reports the metal fluxes to surficial sediments at the time of sampling (1992); they are comparable with the atmospheric fluxes calculated by Cochran et al. (1995b) and from direct deposition (Guerzoni et al., 1995). The contribution of point sources is still very important, although it has decreased recently.

5. Conclusions Our results show that the sediments from the northern part of the Venice Lagoon are heterogeneous in composition and pollutant accumulation. The sediments from the Campalto and S. Erasmo areas have a large silt fraction and high carbonate contents because of the contributions from the Adriatic Sea through the Lido inlet, while the Palude di

Cona area is characterized by the influence of the Dese river which determines high clay contents and a mainly silicoclastic composition. The most polluted sediments are located near the Port0 Marghera industrial area, in the Palude di Cona area and in the Dese river. Some cores present a growth in the carbonate fraction over the last 60 years, which suggests an increasing influence of marine inputs with respect to terrestrial sources. Zn is the major pollutant in the entire area. Zn concentration factors are as much as 13 times the Zn background levels found in cores collected close to the Port0 Marghera industrial area. Maximum concentration factors are 5.2, 2.6, 2.0, and 2.1 for Pb, Cu, Ni and Cr, respectively. Since there are discrepancies between the actual environmental conditions and the modelling assumptions, the results of “‘Pb chronologies are largely uncertain. Therefore, the accumulation rates calculated on the basis of *lOPb and 13’Cs activity-depth profiles had to be validated by means of core correlation and by checking their consistency with other independent information. Apparent accumulation rates vary between 0.31 to 0.75 g cmm2 yr-‘. Inventories of excess trace metals help identifying the sites where metal accumulation occurs. The Zn

M. Frignani et al./Marine

and Pb inventories range from 7.6 to 171 pmol cm-* and from 0.97 to over 7.2 pmol cm-*, respectively, depending on the location. It was possible to calculate the total Pb, Zn, Cu, Ni, and Cr budgets by extrapolating from cores to the whole area. For example, the total anthropogenic Zn stored in sediments from the Campalto, S. Erasmo and Palude di Cona areas is 11.O Mmol, 5.1 Mmol and 0.37 Mmol, respectively. The sediments of most cores recorded a significant increase in anthropogenic metal contamination starting from the second decade of this century, with maximum inputs from 1930 to 1970. The heavy metal concentrations in the surficial sediments from some sampling sites show a decrease which was probably caused by a reduced pollutant input in recent years. However, a similar decline was not found for Hg and Cd which were analyzed in core PCl. The calculated fluxes to surficial sediments, compared to the present time atmospheric delivery reported by Cochran et al. (1995b), show that the atmospheric contribution can account from 10 to nearly 100% of the total excess metal accumulation in the various areas. These results provide the basic knowledge for a quantitative description of the very recent evolution of the lagoon, in terms of sediment composition, degree of pollution, dimension of fluxes, relative importance of sources, and current trends.

Acknowledgements This work was carried out with the financial support of the Project Sistema L.agunare Veneziano. The authors owe thanks to R. Zonta, F. Costa, L. Zaggia, F. Simionato and the other members of the Reparto Misure Ambientali dell’htituto per lo Studio della Dinamica delle Grandi Masse, CNR (Venezia), for providing their scientific and technical support to the selection of sampling sites and operations. M. Ravaioli and G. Marozzi carried out X-ray radiographies; F. Oldfield provided multi-faceted collaborative help in sampling activities and whole-core magnetic susceptibility measurements. S. Lin and E. Lipparini carried out some of the radionuclide analyses and sample preparations, C. Toussaint and M.

Chemistry 58 (1997) 275-292

291

Bianchi the XRF analyses. This is contribution No. 1016 from the Istituto di Geologia Marina, CNR, Bologna, Italy.

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