TRACE ELEMENTS IN THE VENETIAN LAGOON AROUND THE ISLAND OF MURANO (VENICE, ITALY)

Lorenzino Giusti (Dept. of Environmental Sciences, University of the West of England, Bristol, BS16 1QY, UK. Email: Lorenzo.Giusti@uwe.ac.uk); Hao Zhang (Dept. of Environmental Science, University of Lancaster, Lancaster LA1 4YQ, UK).

 

ABSTRACT

An investigation on the trace element distribution in sediments, marine water and mussels in the Venetian Lagoon was carried out. This work focussed on the area around the Island of Murano and is part of an ongoing survey on the impact of the local glassworks on the marine environment. Trace metals analysed included Fe, Mn, Zn, Cu, Cr, Pb, Ni, Ag, and As. In situ measurements of labile species present in marine water were based on the method of diffusion gradients in thin film (DGT). The levels of heavy metals in water and sediments are sufficiently high to cause adverse biological effects. Sediments are mostly contaminated with Zn, Cu, Ag and As. Bioavailable species are significantly accumulated in soft tissue and shells of mussels.

 

INTRODUCTION

The lagoon of Venice is a shallow water basin of about 550 km2 with an average depth of about 60 cm (e.g. Donazzolo et al., 1984). Barrier islands separate the lagoon from the open sea, and water and sediment exchange takes place via three channels. The main source of sediment is the Adriatic Sea via tidal currents. Chemical surveys of the sediments of the entire lagoon of Venice have been carried out previously (e.g. Donazzolo et al., 1984; Albani et al., 1995). Other authors have studied smaller parts of the lagoon. Very few comprehensive studies have been published on trace metals in marine water or biological material. Also, the distribution of elements such as As, Ag, and Cr in water, in sediments, and in marine organisms of the study area have been rarely reported. A preliminary assessment of the environmental impact of the glassworks of Murano on the marine environment was carried out as part of an audit (sponsored by the Gladys Krieble Delmas Foundation) of the waste management practices of the companies operating on this island. Some of the findings of the geochemical study are reported here.

 

MATERIALS AND METHODS

Sediments        Sampling was carried out in the summer of 1999 at ten stations located near the coast of Murano, and from one station at the Lido channel that links the northern part of the lagoon to the Adriatic Sea. Sediment samples (a total of 33 cores, 3 per station) were collected by a diver using a pvc corer. The top 15 cm were processed and analysed. An aliquot of wet sample was wet sieved with de-ionised water mixed with marine water from the Adriatic Sea (50/50 v/v) to separate the < 2 mm fraction. Another aliquot of wet sample was wet sieved to separate gravel (> 2 mm), coarse/medium sand (< 2 mm, > 180 mm), fine sand (< 180 mm, > 63 mm), and pelite (< 63 mm). A third aliquot of dry sediment was ashed at 475 oC in a muffle furnace for 2 hours to estimate the organic matter content as percentage of loss on ignition (% LOI). Aliquots of air-dried sediment fractions were separated, and ground to a fine powder with an agate mortar and pestle; 0.500 + 0.002 g were ashed and digested with aqua regia. All digest solutions were analysed by flame AAS (Varian SpectrAA-10Plus) and by graphite furnace AAS (Varian SpectrAA-300 with GTA96 graphite tube atomiser). Metals analysed included Fe, Mn, Zn, Cu, Cr, Pb, Ni, and Ag. Samples for As (about 0.3 g dry weight) were heated in 5.0 ml of HCl (4M) at 90oC in a water bath for at least 30 minutes. After cooling, the volume was adjusted to 5 ml with Milli-Q water. The extracts were filtered (0.45 mm), diluted (40x), and As determined by ICP-MS (Varian UltraMass).

 

Water  A metal speciation technique named DGT (diffusive gradients in thin-films) (Davison and Zhang, 1994) was used to determine the concentrations of dissolved, labile metal species in water. The DGT technique is based on a simple device that accumulates solutes on a binding agent after passing through a well-defined diffusion layer of hydrogel. The mass of metal ions accumulated in the resin layer was measured by ICP-MS after acid elution in 1 ml HNO3 (1M) for at least 24 hours, and 10 times dilution. Metals analysed included Mn, Zn, Cu, Cr, Pb, Ni, Ag, and As.

 

Mussels   A total of 77 organisms of the common mussel Mytilus galloprovincialis were removed from the wooden pillars (locally called ‘briccole’) located on the edges of the canals around the island.  Most mussels collected were 4.5-5.5 cm long. They were found to be abundant only at 4 of the 11 stations chosen for this work. The hard parts of each organism were analised individually, whereas the dried soft tissue of all organisms from each station was pooled and homogenised. Aliquots of 2 g of soft tissue and 1 g of shell material were dissolved in aqua regia (Giusti et al., 1999) filtered (0.45 mm), and diluted to 100 mL with de-ionised water. Metals analysed included Fe, Mn, Zn, Cu, Cr, Pb, Ni, Ag and As.

The precision and recovery of the procedures for the analyses of sediments and mussels were checked using certified standard reference materials: GBW 08301 stream sediment, GBW 07313 marine sediment, and NIES No. 9 Sargasso seaweed.

 

RESULTS AND DISCUSSION

A summary of the results is given in Table 1.

Sediments        The Lido channel site is characterised by the lowest concentration of pelite (7.7%) and the highest percentage of sand (88%), whereas the average concentration of the pelitic fraction around Murano is 45.4% + 12.4; when shell material > 2 mm is also taken into account, a mean value of 35.3 + 15.8 is obtained, significantly lower than values reported earlier for the lagoon sediments. The pH of the sediment was in the range 7.16-7.64 and the electric potential 8.0-32.1 mV. The mean %LOI of 5.8 indicates a high concentration of organic material.

The pelitic fraction carries the highest load of all heavy metals at most sites except along the southern coast of Murano and at the Lido inlet where fine-grained and coarse sand account for large fractions of Fe, Mn, Zn, Cr, Ni, Ag, and Pb. The interpretation of textural and chemical data is complicated by many factors, such as the large percentages of shells and shell fragments, the extreme depth variation in proximity of artificial canals around the island, and the routine dredging of these canals. These observations undermine the applicability of the data normalisation method based on the % of pelite present in the sample. In general, the metal levels in < 2 mm sediment fraction at the Lido inlet are lower or close to reported background values. The most anomalous data from sites closer to Murano Island include high values of Cu (62-150 mg kg-1), Cr (24-28 mg kg-1), Pb (38-81 mg kg-1), and Zn (69-161 mg kg-1). Arsenic contamination (26.0-38.4 mg kg-1) in the < 2-mm fraction was found at three sites, but As is more associated with the pelite (28.5-110.9 mg kg-1) and the percentage of organic material. The levels of As, Ag, Ni, Pb, Cu, and Zn in the < 2 mm fractions exceed the lower sediment quality guideline value of Long et al. (1995) and Ag always exceeds the maximum guideline value of 3.7 mg kg-1. Adverse biological effects may thus be occurring in all lagoon sediments. Fe and Mn are strongly correlated in the pelite and the fine sand. Both Fe and Mn are significantly correlated with Ni in the pelite and fine sand. Also, Ni, Cr, Pb, and Zn are associated with Fe phases in the coarse/medium sand. In the latter fraction, Cr is also associated with Mn.

 
Mussels   The range of values of trace elements found in soft tissue of Mytilus galloprovincialis are broadly in agreement with those found in the Venetian Lagoon by Zatta et al. (1992) exception made for higher Cu (up to 35 mg kg-1 dry wt) and As (up to 18 mg kg-1 dry wt) burdens. Silver concentrations (up to 6.2 mg kg-1 dry wt) are comparable to those found in other species of Mytilus elsewhere in the world at sites contaminated with sewage. No Ag data for Mediterranean locations were available to the authors for comparative purposes. Most trace elements are more enriched in the soft tissue than in the hard part of the mussels analysed. This applies particularly to Fe and Zn, and less so to Cu, Mn, and Pb. Bioaccumulation of Cr and Ag in the shell is more pronounced than in the soft tissue.

 

Marine water   Trace element concentrations measured by DGT are significantly higher than those typically found in the open ocean (Burton and Statham, 1990) and more in line with reported values from contaminated sites. This applies especially to the data for Zn, Cu, Ni, Cr. These metals are more enriched than in other areas of the Mediterranean previously studied (e.g. Voutsinou-Taliadouri et al., 1997, and ref. therein). Dissolved concentrations of Zn are the highest of all elements analised (up to about 35 μg L-1). The relatively high Mn levels (up to about 8 μg L-1) may be associated with the reduced salinity of lagoon waters and with increased benthic fluxes of Mn (II) species during the sampling period. Levels of dissolved As were relatively high (up to 4.73 μg L-1) at 4 of the sites monitored. Our water quality data refer to a monitoring period of 16-24 hours during the summer period. As trace element concentrations are affected by seasonal factors, a long-term study of their biogeochemical cycles is required to draw firm conclusions.

 

REFERENCE LIST

Albani AD, Rickwood PC, Favero VM, Sarandrei Barbero R (1995), Estratto dagli Atti dell’Istituto Veneto di Scienze, Lettere ed Arti 153: 235-321.

Burton JD, Statham PJ (1990), In: Heavy Metals in the Marine Environ. (R.W. Furness and P.S. Rainbow, Editors), Boca Raton, Florida, CRC Press Inc., pp. 5-25.

Davison W, Zhang H (1994), In situ speciation measurements of trace components in natural waters using thin-film gels, Nature 367: 546-548.

Donazzolo R, Orio AA, Pavoni B, Perin G (1984), Heavy metals in sediments of the Venice Lagoon, Oceanol. Acta 7: 25-32.

Giusti L, Williamson A C, Mistry A (1999), Biologically available trace metals in Mytilus edulis from the coast of northeast England, Environ. Intern. 25: 969-981.

Long ER, MacDonald D.D, Smith SL, Calder, FD (1995), Incidence of Adverse biological effects within ranges of chemical concentrations in marine and estuarine sediments, Environ. Manag. 19: 81-97.

Zatta P, Gobbo S, Rocco P, Perazzolo M, Favarato M (1992), Evaluation of heavy metal pollution in the Venetian lagoon by using Mytilus galloprovincialis as biological indicator, Sci. Tot. Environ. 119: 29-41.

Voutsinou-Taliadouri F, Varnavas SP, Nakopoulou C, Moriki A (1997), Dissolved trace elements in south Aegean seawater, Mar. Poll. Bull. 34: 840-843.

 

 

Table 1. Summary of trace element concentrations in water (μg L-1), mussel Mytilus galloprovincialis (in mg kg-1 dry wt.), and sediments

(in mg kg-1, except for Fe, expressed as wt. %) of the Venetian Lagoon around Murano.

 

Matrix

Fe

Mn

Zn

Cu

Cr

Pb

Ni

Ag

As

WATER

 

 

 

 

 

 

 

 

 

mean

 

5.28

21.3

2.35

5.07

0.26

3.39

 

2.40

sd

 

1.14

10.1

0.70

1.00

1.14

0.81

 

1.32

range

 

3.98-8.02

4.98-34.6

1.38-4.08

3.90-6.33

0.10-0.59

2.18-4.59

 

0.99-4.73

 

 

 

 

 

 

 

 

 

 

MUSSELS

 

 

 

 

 

 

 

 

 

Soft tissue

 

 

 

 

 

 

 

 

 

mean

381.5

16.3

164.5

22.3

2.3

3.2

2.1

5.0

14.6

sd

85.1

2.0

67.0

9.1

0.2

0.7

0.5

1.1

2.9

range

260.0-458.6

14.1-18.6

100.2-241.6

15.8-35.5

2.0-2.4

2.5-3.9

1.6-2.6

3.5-6.2

12.0-18.0

Shell

 

 

 

 

 

 

 

 

 

mean

71.5

7.2

7.3

13.8

30.6

1.3

1.5

9.5

 

sd

55.3

2.3

6.2

7.9

1.9

0.5

0.5

5.4

 

range

20.1-149.7

4.4-9.8

2.9-16.4

8.0-25.5

29.1-33.3

0.9-2.0

1.0-2.2

4.2-16.9

 

 

 

 

 

 

 

 

 

 

 

SEDIMENT

 

 

 

 

 

 

 

 

 

< 2 mm

 

 

 

 

 

 

 

 

 

mean

1.31

268.4

114.0

69.5

22.9

34.8

13.2

9.7

11.9

sd

0.13

31.2

32.6

43.7

3.9

18.9

1.5

1.7

13.5

range

1.13-1.47

200.0-305.0

55.9-161.5

27.7-149.9

17.3-28.1

10.0-80.6

10.5-15.7

7.3-12.4

1.30-38.4

< 63 μm

 

 

 

 

 

 

 

 

 

mean

1.50

310.5

124.7

76.8

22.0

38.2

15.7

8.3

55.7

sd

0.08

22.6

25.5

35.2

2.6

11.0

1.9

1.3

36.4

range

1.35-1.62

279.0-348.1

90.9-163.1

41.5-124.7

17.2-24.9

16.0-48.3

13.3-19.7

6.7-11.2

2.0-110.9

sd = standard deviation