PRELIMINARY ASSESSMENT OF METAL CONTAMINATION IN THE ZADAR BAY (CROATIA)

Roberto Zonta, Luca Zaggia and Flaviano Collavini (National Research Council - I.S.D.G.M., S. Polo 1364, Venezia - 30125, Italy. e-mail: zaggia@isdgm.ve.cnr.it)

ABSTRACT

An investigation on the pollution of Jazine bay sediments (Adriatic Sea, the town of Zadar - Croatia) was conducted in June 1998. Surface sediment samples were collected and analysed for heavy metal concentration (As, Cd, Cr, Cu, Fe, Hg, Mn, Ni, Pb and Zn) as well as grain-size distribution and organic matter content. As a result of the morphology and the slack dynamics of the bay, heavy metals from urban effluents and industrial activities have accumulated in the bottom sediment reaching the highest levels in the more confined sectors. The heavy metal distribution is described and discussed. Given the similarity of the two urban system, the measured concentrations are also compared with those found in the sludge of the Venice canals network (Italy). The investigation represents a preliminary step towards a joint project with the Zadar Municipal Authorities.

INTRODUCTION

The Jazine bay (the town of Zadar - Croatia) is located in the eastern coast of the Adriatic Sea (Figure 1). The complex morphology of this coastal area is the result of the eustatic sea level rise of the Post-Glacial Era and the subsidence induced by tectonic movements. The submersion process combined with the structure of the limestone bedrock give rise to a peculiar topography, characterised by numerous elongated islands and channels parallel to the coastline (Castiglioni, 1982). This pattern is also characterised by the presence of partially closed embayments interfacing with the mainland, which had once been the preferred sites for harbours and the subsequent urban development. The karstic nature of the carbonate terrain prevents the formation of a surface drainage network, giving an arid appearance to the entire area in which only a few rivers are present. By contrast, submarine freshwater springs, many of which exceed the discharge of one cubic meter per second, are disseminated along the shoreline. The studied system is one of the described embay-ments. It has an elongated morpho-logy with a length of approximately 1 km and a width ranging between 100 and 200 m (Figure 1). Its sea-ward opening is partially closed by a mole which limits the water exchanges with the sea by a witdth of less than 50 m. Immediately beneath this artifi-cial enclosure the average depth of the bay is about 12 m, while in its landward half it decreases to about 5 m. The town of Zadar, with approximately 110,000 inhabitants, completely surrounds the Jazine Bay. The historical centre of the town entirely covers the western promontory, while Zadar’s modern urban commercial and industrial districts are primarily concentrated on its eastern border. A considerable amount of urban effluents, mostly from the ancient city centre, are discharged by several outlets located in various positions in the bay’s western margin. Other contamination sources include the harbour traffic and the small industries that, particularly in the past years, have discharged large amount of pollutants. A small stream which discharges on the eastern side of the bay, in a lateral basin occupied by a yacht marina, is the only natural source of freshwater.

As a preliminary step of a joint research project with the Zadar Municipal Authority, an assessment of the contamination levels in sediments of the Jazine Bay was performed in June of 1998. The heavy metal concentrations resulting from this investigation are compared with the average levels found in the sludge from the Venetian canals, which are available from the Venice Municipal Authority’s database (Chiozzotto and Zonta, 1994).

METHODS

The main hydrological characteristics of the water column were surveyed by vertical profiles of temperature, salinity, pH, redox potential and dissolved oxygen, acquired by a multi-parameter probe (Hydrolab H20, USA). Surface sediment samples (0-10 cm) were collected by using a box-corer operated from the surface at six different locations inside the bay area (Figure 1). Materials were homogenised, stored in PE bags and kept at 4 °C until the transfer to the laboratory. An initial aliquot of each sample was mineralised by using HNO₃ and HClO₄ hot digestion. Concentrations of Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn were analysed by Atomic Absorption Spectrophotometry (AAS). A specific HNO₃-H₂SO₄ digestion was performed instead on a second aliquot for AAS determination of As and Hg. Grain-size distribution in both original and organic free samples, after oxidation with H₂O₂ (Zonta et al., 1994) was determined by a laser particle-size analyser. Organic matter content, as loss on ignition at 550 °C (LOI), was also measured in all samples as a control parameter for data interpretation.

RESULTS AND DISCUSSION

Based on the results of the physico-chemical survey, the salinity distribution in the entire bay is quite uniform. However, the vertical profiles do show the existence of a less saline layer at the surface, which extends to the upper first meter of the water column. Besides the minor natural tributary, a detailed investigation on the harbour perimeter also revealed the presence of several sewer outlets distributed along the dock areas. These freshwater outfalls are responsible for the formation of the observed stratification.

A noticeable feature emerging from the results of heavy metal analysis, reported in Table 1, is the marked difference which exists between the metal concentration in the samples collected inside of the bay as opposed to those found in sediments from the external area nearby. Extremely low organic matter contents and heavy metal concentrations were, in fact, measured immediately outside the artificial closure (Site 1), while the levels determined at the other sites were typical of highly polluted environments.

An additional characteristic is the space distribution of the analysed variables within the bay. The organic matter content increases landward, reaching a value of up to 28% in the more confined south-eastern sector (Site 5). A similar trend is demonstrated by a concentration of some anthropogenic heavy metals such as, Zn, Cu, Pb, Ni, Cr, and Cd (Figure 2a) which increases progressively, reaching their highest levels in the inner sector (Sites 4 thru 6). On the other hand, the remaining elements (Figure 2b) are uniformly enriched at the bay’s surface, since they are characterised by a rather constant trend which is particularly evident for As.

Site	Fe	As	Cd	Cr	Mn	Hg	Ni	Pb	Cu	Zn	Org. M.
1	4400	6.8	0.1	21	36	0.47	48	12	9	31	3.6
2	16600	27.9	1.4	49	103	5.33	73	232	218	483	13.8
3	17800	28.0	1.3	56	128	4.67	70	350	353	1082	9.5
4	18900	28.0	3.3	78	99	5.67	96	345	430	980	21.1
5	16800	28.6	4.5	77	104	4.33	89	450	489	1130	27.9
6	17100	27.8	3.9	63	122	4.50	89	510	453	1280	20.6

Table 1. Heavy metal concentrations (mg/kg, d.w.) and organic matter percentage (Org. M.) in the collected samples.

The distribution pattern for the first group of elements is clearly related to the interactions between the morphology of the analysed system and the distribution of pollution sources. The discharge of urban effluents and the slack dynamics of the body of water determine the deposition of large amounts of organic matter in the more confined sectors, where degradation processes induce anoxia. This scenario creates favourable conditions for the processes of heavy metal accumulation by labile reduced sediment phases.

The high resolution provided by the technique adopted for the dimensional analysis, combined with the comparison of distributions in the original and organic-free samples, permit the precise zoning of sediment characteristics, which is essential for the interpretation of chemical data (Zonta et al., 1994). As shown by the results of particle-size analysis in samples collected within the bay area (Sites 2 thru 6, Figures 3 a, b), sediments from the zone closer to the bay entrance (Sites 2 and 3) are characterised by coarser distributions with a prevalence of sand and coarse silt. Conversely, materials from the distal sector (Sites 4 thru 6) are finer and reveal a definite shift of the distribution spectra towards the fine silt-clay fraction upon organic matter elimination (Figure 3b). This shift is determined by the disruption of fine particle aggregates bound by stable organic coatings, and is an expected consequence of the accumulation of organic materials carried by effluents. The prevalence of fine materials, actually present in sediment from this sector and better revealed by the organic free distribution, is the primary reason for the observed landward increase of metal concentrations. The sediment from Site 3, which is mainly constituted by the sand fraction, is the only anomaly in this pattern. Despite the coarser distribution, its metal concentrations are similar to the more polluted area, indicating a local influx from a point source, possibly represented by the commercial dock area nearby.

The information obtained on heavy metal and grain-size distribution show a positive correlation between the pollution and the deposition of fine particles, which is confirmed by the high regression coefficients (even if the small amount of available data is not useful for a significant correlation analysis).

In spite of the different morphologies, the presence of an urban system in a partially confined marine environment and the similarity of pollution sources and processes would suggest a parallel between Zadar Bay and the Venetian canal network crossing the historical centre of the city. As shown by data reported in Table 2, the average concentrations for most of the analysed metals and organic matter in the two systems are surprisingly similar, revealing discrepancies only for the following pairs: Cd, Mn with a concentration ratio of about 0.5, and Cr, Ni with a concentration ratio greater than 2.0.

	Fe	As	Cd	Cr	Mn	Hg	Ni	Pb	Cu	Zn	Org. M.
(a) Jazine Bay	17440	28.1	2.9	65	111	4.9	83	377	389	991	18.6
(b) Venice Canals	18237	20.6	5.5	29	230	4.1	36	216	301	1095	17.2
(a)/(b) Conc. ratio	1.0	1.4	0.5	2.2	0.5	1.2	2.3	1.7	1.3	0.9	1.1

Table 2. Average heavy metal concentrations (mg/kg d.w.) and organic matter percentage in sediments from Jazine Bay and the Venice canal network (from: Chiozzotto and Zonta, 1994). A concentration ratio is also given for each metal.

The results of our investigation in Jazine bay reveal the presence of a spatial zoning of contamination which is determined by the response of the system to sources of pollutants and the morphology of the body of water. Transport and deposition of fine particulate and organic matter in areas of slack dynamics is the primary controlling factor of heavy metal accumulation. Similar problems in the Venetian canal network (direct discharge of effluents from the urban system, commercial/industrial activities, boat traffic, particle settling induced by the slack dynamics) give rise to a close correspondence of contamination levels for most of the analysed species. The bulk of information gathered during the survey represents a storehouse of knowledge which can be useful for future investigations on eventual interventions to improve the environmental quality of the Jazine bay.

REFERENCES

Castiglioni GB (1982), Geomorfologia, Torino, Italy, UTET.

Chiozzotto E and Zonta R (1994), Comune di Venezia Ed.

Zonta R, Zaggia L and Argese E (1994), Sci. Tot. Environ. 151: 19-28.