Elsevier

Chemosphere

Volume 51, Issue 7, May 2003, Pages 603-616
Chemosphere

Estimation of PCDD/F distribution and fluxes in the Venice Lagoon, Italy: combining measurement and modelling approaches

https://doi.org/10.1016/S0045-6535(03)00048-1Get rights and content

Abstract

The available experimental information on the occurrence of PCDD/Fs in the Venice Lagoon, Italy, was compiled and used to calculate fugacities for the environmental compartments of sediment, suspended particulate matter (SPM), water and air and then used to estimate fugacity ratios and assess the likely net direction of flux between media. The bottom sediment: SPM fugacity ratios for different PCDD/Fs indicate conditions close to equilibrium, suggestive of the close coupling of SPM with re-suspended sediment. Sediment/water and the sediment/air fugacity ratios suggest that net flux directions vary depending on the congener and the location within the lagoon. Net sediment–water–air movement (i.e. re-mobilisation/volatilisation) is suggested for the lighter congeners from the industrial canals, where the highest PCDD/F concentrations in the lagoon occur. The tendency to volatilise increases with decreasing congener molecular weight. In contrast, net deposition (air–water–sediment) appears to be occurring for the heaviest (hepta- and octa-) substituted PCDD/Fs. OCDF represents a marker of the industrial district of the lagoon, decreasing in concentration and as a fraction of total PCDD/Fs with increasing distance. The fugacity-based quantitative water air sediment interaction (QWASI) mass-balance model was applied to the central part of the lagoon. The key parameters for the determination of the model output, identified by a sensitivity analysis, were: the sediment active depth, the sediment re-suspension and deposition rates, and the total input of PCDD/Fs to the system. The QWASI model also indicates the tendency for the lighter PCDD/Fs to be released from surface sediment to the water column.

Introduction

The contamination of the Venice Lagoon, particularly with respect to the polychlorinated dibenzo-p-dioxins and -furans (PCDD/Fs) and other persistent organic pollutants (POPs) has received much attention over the last decade. The lagoon has a long history of industrial activity and from the 1950s the inner lagoon area of Porto Marghera expanded as chemical and oil refining plants were developed. During the last two decades, the decline of the chemical industry and the adoption of new technologies for emission control and abatement significantly reduced the aqueous and gaseous emissions from the industries to this area. However, high concentrations of PCDD/Fs, polychlorinated biphenyls, polycyclic aromatic hydrocarbons and heavy metals still affect sediments, especially in the central lagoon, providing a potential source for the lagoon ecosystem and for human exposure, through the ingestion of contaminated seafood. Other sources are the atmospheric deposition, the discharge of untreated municipal effluents from the historical centre and the emissions associated with heavy boat traffic due to local transportation, fishing activities, cruisers and oil tankers. High PCDD/F concentrations were recently recorded in sediments of the industrial canals (Bellucci et al., 2000; Magistrato alle Acque di Venezia, 2000) that serve the Porto Marghera district. The lagoon received untreated municipal and industrial effluents until the end of the 1960s; even today municipal effluents from the historical centre are largely untreated.

The annual consumption of fish and shellfish by Venetians (much of it cultivated/caught from the lagoon) is approximately twice the average consumption recorded in Italy (Zanotto et al., 1999). As a result, high contaminant levels and high consumption of local product have led to public health concerns and a desire to better understand pollutant transfers to the lagoon biota. In order to gain information about the extent of this contamination, extensive sampling campaigns have been conducted in recent years, especially in the central part of the lagoon. Surface sediment, water, suspended particulate matter (SPM) and biota (clams, mussels, crabs and fishes) have all been analysed and physico-chemical data and environmental parameters have been recorded (i.e. temperature, pH, organic carbon fraction in sediment and particulate matter, particulate matter concentration and fluxes; Sfriso et al., 2000). The contaminant contribution of rivers from the catchment area and of atmospheric deposition were also investigated (Bettiol et al., 2001; Rossini et al., 2001). On-going PCDD/F emissions are now much lower than in the past (Marcomini et al., 1999a), but contaminated sediment of the industrial canals and surrounding areas can potentially act as a significant long-term secondary source of PCDD/F as it becomes transported and re-distributed. High re-suspension of bottom sediments is caused by intensive fishing for clams (Tapes philippinarum), shipping activity and tidal currents. These processes favour high sediment–water exchange, whilst the shallow warm waters of the lagoon encourage high biological productivity and air–water exchange.

There are several publications addressing the occurrence and distribution of POPs in the Venice Lagoon (Fattore et al., 1997; Marcomini and Della Sala, 1997; Marcomini et al., 1997, Marcomini et al., 1999a, Marcomini et al., 1999b; Green et al., 1999a; Bellucci et al., 2000; Wenning et al., 2000; Frignani et al., 2001a, Frignani et al., 2001b). However, there are few studies examining the data together and applying modelling approaches to gain insights into the key processes governing the fate of the chemicals in the lagoon. Multi-media environmental fate models, based on the fugacity concept, have been satisfactorily developed and applied elsewhere (Booty and Wong, 1996; Freitas et al., 1997; Suzuki et al., 1998, Suzuki et al., 2000; Connolly et al., 2000; Woodfine et al., 2000; Miller et al., 2001). This approach allows the equilibrium status and net flux direction to be assessed, given information on the environmental conditions and physico-chemical properties of the compounds of interest (Mackay, 2001).

In this study the central lagoon was chosen to evaluate equilibrium status and to develop a mass-balance model to predict the steady state concentration levels and fluxes of individual PCDD/F congeners for key environmental compartments. The quantitative water air sediment interaction (QWASI) model was used, as it allows––once environmental parameters (e.g. total water surface, mean depth, organic carbon content in sediment), physical–chemical characteristics (such as vapour pressure, log(Kow) and the Henry’s law constant), inputs and advective flows are known––to predict concentration values in all environmental media (Mackay et al., 1983). The QWASI model has been successfully applied to Lake Ontario (Mackay, 1989) and has been used widely, being applicable to water bodies with a known water and suspended solids budget.

The objectives of the paper were therefore to quantify the intermedia fluxes of PCDD/Fs given the current present inputs to the lagoon and to predict the steady-state concentrations of these chemicals in the environmental compartments of sediment, water, air, water and air particles. In order to do this we needed to: collect and summarise the experimental data on PCDD/Fs available for the Venice Lagoon; compile environmental budgets for PCDD/Fs for the area; estimate the contemporary flux direction for each congener; obtain the information necessary to develop a steady state mass-balance model.

The Venice Lagoon, located in Northeast Italy, has a half-moon shape and total surface of 550 km2. The water surface open to the tidal exchange is 430 km2 with an average depth of about 1 m and a mean tidal excursion of approximately ±30 cm. It is subdivided in three main basins by three entrance channels allowing water exchange with the Adriatic Sea (111×109 m3/y, i.e. 3.1–4.5×108 m3 per tidal cycle), while the average freshwater inflow is about 35 m3/s. About 1 400 000 people live in the water basin area, i.e. 1970 km2 wide, corresponding to 4 000 000 inhabitant equivalents if industrial and agricultural activities are taken into account (Regione Veneto, 1998). The prevailing wind direction is NE–SW and water temperature ranges from 0 to 30 °C.

Due to low depth, high water exchange and scarce macrophytic coverage compared with the past (Sfriso et al., 2001), the central part of the lagoon is now a rather homogeneous system, with uniform physical–chemical characteristics. In addition, the intense (and currently illegal) fishing activity for clams (Tapes philippinarum) represents a serious threat for the lagoon ecosystem causing a significant increase in sediment re-suspension and favouring the re-circulation of particulate matter and contaminants. The central part of the lagoon, between the Malamocco and Lido channels linking the lagoon to the Adriatic Sea, covers a water surface of approximately 132 km2 and is quite well mixed hydrologically. Four sites in this area were the focus of investigation, selected to represent contamination gradients. These are (Fig. 1):

  • Site A (Alberoni) This site (12°18910E, 45°21180N) is located close to the Malamocco channel, where the tidal exchange with the Adriatic is high.

  • Site B (Sacca Sessola) (12°18512E, 45°24372N) in the middle of the central lagoon, close to the island of Sacca Sessola.

  • Site C (San Giuliano) This site (12°17532E, 45°27735N) is close to the trans-lagoon bridge and the mouth of the Osellino Canal.

  • Site D (Fusina) (12°16075E, 45°24936N) near the industrial area of Porto Marghera, by the western industrial channel. It is also affected by the Naviglio-Brenta river and the proximity to a large power station.

Section snippets

Data analysis

Data on sediments, SPM, water, riverine inputs and atmospheric deposition were obtained from recent investigations (1998–2000) undertaken by the Water Authority of Venice (Magistrato alle Acque di Venezia and its concessionaire Consorzio Venezia Nuova). The sampling campaigns for SPM, water and riverine inputs were restricted to the central part of the lagoon, while surficial sediments were sampled across the whole system.

The analytical procedures for the determination of PCDD/Fs used high

Methods

A convenient way to estimate the net flux direction is to compare fugacity values of a chemical among the different compartments (Mackay, 2001). The higher the fugacity, the higher the tendency of the chemical to transfer to another phase. Fugacity is related to concentration by a linear relationship:C=Zfwhere C=concentration (mol/m3), Z=fugacity capacity (mol/m3 Pa), f=fugacity (Pa).

Z is a proportionality constant, dependent on temperature, on the nature of the compartment and on the

Results and discussion

Sediment-SPM fugacity ratio. The sediment-SPM fugacity ratio (Fig. 4a) was obtained using values calculated for PCDD/Fs at the four sampling stations of the central lagoon for matched samples of sediment and SPM. The fugacity ratio was always in the interval 0.5–1.5, suggesting a close coupling between sediment and SPM. Slight differences in the ratios were noted between sites, with site A ∼0.9, site B ∼1.1 and sites C and D ∼1.3. A likely explanation for this trend is that site A receives SPM

Conclusions

The Venice Lagoon is a complex ecosystem with localised direct (riverine, industrial area and historical centre) and secondary (contaminated sediment) sources. Only limited areas such as the industrial canals have high PCDD/F contamination.

Although predictions of future PCDD/F trends is difficult, it is possible, given an emission scenario, to estimate the concentration values in the different media with a steady-state mass-balance model, such as the fugacity based QWASI model. The advantages

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