PCBs and PAHs in sea-surface microlayer and sub-surface water samples of the Venice Lagoon (Italy)
Introduction
The air–water interface plays a key role in the exchange and distribution of persistent organic pollutants (POPs) and has to be taken into account when their environmental fate and budget calculations are investigated (Wania et al., 1998). In the first 50 μm over 20 properties such as the concentration of suspended particles, density, pH value, complexing capacity and so on change sharply (Zhang et al., 2003a); these unique characteristics contribute to an increase in the interfacial effect of the sea microlayer, which features a unique chemical composition (high content of lipids, fatty acids and protein) that leads to its capacity for accumulating hydrophobic organic pollutants, which may be transferred to the air via volatilisation or bubble bursting.
This paper focuses on an assessment of the sea-surface microlayer and sub-surface water concentrations of two classes of POPs: polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs) which in recent decades have attracted the attention of scientific and policy maker communities (Jones and de Voogt, 1999, Lerche et al., 2002, Eljarrat and Barceló, 2003) due to their persistence, their capacity to bioaccumulate in the food chain and their toxic properties. The investigated area is the Venice Lagoon, a shallow and fragile transitional ecosystem which is strongly influenced by human presence and subject to special laws to preserve and improve its environmental quality. To our knowledge, the first study about contamination of the sea-surface microlayer of the Venice lagoon was done by Cleary et al. (2000) who collected water samples in the lagoon and in the Adriatic Sea on May 1993. They found a clear enrichment of metals and organic pollutants (organotins and polyaromatic hydrocarbons) in the lagoon waters with respect to the Adriatic, particularly near the industrial plants of Porto Marghera. Moreover they supposed that tidal currents coupled with the busy traffic and shallow waters can strongly influence its chemical composition, due to resuspension of particulate matter.
The main aims of this study are to provide updated data about PCB and PAH concentrations in sea-surface microlayer and sub-surface waters to complete the picture of the environmental pollution of the Venice Lagoon and to evaluate the role of this thin air–water interface in the dispersion of POPs in a transitional environment by analyzing the sea-surface microlayer composition in relation to the potential sources of these pollutants.
Section snippets
Sampling
Sea-surface microlayer (SML) and sub-surface (SSW) water samples were collected between July 2001 and June 2003 at two different sites in the Venice Lagoon, as depicted in Fig. 1. Both of them are characterized by shallow waters whose chemical composition may be influenced by processes that perturb the physical and chemical partitioning of pollutants, such as the enhanced resuspension from sediments or the prevention of fine-size particle settling. All these processes have to be taken into
Sub-surface water
Median, minimum and maximum concentrations of PCBs and PAHs in the ‘dissolved’ and ‘particulate’ phases of sub-surface water at stations 1 and 2 are listed in Table 2.
The total (sum of ‘dissolved’ and ‘particulate’) concentration of ∑PCBs in sub-surface water at station 1 varies from 0.45 ng/l to 1.5 ng/l and the median value is 0.8 ng/l. The majority of samples show a preferential accumulation in the ‘dissolved’ phase with respect to the ‘particulate’ one, as can also be observed by comparing the
Conclusions
Ten sampling campaigns have been performed to collect sub-surface and sea-surface microlayer water samples in two areas of the Venice Lagoon. The ∑PCB concentrations at both stations are similar in ‘dissolved’ sub-surface and sea-surface microlayer waters while the enrichment found in ‘particulate’ sea-surface microlayer samples is greater by comparison with the sub-surface water, so it can be hypothesised that there must be diffuse sources contributing to PCB contamination of the Venice
Acknowledgements
This work was supported by CORILA (Consortium for Coordination of Research Activities concerning the Venice Lagoon System) under the project ‘Role of aerosol and secondary pollution in the chemical contamination of the Lagoon of Venice’ and by the National Research Council of Italy (CNR).
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