Organic pollutants in sea-surface microlayer and aerosol in the coastal environment of Leghorn—(Tyrrhenian Sea)
Introduction
During the last decades many investigations have been conducted on the physical properties and chemical composition of the sea-surface microlayer (SML) as well as on exchange processes at the air/sea interface GESAMP, 1995, Liss and Duce, 1997, CIESM Workshop, 1999. These studies have highlighted that a wide variety of natural and anthropogenic contaminants concentrate in the SML because of their surfactant nature, hydrophobic properties, possible association with floatable particles, vertical diffusion mechanisms or bubbles scavenging. The transfer of such materials from the sea-surface to the atmosphere may take place through the volatilisation due to their high vapour tension, the stripping by the bubbles and the formation of wind-generated aqueous aerosols Wallace and Duce, 1978, Hardy, 1982, Buat-Ménard, 1983, Blanchard, 1989, Cini et al., 1994a, Lepri et al., 1995, Gustafson and Dickhut, 1997. In fact, when air bubbles burst at the sea-surface they generate an aerosol which is contaminated and enriched with material scavenged from the sea-surface microlayer and the water column Blanchard, 1975, Tseng et al., 1992. Several studies Blanchard, 1983, Liss and Duce, 1997 have shown that bacteria and viruses in aerosol particles have concentrations significantly higher than those obtained in the bulk-water from which they were generated. Analogous results were found for lipids Gagosian, 1986, Simò et al., 1991 and surfactants Blanchard, 1964, Garrett, 1967 transported via marine aerosol to the coastal regions where hazardous effects on animals and vegetation Giovannelli et al., 1989, Bussotti et al., 1995, together with toxic effects on human health (WHO, 1998), have been observed. In addition, high concentrations of toxic metals Hoffmann et al., 1974, Hunter, 1980, Hardy et al., 1985 and organic pollutants Williams et al., 1986, Hardy et al., 1987, Cross et al., 1987, Sauer et al., 1989, Hardy et al., 1990 have been found in the SML compared to the sub-surface water (SSL) throughout various coastal environments. However, a few investigations have been carried out on the characterisation of organic pollutants in the SML of Mediterranean waters (CIESM Workshop, 1999).
To this end, aliphatic and aromatic hydrocarbons and phthalates have been determined in SML, SSL and aerosol samples collected in the Tyrrhenian Sea near Leghorn. Size fractionated aerosol samples were collected to provide more information on air–sea processes, as well as verify the validity of Spray Drop Adsorption Model (SDAM) (Oppo et al., 1999). Moreover, the compounds measured were particularly useful for evaluating the degree of marine pollution because alkylbenzenes, phthalates and PAHs enter the marine environment only by anthropogenic sources, while n-alkanes may have biogenic and/or anthropogenic origin.
Section snippets
SML and SSL
SML and SSL samples were collected between September and October 1999 at nine different sites in the Tyrrhenian Sea, near Leghorn and Gorgona island. Table 1 gives some information about the sampling sites including location, water depth, distance from the coast, temperature and pH.
Our sampling strategy included stations in areas believed to receive different amounts of pollutants in order to have a representative picture of sea surface contamination in the investigated zone (Fig. 1). SML and
SML and SSL samples
Table 2 reports the mean concentrations, expressed in μg/l, of individual n-alkanes, phthalates, PAHs and alkylbenzenes found in the SML samples and in their related underlying water collected at Quercianella (station 3). Such compounds represent almost all the organic substances extractable with n-hexane or n-hexane-methylene chloride (1:1 v/v).
Dissolved n-alkanes in SML exhibit a different distribution with respect to the particulate hydrocarbons, which reflects the probable loss of low
SML and SSL
Aliphatic hydrocarbons, alkylbenzenes, PAHs, phthalates, anionic surfactants and SFOM were found to be present in the surface film and the underlying water of all the samples. Hydrocarbons concentrate in the particulate phase according to their hydrophobic properties with the exception of alkylbenzenes which have been only detected in the dissolved phase at very low amounts. This behaviour seems to be general and not connected to the pollution level of the investigated sampling site, and
Concluding remarks
The results give a picture of aquatic surface contamination in a coastal zone of the Tyrrhenian sea, near Leghorn, during September–October 1999. Signals of pollution from chemicals of petrogenic origin are evident in SML samples collected in the harbour of Leghorn and at various distances from the Port.
The SML is enriched with n-alkanes, phthalates, PAHs, suspended matter and SFOM with respect to sub-surface waters. The presence of slicks gives rise to unusual dissolved concentrations of these
Acknowledgements
This study was supported by the University of Florence through the National Project “The development of innovative analytical methods for the study of environmental matrices”.
Many thanks to the Director of the Inter University Center of Marine Biology and of the Municipal Aquarium of Leghorn for their hospitality and logistic assistance. We greatly thank Prof. R. Udisti for sodium ion analysis.
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