Partitioning and mixing behaviour of trace elements at the Isonzo/Soča River mouth (Gulf of Trieste, northern Adriatic Sea)
Introduction
Estuaries and river mouths are crucial environments for the transfer of chemicals from land to open sea (Fu et al., 2013; de Souza Machado et al., 2016; Wang and Wang, 2016; Fiket et al., 2018). In this context, mixing between river freshwater and marine saltwater represents a key process for the regulation of the fate and distribution of contaminants, among which trace elements are of major concern (Yiǧiterhan and Murray, 2008; Deycard et al., 2014; Domingos et al., 2015). Indeed, estuaries generally show strong physico-chemical gradients (salinity, density, flow velocity) which may affect trace element speciation and their partitioning between solid and dissolved phases (Savenko et al., 2014). In addition, organic matter production and oxidation (de Souza Machado et al., 2016), the oxygenation state and potential release of trace elements from the sediment-water interface (SWI) (Petranich et al., 2018 and references therein) are also influential factors.
Organic and inorganic matter in natural aquatic systems can be associated with three main fractions: suspended particulate matter (SPM), colloids and the truly dissolved fraction. By convention, particulate matter in suspension is defined as the material retained on a filter with a nominal porosity of between 0.4 and 0.5 μm. The upper dimensional limit of the particles is not well-defined, although Visher (1969) proposed a value of 100 μm, which is the limit between the particles that move by saltation under a laminar flow regime and those transported in suspension. Colloids are generally defined as submicron particles enclosed within the traditionally defined dissolved fraction (<0.45 μm) (Guo and Santschi, 1997).
The role of SPM as the main carrier for trace elements in estuarine and river mouth environments has been investigated in several studies (e.g. Turner and Millward, 2000; Cenci and Martin, 2004; Fu et al., 2013). As a consequence of SPM settling, bottom sediments represent an important sink for contaminants associated with fine particles. However, bottom sediments may act as a secondary source when processes such as bioturbation and resuspension are responsible for the release of trace elements back to the water column (Sigg et al., 2000).
Trace elements bound to the colloidal material may have a different fate. Indeed, very small particles can settle extremely slowly or be trapped in the commonly defined maximum turbidity zone or may eventually form larger aggregates (Guo and Santschi, 2007). Colloidal material can also influence the transport, bioavailability and fate of trace elements and other contaminants in aquatic systems (Benoit et al., 1994; Guentzel et al., 1996). In addition, colloids represent essential intermediaries in several removal processes such as coagulation and flocculation (Savenko et al., 2014) that often take place in estuarine environments where strong physico-chemical gradients occur, especially in terms of salinity.
The Isonzo River represents the primary freshwater input entering the Gulf of Trieste (northern Adriatic Sea) and it is known to be the main source of Hg (Covelli et al., 2001; Covelli et al., 2007) due to long-term cinnabar (HgS) extraction from the Idrija mine, located in the Slovenian sector of the river drainage basin. During mining operations, mineralised tailings generated by HgS roasting processes were dumped along the banks of the Idrijca River, swept away by floodwaters towards the Isonzo River and finally transported into the Gulf of Trieste. According to Baptista-Salazar et al. (2017), the amount of Hg in the soil ranged between 0.08 and 46.3 μg g−1 and the maximum concentrations were found in the vicinity of the mining district, most likely due to the presence of ore tailings. Furthermore, the same authors also stated that the amount of Hg associated with the SPM in the Idrijca River freshwater is significantly affected by hydrological conditions. Indeed, particulate Hg ranged between 4.13 and 37.2 μg g−1 during rain events whereas significant variations of dissolved Hg were not detected during low and high flow conditions (Baptista-Salazar et al., 2017).
In addition, along the Isonzo River drainage basin, the decommissioned PbZn mining district of Raibl (Cave del Predil village, northern Italy) represents a source of Zn, Pb and other trace elements since the major drainage of the mine discharges directly into the Isonzo River in Slovenia (Petrini et al., 2016). Waste water discharge from a cement factory (near Kanal, Slovenia) as well as rainwater runoff from cropping areas of the Isonzo River alluvial plain (Comici and Bussani, 2007; Cozzi et al., 2012) may further contribute to increase the amount of dissolved and particulate trace elements in the river freshwater.
The behaviour of Hg in the Gulf of Trieste has been intensively studied with regard to Hg contamination in coastal sediments (Covelli et al., 2001), transport and dispersion of particulate Hg (Covelli et al., 2007) and Hg cycling at the SWI (Emili et al., 2011). With the only exception being Hg, the occurrence and distribution of other trace elements and their partitioning between solid and dissolved phases have not yet been thoroughly addressed in the eastern sector of the northern Adriatic Sea, as well as the Gulf of Trieste. The aim of the present work is to assess trace element occurrence and partitioning behaviour among SPM, colloidal and truly dissolved fractions in estuarine water at the mouth of the Isonzo/Soča River, where a strong salinity gradient occurs. The behaviour of trace elements was investigated in the very high salinity range under different seasonal conditions at the river mouth and further insight regarding partitioning was provided by the inclusion of the colloidal fraction.
Section snippets
Environmental setting
The Gulf of Trieste is a semi-enclosed marine basin located in the north-eastern Adriatic Sea (Italy). The water depth is generally shallow, reaching a maximum depth of 25 m in the central area of the Gulf where the Isonzo/Soča River represents the main freshwater input (Fig. 1). The average annual flow rate at the river mouth is assessed to be about 82 m3 s−1 in the period from 1998 to 2008 (Cozzi et al., 2012). The Isonzo River mouth can be defined as a microtidal, low-energy and fine-grained
Physico-chemical characterisation of the water column
River discharge was found to be notably higher in winter (275 m3 s−1) and autumn (230 m3 s−1) rather than in summer (20 m3 s−1). Two distinct layers in terms of salinity were detected in the water column at the river mouth, as a consequence of the interaction between the freshwater input and the salt-wedge intrusion (Fig. 2). Indeed, salinity vertical profiles displayed a sharp halocline, as previously reported by Covelli et al. (2004). The surface water layer showed salinity values typical of
Conclusions
At the Isonzo/Soča River mouth, trace elements appear to be mainly associated with the suspended particulate matter, thus confirming its crucial role in the transfer of contaminants from land to coastal and marine environments. Dilution processes between riverine and marine particles occurring at the river mouth are responsible for the decrease in the particulate trace element concentrations along the water column. However, significant levels of particulate Mn were detected in the marine water
Acknowledgements
This research activity was partially funded by Operational Programme 2014/2020 of the European Social Fund (ESF) of the Friuli Venezia Giulia Region (code FP1685623003). We are grateful to Stefano Sponza for his invaluable support during sampling. Special thanks to Karry Close for proofreading the manuscript. The anonymous reviewers are greatly acknowledged for their critical reviews and useful suggestions, which improved the quality of the manuscript.
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