Elsevier

Marine Chemistry

Volume 223, 20 June 2020, 103800
Marine Chemistry

Partitioning and mixing behaviour of trace elements at the Isonzo/Soča River mouth (Gulf of Trieste, northern Adriatic Sea)

https://doi.org/10.1016/j.marchem.2020.103800Get rights and content

Highlights

  • Trace element occurrence and partitioning were evaluated at the Isonzo River mouth.

  • Dilution processes significantly influenced particulate trace element distribution.

  • Fe, Mn and Cu behave non-conservatively due to precipitation/flocculation processes.

  • As and Cs were almost completely partitioned in the dissolved fraction.

  • Colloids play a minor role in trace element partitioning among different fractions.

Abstract

Estuaries are unique water systems and represent a vital link between land and sea. River transported contaminants, among which trace elements are of major concern, are subjected to a variety of physical, chemical and biogeochemical processes in the estuarine mixing zone.

The Isonzo/Soča River is the main source of freshwater into the Gulf of Trieste (northern Adriatic Sea) and is known as the primary source of Hg due to long-term cinnabar (HgS) extraction from the Idrija mining district (Slovenia). Mercury distribution, cycling and speciation have been intensively studied at the Isonzo/Soča River mouth. Still, little information is currently available regarding other trace elements (As, Co, Cu, Cs, Cr, Fe, Ni, Mn, Pb and Zn). Indeed, this research aims to evaluate trace element occurrence and partitioning behaviour among suspended particulate matter (>0.45 μm), colloidal material (0.45 μm – 10 kDa) and the truly dissolved fraction (<10 kDa) at the mouth of the Isonzo/Soča River.

Generally, trace elements are mainly associated with suspended particulate matter, which represents their main effective vehicle to coastal environments. In addition, dilution effects between riverine and marine particles are responsible for the decrease in particulate trace element concentrations along the water column. Mercury was notably present in winter in the freshwater, as expected under conditions of high river discharge. As opposed to other trace elements, particulate Mn was found to be high in the marine water, particularly in summer when high water temperatures could promote oxidation and precipitation processes. The Isonzo/Soča River mouth is characterised by a strong salinity gradient and geochemical processes appear to affect trace element partitioning behaviour. In this context, Fe, Mn and Cu were found to be mainly associated with the suspended particles and displayed the highest concentration in the colloidal material. Conversely, the truly dissolved fraction prevailed for As and Cs, which are often present in ionic dissolved forms in natural water systems.

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 Pbsingle bondZn 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.

References (73)

  • A. Emili et al.

    Does anoxia affect mercury cycling at the sediment-water interface in the Gulf of Trieste (northern Adriatic Sea)? Incubation experiments using benthic flux chambers

    Appl. Geochem.

    (2011)
  • C. Fajon et al.

    The accumulation and release of polysaccharides by planktonic cells and the subsequent bacterial response during a controlled experiment

    FEMS Microbiol. Ecol.

    (1999)
  • F. Floreani et al.

    Diurnal fluxes of gaseous elemental mercury from the water-air interface in coastal environments of the northern Adriatic Sea

    Sci. Total Environ.

    (2019)
  • G.E. Fogg

    Some speculations on the nature of the pelagic mucilage community of the northern Adriatic Sea

    Sci. Total Environ.

    (1995)
  • J. Fu et al.

    Estuarine modification of dissolved and particulate trace metals in major rivers of East-Hainan, China

    Cont. Shelf Res.

    (2013)
  • C. Gagnon et al.

    Distribution and fate of metals in the dispersion plume of a major municipal effluent

    Environ. Pollut.

    (2003)
  • A.G. González et al.

    Redox interactions of Fe and cu in seawater

    Mar. Chem.

    (2016)
  • C. Guéguen et al.

    Partitioning of trace metals between particulate, colloidal and truly dissolved fractions in a polluted river: the upper Vistula River (Poland)

    Appl. Geochem.

    (2003)
  • J.L. Guentzel et al.

    Mercury associated with colloidal material in an estuarine and an open- ocean environment

    Mar. Chem.

    (1996)
  • V. Hatje et al.

    Spatial and temporal variability of particulate trace metals in port Jackson estuary, Australia

    Estuar. Coast. Shelf Sci.

    (2001)
  • M.A. Helali et al.

    Suspended particulate matter fluxes along with their associated metals, organic matter and carbonates in a coastal Mediterranean area affected by mining activities

    Mar. Pollut. Bull.

    (2016)
  • S. Hong et al.

    Arsenic speciation in environmental multimedia samples from the Youngsan River Estuary, Korea: a comparison between freshwater and saltwater

    Environ. Pollut.

    (2018)
  • M.J. Hopwood et al.

    Dissolved iron(II) ligands in river and estuarine water

    Mar. Chem.

    (2015)
  • J. Ingri et al.

    Size distribution of colloidal trace metals and organic carbon during a coastal bloom in the Baltic Sea

    Mar. Chem.

    (2004)
  • M. Juračić et al.

    The role of suspended matter in the biogeochemical cycles in the Adige River estuary (Northern Adriatic Sea)

    Estuar. Coast. Shelf Sci.

    (1987)
  • N. Koron et al.

    Association of macroaggregates and metals in coastal waters

    Mar. Chem.

    (2013)
  • A.M.L. Kraepiel et al.

    Geochemistry of trace metals in the Gironde estuary

    Geochim. Cosmochim. Ac.

    (1997)
  • N.S. La Colla et al.

    Dissolved and particulate metals dynamics in a human impacted estuary from the SW Atlantic

    Estuar. Coast. Shelf Sci.

    (2015)
  • A.W. Morris et al.

    Chemical variability in the Tamar Estuary, south-west England

    Estuar. Coast. Shelf Sci.

    (1982)
  • S.A. Sañudo-Wilhelmy et al.

    Distribution of colloidal trace metals in the San Francisco Bay estuary

    Geochim. Cosmochim. Ac.

    (1996)
  • P. Serratore et al.

    Some observations about bacterial presence in seawater related to mucilaginous aggregates in the Northwest Adriatic Sea

    Sci. Total Environ.

    (1995)
  • P.L. Smedley et al.

    A review of the source, behaviour and distribution of arsenic in natural waters

    Appl. Geochem.

    (2002)
  • I. Sondi et al.

    Particulates and the environmental capacity for trace metals. A small river as a model for a land-sea transfer system: the Raša River estuary

    Sci. Total Environ.

    (1994)
  • A.A. de Souza Machado et al.

    Metal fate and effects in estuaries: a review and conceptual model for better understanding of toxicity

    Sci. Total Environ.

    (2016)
  • S.P.C. Tankéré et al.

    Distribution of dissolved Cd, Cu, Ni and Zn in the Adriatic Sea

    Mar. Pollut. Bull.

    (1996)
  • S.P.C. Tankéré et al.

    Mass balance of trace metals in the Adriatic Sea

    J. Mar. Syst.

    (2000)
  • Cited by (18)

    • Occurrence, contamination level and ecological risk assessment of dissolved and particulate trace elements in rivers entering the southwestern Mediterranean Sea

      2022, Marine Pollution Bulletin
      Citation Excerpt :

      In both river's water, Fe, Pb, and Cu had the highest Log Kd values, followed by Cr, indicating that these elements are highly reactive to the SPM. The same observation reported in Isonzo/Soča river (Gulf of Trieste, northern Adriatic Sea) by Pavoni et al. (2020). According to Tomczak et al. (2019), the metals characterized by high Log Kd values form stable complexes with the particulate phase, which implies that the particulate metals are predominant at the both river.

    • Dissolved concentrations and organic speciation of copper in the Amazon River estuary and mixing plume

      2021, Marine Chemistry
      Citation Excerpt :

      Despite these anthropogenic impacts to the Amazon basin, our study finds remarkably little change compared to 1976 (Boyle et al., 1982) for dissolved Cu concentration in the Amazon River estuary region and mixing plume during the high-discharge period (Fig. 8). Deviations from conservative mixing behavior of Cu and other metals has been attributed to colloidal flocculation upon river-seawater mixing in a variety of different estuaries (Sholkovitz, 1976; Abdel-Moati, 1990; Zhang, 1995; Tang et al., 2001; Pearson et al., 2017; Pavoni et al., 2020). In contrast, we observed mostly conservative behavior in Cu concentrations in the dissolved and soluble fraction (Fig. 4, Supplemental Fig. S2a).

    View all citing articles on Scopus
    View full text