Special Issue on Contaminants in Coastal Environments: From the Sediment-Water Interface to the Trophic Chain

Rapid industrialization and urbanization have led to a decline in environmental quality, especially in coastal aquatic environments (i.e., lakes, estuaries, lagoons, bays, and harbors), which are subjected to several various form of pressure (i.e., industrial, agricultural and sewage effluents, shipping, oil spills, nutrient inputs in rivers and atmospheric depositions). In these environments, sediments represent the final sink and the potential secondary sources, for the water column and biota, of several contaminants. Thus, potential toxic elements (PTEs), nutrients, persistent organic pollutants (POPs), and contaminants of emerging concern (CECs) harm aquatic life, endanger human health, and often require expensive mitigation procedures. The solutions to prevent and to mitigate the harmful effects of contaminants upon the aquatic environments cannot ignore relevant investigations, the transport and mobility of contaminants and their interactions with sediments, the water column and biota. This Special Issue is aimed at examining both the local and large-scale effect interactions and management of potential contaminants in the coastal aquatic environment. Five research articles and one review article were collected which report on various approaches used to assess the bioavailability, fate, and transport of contaminants, along with the risk assessment and management of the contaminated material from various anthropogenic sources

Behaviour of Metal(loid)s at the Sediment-Water Interface in an Aquaculture Lagoon Environment (Grado Lagoon, Northern Adriatic Sea, Italy)

The cycling of metal(loid)s at the sediment\u2013water interface (SWI) was evaluated at two selected sites (VN1 and VN3) in an active fish farm in the Grado Lagoon (Northern Adriatic, Italy). In situ experiments using a transparent benthic chamber and the collection of short sediment cores were performed, to investigate the behavior of metal(loid)s in the solid (sediments) and dissolved (porewaters) phases. Total and labile concentration of metal(loid)s were also determined in sediments, to quantify their potential mobility. Comparable total concentrations were found at both sites, excluding As, Mn, Pb and V, which were higher at VN3. Metal(loid) porewater profiles showed a diagenetic sequence and a close dependence with redox (suboxic/anoxic) conditions in the surface sediments. Positive diffusive fluxes along with benthic fluxes, particularly at the more oxic site, VN1, were found for almost all metal(loid)s, indicating their tendency to migrate towards the overlying water column. Despite sediments at two sites exhibiting high total metal(loid) concentrations and moderate effluxes at the SWI, the results suggest that they are hardly remobilized from the sediments. Recycling of metal(loid)s from the SWI would not constitute a threat for the aquatic trophic chain in the fish farm

Dissolved gaseous mercury production and sea-air gaseous exchange in impacted coastal environments of the northern Adriatic Sea

The northern Adriatic Sea is well known for mercury (Hg) contamination mainly due to historical Hg mining which took place in Idrija (Slovenia). The formation of dissolved gaseous mercury (DGM) and its subsequent volatilisation can reduce the amount of Hg available in the water column. In this work, the diurnal patterns of both DGM production and gaseous elemental Hg (Hg0) fluxes at the water-air interface were seasonally evaluated in two selected environments within this area, a highly Hg-impacted, confined fish farm (VN: Val Noghera, Italy) and an open coastal zone less impacted by Hg inputs (PR: Bay of Piran, Slovenia). A floating flux chamber coupled with a real-time Hg0 analyser was used for flux estimation in parallel with DGM concentrations determination through in-field incubations. Substantial DGM production was observed at VN (range = 126.0–711.3 pg L−1) driven by both strong photoreduction and possibly dark biotic reduction, resulting in higher values in spring and summer and comparable concentrations throughout both day and night. Significantly lower DGM was observed at PR (range = 21.8–183.4 pg L−1). Surprisingly, comparable Hg0 fluxes were found at the two sites (range VN = 7.43–41.17 ng m−2 h−1, PR = 0–81.49 ng m−2 h−1), likely due to enhanced gaseous exchanges at PR thanks to high water turbulence and to the strong limitation of evasion at VN by water stagnation and expected high DGM oxidation in saltwater. Slight differences between the temporal variation of DGM and fluxes indicate that Hg evasion is more controlled by factors such as water temperature and mixing conditions than DGM concentrations alone. The relative low Hg losses through volatilisation at VN (2.4–4.6% of total Hg) further confirm that static conditions in saltwater environments negatively affect the ability of this process in reducing the amount of Hg retained in the water column, therefore potentially leading to a greater availability for methylation and trophic transfer

Legacy of Past Mining Activity Affecting the Present Distribution of Dissolved and Particulate Mercury and Methylmercury in an Estuarine Environment (Nalón River, Northern Spain)

At the Nalón River estuary (Asturias, Northern Spain), the occurrence of Hg is due to historical mining activity which has resulted in environmental issues of great concern. Although several studies have investigated the sediment compartment regarding Hg contamination, no information is currently available on the fate of Hg and MeHg in the water column. Considering different hydrodynamic/seasonal conditions, water samples were collected along the estuary to evaluate Hg and MeHg distribution and partitioning behaviour between solid and aqueous phases. The complementary effect of the river discharge and tidal currents contributed to the prevalence of the dissolved (4.02 +/- 1.33 ng L-1) or particulate (8.37 +/- 4.20 ng L-1) Hg under different conditions of discharge in summer and autumn, respectively. Conversely, particulate MeHg prevailed when the river flow was low, especially at the estuary mouth (25.8 +/- 19.1 pg L-1) and most likely due to the resuspension of fine particles promoted by a stronger tidal current. In comparison with the total Hg concentration, extremely low amounts of dissolved and particulate MeHg were observed, and strong interactions between MeHg and organic carbon highlighted a negligible risk of increased mobility and potential bioaccumulation of MeHg

Gaseous Mercury Exchange from Water-Air Interface in Differently Impacted Freshwater Environments

Gaseous exchanges of mercury (Hg) at the water-air interface in contaminated sites strongly influence its fate in the environment. In this study, diurnal gaseous Hg exchanges were seasonally evaluated by means of a floating flux chamber in two freshwater environments impacted by anthropogenic sources of Hg, specifically historical mining activity (Solkan Reservoir, Slovenia) and the chlor-alkali industry (Torviscosa dockyard, Italy), and in a pristine site, Cavazzo Lake (Italy). The highest fluxes (21.88 ± 11.55 ng m-2 h-1) were observed at Solkan, coupled with high dissolved gaseous mercury (DGM) and dissolved Hg (THgD) concentrations. Conversely, low vertical mixing and saltwater intrusion at Torviscosa limited Hg mobility through the water column, with higher Hg concentrations in the deep layer near the contaminated sediments. Consequently, both DGM and THgD in surface water were generally lower at Torviscosa than at Solkan, resulting in lower fluxes (19.01 ± 12.65 ng m-2 h-1). However, at this site, evasion may also be limited by high atmospheric Hg levels related to dispersion of emissions from the nearby chlor-alkali plant. Surprisingly, comparable fluxes (15.56 ± 12.78 ng m-2 h-1) and Hg levels in water were observed at Cavazzo, suggesting a previously unidentified Hg input (atmospheric depositions or local geology). Overall, at all sites the fluxes were higher in the summer and correlated to incident UV radiation and water temperature due to enhanced photo production and diffusivity of DGM, the concentrations of which roughly followed the same seasonal trend

Fluxes of settling sediment particles and associated mercury in a coastal environment contaminated by past mining (Gulf of Trieste, northern Adriatic Sea)

Purpose: As the result of historical mining at Idrija (Slovenia), mercury (Hg) contamination in the Gulf of Trieste (northern Adriatic Sea) is still an issue of environmental concern. The element has been conveyed into the coastal area by the Isonzo/Soča River inputs of freshwater and suspended particles for centuries. This research aims to investigate the occurrence of Hg bound to the settling sediment particles (SSP) in the coastal water and to assess the sedimentary Hg fuxes. Methods: Settling sediment particles were collected at four sites located in the innermost sector of the Gulf, a shallow and sheltered embayment where the accumulation of fine sediments is promoted. Six sampling campaigns were performed under different environmental conditions in terms of discharge from the Isonzo River and 12 sediment traps were installed in the upper and bottom water column for SSP collection. Settling sediment particles (SSP) were collected approximately every 2 weeks and analysed for grain size and total Hg. Results: Settling sediment particles (SSP) consisted predominantly of silt (77.7±10.1%), showing a concentration of Hg ranging overall between 0.61 and 6.87 µg g−1. Regarding the daily SSP fuxes, the minimum (7.05±3.26 g m−2 day−1) and the maximum (92.4±69.0 g m−2 day−1) values were observed under conditions of low and high river discharge, respectively. The daily Hg fluxes displayed a notable variability, up to an order of magnitude, both in the surface water layer (3.07–94.6 µg m−2 day−1) and at the bottom (11.3–245 µg m−2 day−1), reaching the maximum values following periods of high river flow. Conclusions: The Isonzo River inputs of suspended particulate matter continue to convey Hg into the Gulf of Trieste, especially following river flood events, which represent one of the most relevant natural factors affecting the variations of the Hg flux in the investigated area

Benthic nutrient cycling at the sediment-water interface in a lagoon fish farming system (northern Adriatic Sea, Italy)

Metabolism and carbon, oxygen, and nutrient fluxes (DIC, DOC, DO2, NO2 12, NO3 12, NH4+, PO4 3 12 and SiO4 4 12) were studied during three surveys at two sites (VN1 and VN3) located at a fish farm at theMarano and Grado Lagoon (northern Adriatic Sea), using an in situ benthic chamber. Field experimentswere conducted in July and October 2015 and March 2016 at a depth of approximately 2 m along the main channels of the fish farm.Water samples were collected by a scuba diver every 2 h in order to investigate daily fluxes of solutes across the sediment-water interface (SWI). Regarding the solid phase, Corg/Ntot and Corg/Porg molar ratios suggested an autochthonous marine origin of the organic matter and a minor preservation of P in the sediments, respectively; high values of sulphur (Stot) were also encountered (0.8\u20132%). The conditions at VN3 were mostly anoxic with high NH4 + levels (30\u20131027 \u3bcM) and the absence of NO3 12. Substantial daily patterns of all solutes occurred especially in autumn andwinter. On the contrary, fluxes at VN1were less pronounced. Usually, inverse correlations appeared between dissolved O2 and DIC trends, but in our systemthis was observed only at VN3 in autumn and accomplished by a parallel increase in NH4+, PO4 3 12 and SiO4 4 12 during intense nutrient regeneration. These results are significantly different than those reported for open lagoon environments,where nutrient regeneration at the SWI and in surface sediments is the primary source of nutrients available for assimilation processes, especially during the warmer period of the year when the natural nutrient input by fresh water inflows is limited. Due to the importance of this site for aquaculture, biodiversity and ecosystem services, useful suggestions have been provided from this study in order to improve the quality of this unique aquatic system

Effects of hypoxia on biogeochemical cycling of nutrients and trace elements in a stratified estuarine system (Gulf of Trieste, northern Adriatic Sea)

Estuaries can be thought as a sedimentary trap leading to the accumulation of potentially toxic trace elements (PTEs) in sediments. However, biogeochemical processes at the sediment-water interface (SWI) may also be responsible for the release of dissolved PTEs and nutrients in the overlying water column affecting the water quality. The estuarine system of the Timavo River (Gulf of Trieste, northern Adriatic Sea) is a semi-closed aquatic environment where a long-lasting oxic-hypoxic interface along the water column occurred due to the scarce water circulation in the innermost sector. To prevent bloom-forming and potential production of toxins and off-flavours, artificial mixing has been provided with a bubble plume installation connected to pressurised air and built on the sediment surface aiming at reoxygenating the water column. The aim of this research was to evaluate the behaviour of PTEs (As, Cr, Hg, Fe, Mn, Ni, Pb, V) and nutrients (NO3, NO2, NH4 and SRP) along the water column and at the SWI before (June) and during (September) the activation of the forced aeration system. Water samples were collected at different depths along the water column, in situ benthic chamber experiments were performed at the SWI and short sediment cores were sampled to investigate both the sediment and porewater. Dissolved oxygen decreased along the water column, especially in June when hypoxia (2.29 mg/L) and reductive conditions (58 mV) were observed at the bottom resulting in increasing dissolved PTE and nutrient concentrations. Accordingly, a gradual oxygen depletion was observed in the benthic chamber testifying to intense organic matter remineralisation processes. Moreover, the highest concentrations of dissolved PTEs in porewater were restricted to the top of the sedimentary sequence, especially in June when hypoxic conditions may promote PTE and nutrient effluxes from the sediment to the water column

Two-year monitoring of water hydrochemistry in a Pb-Zn Mississippi Valley-Type mine (MVT) in the Southeastern Alps (Raibl, Friuli Venezia Giulia)

The recent and past mining activities are among the main anthropic sources of dispersion of potentially toxic trace elements (PTEs) in the environment. In this study, a two year monitoring of different water bodies in a decommissioned mining site located in the Southeastern Alps (Friuli Venezia Giulia, Raibl mine) was performed. Results have allowed to provide a characterisation of the hydrogeochemistry, the chemical signatures and the temporal-spatial variations of PTEs in a carbonate-hosted Pb-Zn Mississippi Valley-Type (MVT) mine, where no acid mine drainages (AMD) occur. Besides mineralogy and pH-Eh conditions, strong rainfalls and high-flow events are the main factors affecting the temporal variability of dissolved PTEs, promoting their dissolution and dispersion. Anomalous concentrations of trace metals (Zn, Pb, Tl) were found in near neutral pH-buffered groundwaters entrapped in tailings impoundments, whereas concentrations of metalloids (As, Sb and Ge) were more abundant in low-flow water drainage from mine adits. High concentrations of Tl were found in the saturated area of the tailings impoundments, related to relatively lower pH and sulfate ions contents, thus suggesting Tl-bearing pyrite/marcasite oxidation. At the same time, low concentrations of dissolved Ge and Cd in groundwaters entrapped in tailings are possibly associated to sphalerite-depleted post-flotation tailings. Based on chemical data, modeling and literature, attenuation processes of dissolved PTEs (mainly Pb) are mainly attributed to sorption onto Fe-oxy-hydroxides, which is pH-dependent, and precipitation of mineral phases (e.g., dissolved Zn to hydrozincite: Zn5(CO3)2(OH)6). The Tl/Zn and Tl/Pb ratios show that enrichments occur without notable attenuation inside the tailings impoundments, possibly indicating that Tl attenuation needs higher pH to effectively promote adsorption onto Fe-oxy-hydroxides, as, conversely, occurs in the Rio del Lago stream waters

Environmental impact of potentially toxic elements on soils, sediments, waters, and air nearby an abandoned Hg-rich fahlore mine (Mt. Avanza, Carnic Alps, NE Italy)

The decommissioned fahlore Cu-Sb(-Ag) mine at Mt. Avanza (Carnic Alps, Italy) is a rare example of exploited ore deposits, as the tetrahedrite (Cu6[Cu4(Fe,Zn)2]Sb4S13) is the main ore mineral found. This multi-compartmental geochemical characterisation approach provides one of the first case studies regarding the geochemical behaviour and fate of Hg, Sb, As, Cu, and other elements in solid and water matrices and of Hg in the atmosphere in an environment affected by the mining activity of a fahlore ore deposit. Elevated concentrations of the elements (Cu, Sb, As, Pb, Zn, Hg) associated with both (Zn-Hg)-tetrahedrite and to other minor ore minerals in mine wastes, soils, and stream sediments were observed. Concentrations in waters and stream sediments greatly decreased with increasing distance from the mining area and the Igeo index values testify the highest levels of sediment contamination inside the mine area. Thallium and Ge were associated with the "lithogenic component" and not to sulfosalt/sulphide minerals. Although mine drainage water often slightly exceeded the national regulatory limits for Sb and As, with Sb being more mobile than As, the relatively low dissolved concentrations indicate a moderate stability of the tetrahedrite. The fate of Hg at the investigated fahlore mining district appeared similar to cinnabar mining sites around the world. Weak solubility but the potential evasion of gaseous elemental mercury (GEM) into the atmosphere also appear to be characteristics of Hg in fahlore ores. Although GEM concentrations are such that they do not present a pressing concern, real-time field surveys allowed for the easy identification of Hg sources, proving to be an effective, suitable high-resolution indirect approach for optimising soil sampling surveys and detecting mine wastes and mine adits