Mercurial, the metaphor for volatile unpredictable behavior, aptly reflects the complexities of one of the most insidiously interesting and scientifically challenging biogeochemical cycles at the Earth’s surface. At the base of this toxic metal cycle there is the conversion between the different Hg chemical species, in which the balance between the reduced and oxidized forms depends primary on redox system conditions. The potential risks of human exposure to Hg, especially in the form of monomethylmercury (MMHg), particularly prenatally, and the potential deleterious ecological consequences from localized to global scale Hg pollution, have given much impetus to mercury studies and regulatory activities at international level. Much of this advancement has come since the early 1970s, and the growth in mercury research continues at breakneck pace. The menace of this item for environment and human health deserves further information concerning the geochemistry of mercury, especially in coastal marine system. The Augusta Bay is a semi-enclosed marine area, located in the SE of Sicily (southern Italy), well-known because of the high Hg pollution. The area indeed has experienced, since the early 60s, a significant industrialization phase that put in several chemical and petrochemical plants and oil refineries resulting in a severe pollution of the surrounding environment. In particular, the petrochemical district of Augusta Bay hosted one of the most important chlor-alkali plant in Italy, that produced chlorine and caustic soda by electrolysis of sodium chloride aqueous solution in electrolytic cells with a graphite anode and metallic mercury cathode. Uncontrolled chemical discharge of Hg occurred in the Augusta Bay until 1978, when restrictions were imposed by the Italian legislation. For this reason, in the last decade, several studies have provided detailed information on the pollution levels and risks for human health of resident populations of Augusta Bay. The effects of this indiscriminate Hg discharge include the alarming high concentrations of the element recently measured in sediments of the basin, prompting the Italian government to include the Augusta basin in the National Remediation Plan. The “Augusta case“ menaces to not remain confined to a “local problem”, but to became a large-scale threat. Indeed, the effects of meso-scale circulation of the Ionian Sea create a higher potential risk for HgT contamination of the basin, being affected by the transit and transformation of the major water masses, which regulate the general thermohaline circulation in the upper, intermediate and deep layers, respectively. Owing to the geographical location of the Augusta basin, its outflowing shelf waters are immediately intercepted by the surface Atlantic Ionian Stream (AIS) and mixed with the main gyres of the eastern Mediterranean Sea, thus representing a risk for the large-scale marine system. This complex water circulation system, together with the closeness with the steep continental slope (part of the Malta escarpment), make the area a potential point sources of mercury for the entire Mediterranean sea, as previously speculated by Sprovieri et al. (2011). All this features make the Augusta Bay an ideal natural laboratory for deeper insights on the biogeochemical cycle of mercury in a coastal marine environment and the need to investigate the large-scale effect of Augusta Bay pollution has become imperative! With the aim to fill this requirement, an integrated model on the biogeochemical cycle of Hg has been created. Hg cycle is a very articulated topic. Once introduced in the aquatic system environment the fate of Hg in the marine system is affected by sorption/ desorption processes onto suspended particulate matter and, based on associated kinetics, it may be partially transferred from surface waters to bottom sediments. Microorganisms, at the water/sediments interface such as sulfate reducing bacteria (SRB), mediate the transformation of inorganic Hg to MMHg with high rates of methylation favored by the presence of high content of organic matter under reducing environmental conditions. Therefore, sediments are considered key contributors of MMHg to the marine ecosystem. Clearly, this analysis stresses the necessity for better knowledge of the specificity of the mercury biogeochemical cycle in this particular environment through the gathering of more data on the distribution and fluxes among the various compartments including the water column, sediment, atmosphere and biota. This multidisciplinary approach offers a nice opportunity to explore the biogeochemical dynamic of mercury in highly complex coastal areas under important anthropic impact and the potential on larger scale diffusion. Multiple oceanographic cruises, realized during 2011-2012 period, permitted to collect samples of sediments, seawaters and fishes inside and outside the Augusta Bay. Furthermore in order to trace the entire chain, from sources (polluted sediments) to sink (man), analysis of Hg in fishes (the main route of Hg uptake for humans), and toxicological aspects have been addressed. Analysis of THg in muscles and liver of some pelagic, demersal and benthic species captured inside and outside the semi-enclosed area, has been analysed in order to explore the effects of HgT pollution on fish compartment and to assess the potential health risks associated with the consumption of contaminated fish. THg content of fishes shows a wide range of concentration (range: 0.02 - 2.71 μg g-1and 0.03 -9.72 μg g-1 in muscles and in livers respectively), with highest values measured in benthic species and the lowest in pelagic ones. This increasing trend along the habitat depth suggests an active release mechanism of mercury from polluted sediments to the water column, with consequent effects of bioaccumulation in the trophic web. Anomalous THg content measured in pelagic species captured in the external zone of the bay confirms the role of the Augusta marine environment as pollutant source of Hg for the surrounding area and underscores the crucial risk associated with contaminant transfer from the basin to the open sea. Finally, values of hazard target quotient (THQ) and estimated weekly intake (EWI) demonstrate that consumption of fishes caught inside the bay represents a serious risk for human health and suggests caution in consuming demersal and benthic fishes from outside the Augusta Bay, definitively demanding for appropriate social actions. Hg distribution in sediments (range: 1.77 - 55.34 mgKg-1; mean: 13.78 ±10.72 mgKg-1) clearly divides the area into three parts, with the lowest values recorded in the northern Augusta Bay, intermediate value in the center and the highest HgT concentrations recorded in the southern part of the Augusta basin (from the Pontile Cementeria down to the dam) with decreasing values from the coastline. Despite sequential extraction procedure (SEP) documented that the most part of HgT in sediments consists of strong complexes (~80% of HgT as strong complex, ~15% of Hg as less strong forms and ~ 2% of HgT as more soluble and bioavailable forms), some anaerobic microorganisms can manage these stable Hg trapped in minerals structures as substrate for their metabolism, making Hg more easily bioavailable for the environment. Analysis of THg and DHg content at different quote of the water column, provided significant information on Hg distribution along space and depth. The HgT distribution in seawaters (range: 0.45 - 129.27 ngL-1 and >0.01 - 21.3 ngL-1 for THg and HgD respectively), putted in light an evident increasing trend of Hg content toward the southern and more contaminated part of the Augusta Bay, where waste spillage from chlor-alkali plant occurred. A clear trend was also observed on the vertical, with Hg concentration increasing near the bottom and reducing in surface water, strengthening the role of Augusta sediments as sources for the overlying water. Moreover the unexpected THg concentrations measured in seawater outside the bay (range: 2.62-11.95 ng L-1; mean: 6.46±2.95 ngL-1), confirmed the hypothesis of transport of Hg from Augusta harbor to the open sea representing a vehicle of contamination for the entire Mediterranean basin through the complex circulating currents affecting the western Ionian. In this scenario, fluxes assessment at the interface sediments-seawater-air became crucial in order to create a mass balance of Hg in the study area and to determine the net outflow for the Mediterranean sea. Detailed information on the mobilization processes from sediment to seawater and consequent escape to the atmosphere has been investigated. For this reason, for the first time in this area, a benthic chamber and a dynamic accumulation chamber, have been employed in order to evaluate fluxes at the interfaces sediments/seawater and seawater/air and to recognize equilibrium of exchanges among phases. Using in situ accumulation chamber Bagnato et al., 2013 reported an estimated sea–air Hg evasion for the entire Augusta basin (~23.5 km2) of about 9.7 ± 0.1 g d-1 (~0.004 t yr-1), accounting for ~0.0002% of the global Hg oceanic evasion (2000 t yr-1). Simultaneously using in situ benthic chamber, a total flow from sediment to seawater for the whole Augusta Bay has been estimated in 0.22 kmol y-1 in 2011 (0.05 ty-1) and 0.38 kmol y-1 in 2012 (0.11 ty-1). The mass balance calculation permitted to estimate a HgT output from the Augusta basin to Ionian surface waters (O) corresponding to an average of 1.29 kmol y-1. Analysis of Hg isotopes in sediments and fishes of the Augusta Bay, provided unique information on Hg sources in the environment and processes influencing Hg cycling. The success of such an approach strongly depends on two factors. First, different natural and anthropogenic Hg sources must have analytically discernible Hg isotope signatures. Second, the processes that transport and transform emitted or discharged Hg into the environment must not obscure the original Hg source isotope signatures. This requires that fractionation of Hg isotopes after release is either small relative to source differences or is predictable enough to be corrected for, allowing estimation of the source isotopic composition. The magnitude of mass-dependent (MDF) and mass-independent fractionation (MIF) has been described primarily as δ 202Hg and Δ201Hg. The positive MIF fractionation in fishes, especially in pelagic one, demonstrated photochemical reaction of Hg(II) prior of the intake in the marine food web. Sediments isotopes fractionation demonstrated reaction of methylmercury production biological mediated. A geographic pattern in δ202Hg and Δ199Hg values suggests that the sources of Hg to the sediment are locally controlled. Hairs exhibit positive MIF fractionation, suggesting reaction of photochemical reduction of MeHg in presence of organic matter. The overlapping δ 202Hg values of both sediments and fishes suggested sediments represent the source of Hg for fish. The positive relationship obtained by plotting Δ201Hg vs. Δ199Hg of both hairs and sediments demonstrate fish consumption represents the first pathway of exposure for human. Difference of 2‰ between δ 202Hg in fishes and values, could be due to could suggest that substantial MDF takes place during MMHg human metabolism Rare Earth Elements (REEs) are important because their geochemical properties enable them to be powerful tracers of chemical processes. Their distribution has been investigated in seawater of the Augusta Bay in order to verify if anthropogenic sign can also transpire through the investigation of REE. The REEs distribution along the water column suggests that the high dissolved organic matter created ideal condition for an increasing of REE in dissolved phases, much to hide the negative Ce anomaly usually recorded in the oligothropic water. Gd anomaly, expressed as Gd/Gd*>1, suggests significant contributions of the petrochemical industries, using gadolinium, in the form of gadolinium oxide, as petroleum cracking catalyst. A common thread, started from the evaluation of Hg in the key component of the cycle, the study of fluxes at the interfaces, the evaluation of Hg isotopic fractioning and the REE distribution in water column, permitted to evaluate the fate of Hg in the Augusta Bay and the main processes rule the Hg biogeochemical cycle.
|Titolo:||The biogeochemical cycle of mercury in the Augusta Bay|
|Data di pubblicazione:||25-mar-2014|
|Citazione:||(2014). The biogeochemical cycle of mercury in the Augusta Bay. (Tesi di dottorato, Università degli Studi di Palermo, 2014).|
|Tipologia:||Tesi di dottorato|
|Nome editore:||Università degli Studi di Palermo|
|Luogo di pubblicazione:||Palermo|
|Appare nelle tipologie:||Tesi di dottorato|