Mercury Behaviour in Estuarine and Coastal Environment M. Horvat

Mercury behaviour in estuarine and coastal

environment M. Horvat

Department of Environmental Sciences, Jozef Stefan Institute, Jamova 39, Ljubljana, Slovenia

Abstract

General facts about the global mercury cycle with particular emphasis on the coastal and ocean environment are summarized. In the coastal environment the largest source of mercury is river-born particulate bound species. This portion of mercury is unreactive and is quickly buried in nearshore sediments. Only a small fraction of reactive mercury (ionic mercury in solution that is immediately available for reaction) originates from river inputs. The most important source of reactive mercury in the coastal and oceanic environment is through atmospheric input and via upwelling. Biologically-mediated processes, mainly connected to

primary production, are responsible for active redistribution of reactive mercury. In this process a large part of reactive Hg is reduced to elemental mercury which is returned to the atmosphere by evasion, while the rest is scavenged by particles and transported to deeper oceanic waters. Because of the active atmospheric mercury cycle oceans acts as a source and a sink of atmospheric mercury and the global oceanic evasion is balanced by the deposition. Current studies show that methylated species are primarily formed in the deeper ocean and the mam source of monomethylmercury (MMHg) compounds in coastal areas is through upwelling of oceanic waters and from in-situ methylation in coastal waters. All these environmental processes occur at extremely low concentration levels of mercury species; however MMHg in marine organisms accounts for a high proportion of this toxic compounds owing to its property for bioaccumulation and biomagnification. Coastal areas on the local scale may account for geochemical differences that significantly influence the conversion between various Hg species. In order to assess the impact of mercury in contaminated and non- contaminated coastal areas on man and his environment, it is of the greatest importance to understand these processes. The paper also identifies uncertainties and gaps in current

knowledge of mercuy cycling

1 General facts

Mercury and its compounds are extremely hazardous. The most toxic are monomethylmercury (MMHg) compounds which represent a health risk, particularly to the foetal neurosystem^. The risk to public health is evidenced in fish consumption regulations that have been issued in Canada, Scandinavia, by

more than 30 states, the US FDA, the World Health Organization (WHO) and numerous other governments. In the USA, the Clean Air Act Amendments require an assessment of health risk to humans and wildlife caused by Hg emissions. Also, the potential adverse effects of atmospheric Hg deposition on coastal waters is contained in a planned Protection Agency report to Congress^. In the environment mercury can exist in a number of different physical and chemical forms with a wide range of properties. Biogeochemical conversion between these different forms provides the basis for mercury's complex distribution pattern in local and global cycles and for its biological enrichment and effects. The most important chemical forms are: elemental mercury (Hg°), divalent inorganic mercury (Hg(II)), monomethylmercury (MMHg), and dimethylmercury (DMHg). There is a general biogeochemical cycle by which these different forms may interchange in the atmospheric, aquatic and terrestrial environments. Most studies of Hg cycling have, so far, been made in terrestrial systems, with the biogeochemistry of Hg in fresh water systems^, even though it is well known that the primary exposure of humans to MMHg is through the consumption of marine fish and fish products^. In principle, Hg cycles in terrestrial and marine aquatic systems are similar with some distinct differences. The most important feature in both systems is the in-situ bacterial conversion of inorganic Hg species to the more toxic MMHg, which concentrates in fish muscle?". The key feature that influences mercury distribution in aquatic environments is the high stability of its associations with sulphur and carbon, the stability of its volatile elemental form and its strong affinity to particles. Consequently, most inorganic and organic Hg appears to be bound to particles, colloids and high molecular weight organic matter, where it is probably coordinated with sulphur ligands on particles (distribution coefficients are in the 10^-10^ ml/g range). In turbid rivers most mercury, therefore, is transported by suspended matter. Only a small part of Hg in fresh, estuarine and sea water is likely to be present in dissolved form **"**. After entering the atmosphere mercury exchanges and cycles through the atmosphere to be deposited in the ecosystem, almost exclusively as Hg(II). When Hg enters a surface (soil or water) Hg(II) can be methylated to MMHg. This is the first step in aquatic and terrestrial bioaccumulation processes. The mechanism of synthesis of MMHg is not very well understood. The main factors that affect the levels of MMHg in fish are the dietary trophic level of the species, the age of the fish,microbia l activity and the mercury concentration in the upper layer of the local sediment, its dissolved organic carbon content, salinity, pH, and redox potential^. Recent work suggest that sulphato-reducing bacteria are the most important methylating agents along with environmental conditions existing in transition regions between oxygenated and anoxic conditions^. Methylation-demethylation reactions are assumed to be widespread in the environment and each ecosystem attains its own steady state equilibrium with respect to the individual species of mercury. However, owing to

environment than demethylation. Once MMHg is formed, it enters the food chain by rapid diffusion and tight binding to proteins in aquatic biota, and attains its highest concentrations in the tissues of fish at the top of the aquatic food chain due to biomagnification through the trophic levels. For example, most predatory fish species show MMHg values > 1 ppm, while concentrations in water are commonly < Ippt (e.g. amplification factor of about 10^'^^. Although MMHg is the dominant

form of mercury in higher organisms, it represents only a very small amount of the total mercury in aquatic ecosystems and in the atmosphere.

2 Global mercury cycle

Mercury vapour is released into the atmosphere from a number of natural sources (e.g. the ocean surface and other water surfaces, soils, minerals, and vegetation located on land, forest fires and volcanoes) and through

anthropogenic emissions (metal extraction processes, agricultural uses, paints, waste disposal, burning of fossil fuels, smelting of ores and other industrial minerals, and from power plants burning fossil fuels for electricity generation).

Recent studies ^^ show that human activity contributes about 50 to 75% (e.g. 3500 to 4500 tons) of the total yearly input from all sources (7000 tons/y) (Figure 1). About half of the anthropogenic emissions (approx. 2000 tons/y) appear to enter the global Hg cycle, while the other half is deposited locally. As

a consequence human activities have tripled the concentrations of Hg in the atmosphere and in the surface ocean. It is estimated that 60% (approx. 5000 tons) of the total Hg is deposited on terrestrial environments (30% of the surface of the Earth) and the remainder to the ocean. This is due to the oxidation

of Hg in the abundant terrestrial aerosols. The ocean receives about 90% of its Hg through wet and dry deposition as Hg(II) and the remaining (200 tons/y) from river inflows. The particulate scavenging and removal to the deep ocean is

equal to the riverine Hg flux (200 tons/y). Due to biological reduction of deposited Hg(II) in the mixed layer of the ocean and its evasion most Hg° deposited (2000 ton/y) is re-emitted to the atmosphere. This active process and the minimal removal of Hg to the deep ocean makes terrestrial systems the

dominant sink. The model presented in Figure 1 is based on rather limited data and uncertainties may account for a factor of two or more. The main reasons for this uncertainty are the limited data for Hg concentrations and speciation in precipitation, both temporally and spatially. Almost no data are available for the Southern Hemisphere, Asia and the Arctic coastlines. Tropical regions also need to be assessed, as the impact of current activities (e.g. biomass burning and gold extraction^) on the global Hg cycle and budget may be significant. The flux of

MMHg to the oceans is also unknown. In particular, coastal rain could be an important source of MeHg. Another reason for uncertainty is the limited data available on evasion of Hg from the ocean surface. This process is controlled by the ability of the system to

reduce reactive Hg. Current studies show that this process is directly connected to primary productivity. It has also been shown that there are ocean areas where's atmospheric deposition exceed the evasion ( e.g. the Siberian Arctic) and other where evasion exceeds the deposition (e.g. the productive zones at lower latitudes). Probably, net atmospheric transfer of mercury to higher latitudes is occurring. More studies need to be done in productive and non- productive ocean areas in order to understand better global Hg transport and cycling. The current model also does not take into account some other potential sources of Hg in the ocean. These are hydrothermal inputs and oil and gas deposits. It is assumed that most inorganic mercury from hydrothermal vents would be trapped on particles and precipitated. However, the extent to which these deposited minerals are remobilized is unknown. Many more studies need to be done, particularly in regions where such activity occurs (Pacific rim,

Aleutian islands, Mediterranean, etc.). Oil and gas deposits also contain elevated Hg concentration and the flux from such deposits may be significant in coastal oceans and shelves where these deposits formed seeps into the ocean.

The mercury flux to the deeper ocean only accounts for about 10% of the ocean input**. Some recent studies suggest that this flux may be much smaller, but no reliable study has been done so far. Sedimentation as well as remobilization of Hg from sediments in different geological parts of the deep ocean and continental shelves should be studied to improve these figures. It is important to note that due to extremely low mercury concentrations in water samples (Table 1) the measurement (sampling and analysis) protocol must be carefully designed, in particular, if speciation of mercury forms is intended.

Due to possible erroneous analytical data caused by sample contamination and/or speciation change during storage of samples a number of studies in the past should, therefore, be treated with great reservation. Due to remarkable improvements in analytical techniques over the last 10 years the reliability of data have improved significantly^. However, further development and improvements of techniques for Hg speciation, particularly for identification of Hg-organic associations, are needed.

3 Mercury in the oceanic environment

Concentration levels of dissolved Hg in the oceans waters are very low (Table 1), usually below 1 ng/1, of which a portion can be present as dissolved chlorocomplexes and the remaining is bound to particles, which are not abundant in the ocean environment, except in highly productive zones. Other forms of Hg in ocean waters are the elemental Hg form, particularly in highly productive surface waters, and the organic mercury forms (MMHg and DMHg) which are found in deeper waters^^. It was shown that in the ocean

AIR 25 Mmol HEP 98% Hg' Kg' 2% Hg,

DEPOSITION Hg(ll)

Figure 1. The current global Hg cycle, (adapted from Fitzgerald and Mason**. (1 Mmol is equal to 200 tons of Hg)

environment DMHg is the dominant methylated compound in contrast to freshwater where MMHg predominates. Even more, it was hypothesized that DMHg may be the primary source of MMHg in the oceans, which means that the mechanisms of MMHg production may be different from fresh water 1 8 74 systems. But this still needs further investigation ' . The present oceanic data are very sparse. As already mentioned one of the most important components of the Hg cycle the ocean environment is the in-situ production and water-air transfer of elemental Hg*'**'^\ In this process most of the deposited Hg(II) is reduced to Hg° and returned to the atmosphere and only a small fraction of Hg is trapped in the ocean. One part of this fraction is removed by sedimentation, while the other enters the Hg cycle including in-situ methylation. It was estimated that only 2% of the mercury entering the oceans is

enough to account for the significant accumulation of MMHg in fish*. The importance of Hg° in controlling the production of MMHg in ocean environment is clearly evidenced from Figure 2.

4 Mercury in coastal areas

Mercury concentrations in coastal waters tend to be higher than in the open sea, which is due to the greater abundance and deposition of particulate mercury (Table l/\ Particulate Hg dominates Hg partition due to the high suspended

solids load. Dissolved Hg concentrations are low and vary in the range from 0.3 to 1.6 ng/1. Particulate Hg concentrations are rather variable ranging from 0.05 Hg/g to 1 |ig/g in non-contaminated areas and reach up to several |ig/g in contaminated coastal regions**. Hg enrichment in particles is proportional to

their organic carbon content, particularly in areas where phytoplankton is abundant. MMHg compounds can be found as up to 3% of total particulate Hg, while dissolved MMHg is present at very low concentrations (< 0. DMHg is undetectable in coastal waters.

It is estimated that the Hg flux through rivers represents about 10% of the global Hg input to the oceans. This figure needs further refinement as it is calculated on the basis of limited data. For example there is no information on inputs through the largest rivers, such as the Amazon, and others in the Southern

Hemisphere, and Asian and Siberian rivers. Very little is known about mercury behaviour in rivers,eve n though rivers represent an important mechanism for the transport of Hg species to other ecosystems. Also the biogeochemistry of Hg in a river system may be different from that in other water bodies.

Table 1. Mercury concentrations in ocean and coastal waters

Dissolved Hg Participate Hg Other Hg compounds (0.45 jim filter) pg/g (range) ng/1 (range) Rivers non-contaminated 0.3- 1.0" 0.044- 1.40" MMHg: 1-10% of paniculate Hg contaminated 2_4^» 1.2-30^ < 1 % of dissolved Hg Hg° : < 0.2 ng/1 DMHg : not detected Coastal areas near shore 0.5-2.0^* 0.04- 1.88 " MMHg: 3% of part. Kg" off shore 0.2 - 0.4^* 5-50 pg/1 in

contaminated area 0.6 - 2.3^* 0.1-2.14^ contaminated areas ^ DMHg: up to 58 pg/1 "'^ Ocean NW Atlantic 0.6-1.3^ low abundance DMHg: < 1 0% of total Hg *-"'**" NE Pacific 0.2 - 0.6*""^ of participates Hg°: 40 - 80 pg/1 *"'*^ Mediterranean Sea 0.1-0.8^° MMHg: below detection limits

Estuaries are very important parts of the ocean margin and only a few studies

of Hg behaviour have been done so far ^,28,3132 Mercury cycling in estuaries is more complex and variable due to differences in chemical and physical gradients. The concentrations of Hg are more variable than in other coastal areas because

of a direct relationship to the source of contamination. Mercury behaviour significantly differs from one estuary to another. In some estuaries dissolved Hg behaves conservatively (it is not removed from water), while in other it behaves non-conservatively due to coagulation of colloids and subsequent sedimentation.

Colloidal ligands, particularly those with thiol functional gropus play an important role in mercury cycling in estuaries ". Elemental Hg is present outside the turbidity plumes and is related to the presence of phytoplankton^. Maximum concentrations of MMHg occur in hypoxic conditions and in the fluvial part that is rich with organic particulates. DMHg was also detected at very low concentrations (<20 pg/1)^. In order to calculate accurately the input of Hg from rivers and coastal areas more work needs to be done to understand processes in estuaries, particularly, the relationship between Hg and colloidal and participate phase, and the fate of Hg during estuarine mixing.

!Hg'!<

r , \ ^.,- , k 7+iW • \ -*!" _^.!H g 1 " Hg i*^ ^/* t ^^^^^^ i *N dry (?) and wet and |(CHj)2Hg! |(CH^Hgi dry i evasion wet \ deposition r* deposition

run-off

Hgp - paniculate Hg Hg - divalent, reactive Hg

CHgH g - monomethyl Hg (CH^^H g - dimethyl Hg burial

Fig. 2: Cycling of mercury in coastal and ocean environment.

4.1 Mercury in coastal sediments

In general, sediments are considered to be an ultimate sink of particulate Hg. However, they also play an important role in mercury cycling (Figure 2). It is

well documented that MMHg and DMHg can be produced in marine sediments^. As important factor influencing MMHg production is the concentrations of sulphides^. Net methylation rates were favoured in anaerobic oxic sediments. With the increasing sulphide concentration MMHg is lost by

dismutation - a process by which MMHg is transformed into DMHg and HgS via decomposition of dimethylmercury sulphide. This process has been experimentally proven in natural environments such as highly reducing estuarine sediments and flood plain soils of polluted rivers^. Such reactions could also

generate gaseous Hg°, due to reduction of Hg(n)^^ Both elemental Hg and DMHg are in gaseous form and are lost from anoxic coastal sediments by volatilization (Figure 2). MMHg in sediments accounts for less than 1% of total

Hg and originates from settling particles and from internal production associated with the sulphato-reducing bacterial activity *^\ The diffusion of MMHg and Hg(II) from pore to overlying waters is generally low^^. It is governed by molecular diffusion and biological mobilization by trophic transfer.

5 Bioaccumulation

It is well known that mercury is an accumulative toxic trace metal. The mercury

concentration in an organism depends on environmental factors such as its concentration in seawater, its position in the food chain, and in particular, on the chemical species of Hg to which the organism is exposed. The uptake efficiency of inorganic Hg is much smaller (less than 10%) than for MeHg (near

100%). Phytoplankton and seaweeds constitute the firstleve l of the food chain and take up inorganic and organic mercury directly from seawater. Assuming an average concentration of Hg in seawater, the concentration factor is about 5000 -10 000. Uptake at higher trophic level may occur primarily through the food

chain. However, other mechanisms should also be investigated such as direct uptake of MMHg and DMHg through gills. This process may account for considerable uptake in open ocean fish, especially long-lived species and marine mammals Current understanding of the marine food chain is limited and many

more studies need to be done. Even though numerous papers on mercury levels in marine biota were published, typical mercury concentrations are difficult to identify^. Various organisms have differerent mercury concentrations, and biological tissues within the same organism differ considerably. Highest Hg

levels are found in long-lived fish in which its concentration frequently exceeds 500 ng/g, FW. Consumption of 200 g of such fish will result in an intake of 100 j^g of mercury (predominantly in methylated form). This represents one-half of the provisional tolerable weekly (PTWI) intake in humans^. Problems associated with Hg in the marine environment are directly related to accumulation of MMHg in the aquatic food chain; however this process in the marine environment is poorly understood. Even more, the budget for MMHg in

the oceans is rather uncertain, and much more work needs to be done to improve the accuracy of these figures, which are essential for accurate assessment of mercury impact on man and ecosystems.

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