Marine mercury pollution

Mercury is a heavy metal that cycles through the atmosphere, water, and soil in various forms to different parts of the world. Due to this natural mercury cycle, irrespective of which part of the world releases mercury it could affect an entirely different part of the world making mercury pollution a global concern. Mercury pollution is now identified as a global problem and awareness has been raised on an international action plan to minimize anthropogenic mercury emissions and clean up mercury pollution. The 2002 Global Mercury Assessment concluded that "International actions to address the global mercury problem should not be delayed". Among many environments that are under the impact of mercury pollution, the ocean is one which cannot be neglected as it has the ability to act as a "storage closet" for mercury. According to a recent model study the total anthropogenic mercury released into the ocean is estimated to be around 80,000 to 45,000 metric tons and two-thirds of this amount is estimated to be found in waters shallower than 1000m level where much consumable fish live. Mercury can bioaccumulate in marine food chains in the form of highly toxic methylmercury which can cause health risks to human seafood consumers. According to statistics, about 66% of global fish consumption comes from the ocean. Therefore, it is important to monitor and regulate oceanic mercury levels to prevent more and more mercury from reaching the human population through seafood consumption.

Sources Mercury release occurs through both natural and anthropogenic processes. Natural processes are mainly geogenic such as volcanic activities and land emissions through the soil. Volcanoes release mercury from the underground reservoirs upon eruption. Land emissions are usually observed in regions closer to plate-tectonic boundaries where soils are enriched with minerals such as cinnabar (insoluble mercury sulfide, HgS). This mercury is released, usually as a salt, either by natural weathering of the rocks or by geothermal reactions. While natural phenomena account for a certain percentage of present-day emissions, anthropogenic emissions alone have increased mercury concentration in the environment by threefold. Global Mercury Assessment 2013 states main anthropogenic sources of mercury emission are artisanal and small-scale gold mining, fossil fuel burning, and primary production of non-ferrous metals. Other sources such as cement production, consumer product waste, crematoria, contaminated sites, and the chloralkali industry also contribute in relatively small percentages. Mercury enters the ocean in different ways. Atmospheric deposition is the largest source of mercury in the oceans. Atmospheric deposition introduces three types of mercury to the ocean. Gaseous elemental mercury (Hg0) enters the ocean through air-water exchange. Inorganic mercury (Hg2+/HgII) and particle-bound mercury (Hg(P)) enter through wet and dry deposition. In addition, mercury enters the ocean via rivers, estuaries, sediments, hydrothermal vents, etc. These sources also release organic mercury compounds such as methylmercury. Once they are in the ocean they can undergo many reactions primarily grouped as; redox reactions (gain or loss of electrons), adsorption processes (binding to solid particles), methylation, and demethylation (addition or removal of a methyl group).

Sedimentary mercury Mercury can enter seas and the open ocean as a result of the down stream movement and re-deposition of contaminated sediments from urban estuaries. For example, high total Hg content up to 5 mg/kg and averaging about 2 mg/kg occur in the surface sediments and sediment cores of the tidal River Mersey, UK, due to discharge from historical industries located along the banks of the tidal river including industries such as historical chlor-alkali industry. Sediments along a 100 km stretch of the Thames Estuary have also been shown to have total Hg contents of up to 12 mg/kg and a mean of 2 mg/kg with the highest concentrations found at depth in and around London. A gradual and statistically significant decrease in sedimentary Hg content occurs in the Thames as a results of greater distance from the historical and current point-sources, sorption and in-river deposition in the mud reaches, as well as dilution by marine sands from the Southern North Sea. In contrast, sediments entering the ocean from the marsh creeks of east coast US and mangroves fringing the South China Sea generally have moderate sedimentary Hg (<0.5 mg/kg).

Submarines Many tonnes of liquid mercury reside in steel cylinders in the keels of sunken submarines around the world. Some have begun to leak and create environmental problems, for example German submarine U-864, sunk in 1945 near the coast of Norway, containing 67 tonnes of mercury.

Chemistry

… excerpt ends here. Continue reading the full article.