is emitted to the atmosphere results in large overall emissions from this sector. The mercury content of coal varies widely, making emissions estimates highly uncertain (UNEP, 2013).
Much of the mercury released into aquatic environments comes from ASGM. However, the latest findings suggest that even deforestation can be a source of mercury emissions through extensive erosion, which releases mercury previously held in soils. Using 2010 figures for global deforestation rates, it is estimated that around 260 tonnes of mercury may have been released into rivers that year (UNEP, 2013).
Assessing the global spread and fate of mercury is a challeng- ing task, as there are few studies available about net deposi- tion of different forms of mercury in air, water and land. For example, when mercury moves from air to water and land it is generally in an oxidized gaseous or particle form, where- as when it is re-emitted to air it has been converted back to gaseous elemental mercury. These complicated mechanisms make final calculations a challenging task.
Much of the mercury in the Arctic has been carried over long distances from human sources at lower latitudes. The main
way mercury is transported to the Arctic is by the atmosphere, which contributes slightly less than half. Oceanic transport, mainly from the Atlantic, makes up around 23 per cent, with a similar amount coming from coastal erosion. The remainder comes from rivers. Mercury reaches the Arctic on air currents within days, while on ocean currents it may take decades. The form in which mercury is released and the processes that change it from one chemical form to another are the key to determining its spread and fate. The aquatic environment is of critical importance to mercury pathways to humans and wild- life, because inorganic mercury in water is transformed into highly toxic methylmercury.
About 100 tonnes are estimated to reach the Arctic Ocean by air annually, with about the same amount from the Atlantic and Pacific Oceans, rivers and erosion combined. Recent cal- culations suggest that the water in the Arctic Ocean accumu- lates about 25 tonnes of mercury a year (AMAP, 2011). Less is known about mercury dynamics and pathways in the ocean than in the atmosphere, but about 75-90 tonnes annually are thought to leave the Arctic in ocean outflow, with about 110 tonnes deposited in Arctic Ocean shelf and deep ocean sedi- ments (AMAP, 2011).