Technical guidance on normal levels of contaminants in Welsh soil : Mercury (Hg) : January 2013

Mercury (Hg) is a metallic element present in small quantities (typically 0.02 mg/kg) in the rocks of the Earth’s crust. It is a highly toxic element, although its toxicity depends on its chemical form and the route of exposure. Several forms of Hg occur naturally in the environment, the most abundant natural forms of Hg being metallic Hg, cinnabar (HgS), Hg chloride (HgCl2), and methylmercury (CH3Hg). Mercury associates strongly with organic matter in soils and is relatively immobile and persistent. The major loss pathway from soils is as gaseous mercury. Generally, atmospheric deposition is the prevalent source of background Hg entering the soil, with natural sources, particularly volcanoes, accounting for about half the background Hg input to soils, and the remainder coming from human activities, largely combustion (e.g. coal-burning power stations).