Sources and Forms of Mercury

Mercury reaches the waters of the Great Lakes directly, through discharges into the waters, and indirectly, through disturbances of previous mercury deposition and through atmospheric deposition. This report focuses on contributions from atmospheric sources to the Great Lakes.

Mercury can be released into the air by human activities such as metallurgical processing, municipal and medical waste incineration, and electrical power generation such as from coal combustion. It is also released to the atmosphere by various natural phenomena, including volcanic eruptions, forest fires, and the weathering of geological formations.⁴

Mercury occurs principally in three different chemical forms, or species: elemental mercury, reactive gaseous mercury, and mercury associated with particulate. Different forms of mercury have different solubility, reactivity, and toxicity, behave differently in the atmosphere and the environment, and have different impacts on the ecosystem and on human health. ⁵

Elemental mercury can persist for over a year in the atmosphere in a vapor state and, thus, can travel globally with the prevailing winds. Most mercury reaching the Great Lakes from distant sources is in this form.⁶ Elemental mercury has limited solubility in water and, as a result, is largely unavailable to fish and other living things. It can be transformed to the other forms of mercury, including the reactive form; however, this reaction proceeds very slowly.

Reactive gaseous mercury (or the ionic form of mercury) is both substantially more soluble in water and more reactive than elemental mercury. It remains in the atmosphere from one to ten days, and therefore tends to be deposited locally and regionally - from a few miles to a few hundred miles from its source. Its limited range of travel, solubility, and high reactivity contribute to its ultimate presence in biota on a regional basis.⁷

Mercury particulate is mercury bound to airborne particles. Mercury particulate can remain in the atmosphere for one to ten days - comparable to reactive gaseous mercury - and thus is deposited regionally and locally. However, it is less available to living organisms than the reactive gaseous form. ⁸