The industrial use of mercury in the United States has been dramatically reduced in recent years, with releases decreasing more than 75 percent between 1980 and 1997.[2] Mercury deposition has likewise decreased substantially.[3] This has been achieved through a combination of phasing out some uses, improved control technologies and product recycling. For example, the mining of mercury in the United States ended in 1991, thereby eliminating emissions from the milling and roasting of ores. The use of mercury in fungicides and biocides has also been reduced, and the removal of mercury from paints in 1990 and 1991 cut mercury emissions by approximately 140 tons per year.[4]

The mercury emissions that remain today largely come from combustion, in which mercury is a trace element of the fuel or feedstock such as coal. These sources are undergoing control. Federal emissions regulations governing municipal, medical and hazardous waste incineration were issued in the 1990s.

The amount of mercury currently released worldwide each year is estimated to be between 4,000 tons and 7,000 tons. Total emissions are produced in roughly equal measure by "natural" sources such as volcanic eruptions; human activity such as industrial processes; and the re-release of mercury from past deposits.

In the United States, the volume of mercury releases that are related to human activity (anthropogenic) was estimated by the EPA to be 115 tons in 2001. Current anthropogenic releases in Michigan are estimated to be 2.3 tons per year.

As noted earlier, not all releases of mercury result in depositions on land and water near the source of emissions. But a precise measurement of actual deposits from local releases is virtually impossible because of the complex chemical and physical interactions that affect deposition patterns. Consequently, scientists rely on models of the atmosphere to estimate the amount of deposition that results from various releases. Unfortunately, it is difficult at present to test the accuracy of these models because there are very few monitors that measure actual "wet" depositions (precipitation), and no monitoring of "dry" depositions (when gases or particles stick to surfaces).

There is wide agreement, however, that Michigan sources contribute only a small percentage of the mercury deposited in the state. The firm Atmospheric and Environmental Research Inc. conducted a modeling study in 2004 to determine the sources of mercury depositions in Michigan. Based on measurements of deposits at the Sleeping Bear Dunes National Lakeshore, the AER researchers determined that 30 percent of the mercury deposited there originated from anthropogenic sources throughout North America.[5] Emissions from Asia as well as natural sources contributed about 25 percent each, while the balance originated in Europe, Africa, and South and Central America. The myriad sources of mercury depositions must be taken into account when crafting regulatory controls.

Michigan power plants contribute only a fraction of the mercury deposited in the state. In recent years, coal-fired power plants in Michigan have released an estimated 1.25 tons to 1.55 tons of mercury annually.[6] But only about 2 percent of the four tons of mercury deposited annually in Michigan is the result of emissions from in-state utilities.

The AER researchers also calculated that Michigan utilities only contribute between 0.5 percent and 3 percent of the mercury deposits over the entire Great Lakes. A large portion of the in-state utility releases are gaseous elemental mercury that is transported downwind.