Mercury has always held fascination for humans. Its red sulfide ore is known by the romantic name of cinnabar. Mercury metal, which we played with as kids in the 1950s, is quicksilver. It is clearly not very toxic, or I wouldn’t be writing to you. Some salts of mercury, such as mercuric chloride, are good bacterial poisons and were once used to preserve leather. The Mad Hatter in Alice in Wonderland clearly suffered from an overdose of mercury.
When added to the environment, mercury can find its way to lake and stream sediments, where it is converted into methylmercury—an extremely toxic form. I know of one chemist who died from getting just a small drop on her skin. People have paid a lot more attention to mercury in the environment since the poisoning of more than 2000 people by methylmercury in Minamata Bay, Japan in the 1960s.
Some mercury is found in nature from natural sources. The crust of the Earth contains about 50 micrograms of mercury per kilogram of rock. It is released to streamwater during rock weathering and to the atmosphere during the eruption of volcanoes. Nearly 40 years ago, I measured mercury in the rain on Mt. Moosilauke in New Hampshire at 0.06 micrograms per liter—way below the health standard for mercury in drinking water (2.0 micrograms per liter).
There are a variety of human-derived sources of mercury in the environment, including mercury released to surface waters during the mining of gold and silver and mercury released to the atmosphere by coal-fired power plants. For the latter, the mercury is derived from the coal, where it accumulated as a component of organic sediments millions of years ago.
Unfortunately, the mercury found in freshwaters can be converted to methylmercury in their sediments and accumulate in that form in the fatty tissues of fish and the birds that eat fish. In many areas of the Northeast, freshwater fishes are not recommended for human consumption due to their mercury content, and mercury is suspected in the reproductive failure of some waterfowl, such as the Common Loon. In the U.S., some of the highest concentrations of mercury are found in the fishes in the southeastern states. It pays to consult the local fish and game department before eating freshwater fishes you may have caught on vacation.
A variety of archival sources indicate that humans have made a substantial impact on the amount of mercury in the environment. Over the past 130 years, mercury has increased by 45X in the feathers of the Ivory Gull—a scavenger at high Arctic latitudes. In many regions, higher concentrations of mercury are also found in recent tree rings, lake sediments, and the upper layers of peat bogs. On the other hand, mercury levels in Blue Fish caught off the North Carolina coast have declined during the past 40 years, indicating that some remediation of coastal waters is in progress.
Globally, about 30% of the mercury in the atmosphere seems derived from human activities. So when the EPA suggests regulating the emissions of mercury from power plants, the immediate question is how much of the mercury found in downwind environments is from the powerplants and how much is natural. And, of the mercury in the atmosphere, how much is regional and how much is involved in a global circulation that might deliver mercury from as far away as China. Within the U.S., the highest concentrations of mercury in rainfall are found in the West, but the highest deposition is found in the Southeast, where it rains more.
These are good questions and fortunately due to the hard work of environmental scientists, we have good answers to many of them. With an atmospheric life-time of more than one year, there is no doubt that some mercury deposited within the U.S. is derived from China. By one estimate only 20% of the mercury deposited in the U.S., is derived within the U.S., but there are large regional differences. The mercury content in the rain of New Hampshire is now about 0.007 micrograms per liter, or about 10X lower than what I measured in 1971. It’s nice to think that change is due to regulations that reduce mercury emissions within the U.S.
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