Mercury levels in parts of the western and central United States' rainwater have been increasing over the past 20 years. However, the East Coast’s rain has seen a decline in these levels. This is despite nationwide cuts in coal emissions and moving away from coal-fired power plants. So what's causing this disparity between the eastern and western states?
A study by environmental toxicologists at the University of California, Santa Cruz, has suggested that the increasing mercury levels are due to emissions from other parts of the world seeping over the Pacific. Their findings were recently published in the journal Science of the Total Environment.
The long-term study collected samples of rainwater from 19 separate sites across the United States and Canada between 1997 and 2013.
Mercury ends up in rainwater primarily through the burning of coal and industrial activities, as well as from small concentrations produced through natural processes. The researchers believe the east-west discrepancy is caused by emissions from industrializing parts of Asia – primarily China – that have been traveling across the Pacific through the upper atmosphere. Weather systems around the Sierra Nevada and the Rocky Mountains then allow the mercury to be deposited through rain in the central and western states.
"A lot less mercury is being emitted to the atmosphere in the U.S. and Canada than 20 years ago as a result of regulations, efforts by industry, and the economic realities of cheap natural gas," study author Peter Weiss-Penzias said in a statement. "In spite of that, there are other factors, including emissions from other parts of the world, that are causing an increase in the amount of mercury being deposited in certain locations in North America."
This is problematic, because bacteria in the environment can convert this elemental mercury into something far more dangerous – methyl mercury, which builds up in the food chain and can therefore represent a health hazard if humans use affected animals as a food source.
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