Mercury levels in rainfall are rising in parts of North America, study finds

Public Release: 27-Jan-2016
Mercury levels in rainfall are rising in parts of North America, study finds
Positive trends in central regions are consistent with increased mercury emissions in Asia and long-distance transport in the upper atmosphere
University of California - Santa Cruz

An analysis of long-term trends in the amount of mercury in rainfall and other forms of precipitation in North America found recent increases at many sites, mostly in the center of the continent. At other sites, including those along the East Coast, mercury levels in rainfall have been trending steadily downward over the past 20 years.

The findings are consistent with increased emissions of mercury from coal-burning power plants in Asia and decreased emissions in North America, according to Peter Weiss-Penzias, an environmental toxicologist at UC Santa Cruz. Weiss-Penzias is first author of a paper on the findings published in the journal Science of the Total Environment (in press, available online).

Mercury is a toxic element released into the environment through a variety of human activities, including the burning of coal, as well as by natural processes. Rainfall washes mercury out of the atmosphere and into soils and surface waters. Bacteria convert elemental mercury into a more toxic form, methyl mercury, which becomes increasingly concentrated in organisms higher up the food chain. Mercury concentrations in some predatory fish are high enough to raise health concerns.

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The researchers also looked at trends in sulfate concentrations in rainfall, which is an indicator of local sources of emissions from coal combustion. While mercury and sulfate concentrations were closely correlated at East Coast monitoring sites, other sites showed a stark contrast between rising mercury concentrations and falling sulfate concentrations.

"This indicates that mercury emissions in the U.S. and Canada are not the cause for the observed upward tendencies in mercury concentrations," Weiss-Penzias said.

Mercury emissions have declined in North America and Europe for several reasons, including better emissions controls and increasing use of natural gas rather than coal as a fuel for power plants. The "scrubber" systems installed at coal-fired power plants in North America and Europe since the early 1990s to combat acid rain also reduce mercury emissions.

Emissions from Asia have been increasing, however, and are transported over long distances in the upper atmosphere. The influence of the Sierra Nevada and Rocky Mountains on weather systems results in mercury from the upper atmosphere being deposited in precipitation in western states such as Nevada and Idaho and in the central United States, Weiss-Penzias said.

The researchers also looked at trends for concentrations of gaseous mercury in the atmosphere, although less data are available than for mercury in rainfall. By merging several short-term datasets for air concentrations in North America, they found the trends were similar to the pattern for mercury in rainfall: a downward trend for the early time period (1998-2007), shifting to a flat slope for the recent period (2008-2013).

"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," Weiss-Penzias said. "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."