Fallen autumn leaves transfer as much, if not more, hazardous
mercury from the atmosphere to the environment as does
precipitation each year, according to recent U.S. Geological Survey
Mercury is an environmental contaminant that accumulates in fish
and food webs and poses a health risk to humans and wildlife.
Precipitation is a major avenue by which mercury is transferred
from the atmosphere into the environment, but new studies by the
USGS and partners show that litterfall-the leaves and needles that
drop to the forest floor each year-delivers at least as much
mercury to eastern U.S. ecosystems as precipitation, and
precipitation has been increasing in the Great Lakes region.
“Before these studies, we didn’t know the extent of litterfall
as a mercury pathway in different types of forests across the
eastern U.S.,” said USGS research hydrologist Martin Risch. “Our
research found that annual amounts of mercury deposited in autumn
litterfall from deciduous forests were equal to or exceeded the
annual amounts deposited in precipitation.”
Most of the mercury that eventually ends up in fish and food
webs comes from the air, and much of the mercury in the air comes
from human sources such as coal-fired power plants, industrial
boilers, cement manufacturing, and incinerators. Forest canopies
naturally remove mercury from the air and incorporate the mercury
into and onto the leaves and needles of trees.
USGS scientists researched mercury levels in litterfall from
forests over a three-year period in 15 eastern U.S. states. When
they compared the results to those from a separate study of mercury
in precipitation within the Great Lakes region, they found similar
geographic patterns for mercury in litterfall and mercury in
precipitation: Both types of mercury deposition were generally high
in the same areas and low in the same areas.
“The similar geographic patterns indicate that the same mercury
emissions sources affecting mercury levels in precipitation in an
area also may affect mercury levels in forests and litterfall in
that same area,” Risch said.
Furthermore, the precipitation study found no improvement in the
amount of atmospheric mercury deposited by precipitation in the
Great Lakes region over a 7-year period, and found that the amount
of precipitation in the region had increased during this time. This
precipitation study covers a time period that precedes new
regulations by the U.S. Environmental Protection Agency to reduce
mercury emissions in the U.S.