Mercury contamination widespread across western North America

An international research team has found that mercury contamination is widespread across western North America in the air, soil, lake sediments, plants, fish and wildlife. The team, led by researchers from the USGS, evaluated potential risks from mercury to the health of humans, fish and wildlife.

Mercury is a naturally occurring metal that poses a health threat to humans, fish and wildlife. Its most toxic form, methylmercury, primarily affects the nervous and reproductive systems and is particularly harmful during early development.

Inorganic mercury moves from the atmosphere and the land surface into waterways where, under the right conditions, it is converted to methylmercury by bacteria. Although methylmercury levels in water generally do not pose a direct threat to fish, wildlife or humans, its concentration increases as it moves up the food chain, reaching its highest levels in predators and long-lived species.

In North America, human exposure to methylmercury primarily occurs through the consumption of fish, which complicates public health guidance because eating fish provides numerous health benefits, the team found.

Key findings from the western North America mercury study, reported recently in a series of articles in Science of Total Environment, include:

• Contamination with methylmercury, the toxic organic form of the metallic element, in fish and birds is common in many areas across western North America.
• Fish and birds in many areas were found to contain mercury concentrations above levels considered toxic to them.
• Forest soils typically contain more inorganic mercury than soils in semi-arid environments, yet the highest levels of methylmercury in fish and wildlife tend to occur in semi-arid areas.
• Land disturbances, such as urban development, agriculture and wildfires, are important factors in releasing stored mercury from the landscape, potentially making it available for biological uptake.
• Land and water management activities can strongly influence how methylmercury is created and transferred to fish, wildlife and humans.

University of Michigan biologist Paul Drevnick led a group that compiled mercury records from 165 dated sediment cores collected from 138 natural lakes across western North America.

They found that mercury accumulation rates in western lake sediments have increased, on average, by four times from 1850 to 2000 and continue to increase today.

Atmospheric deposition from human activities, especially emissions from coal-fired power plants and artisanal gold-mining operations, is responsible for much of the mercury that ends up in western lake sediments. Other sources include industrial and municipal wastewater.

"Mercury emitted from power plants in Asia is incorporated into the hemispheric pool of atmospheric mercury and is affecting all of western North America," Drevnick said. "That is the reason why, despite local, regional and national efforts to reduce mercury emissions in North America ,we continue to observe increased mercury loading to lakes in the West."

Drevnick has also been involved in efforts to compile, analyze and interpret mercury data from the Great Lakes, a region that offers a stark contrast to the U.S. West. In the Great Lakes region, mercury levels in lake sediments peaked in the 1980s and have been declining since then.

"As far as mercury in the Great Lakes region, we are in a recovery phase," he said. "We have a good understanding of the problem here and have eliminated point sources to water bodies, such as chlor-alkali plants and pulp and paper mills that used mercury in industrial processes. Also, we have controlled emissions to the atmosphere."

• Read the Science of the Total Environment papers

Source: University of Michigan

Featured image: A forested wetland in Mount Rainier National Park, Washington. Sites in the park were part of a comprehensive new study that found mercury contamination is widespread across western North America. Image credit: Collin Eagles-Smith, USGS