Widespread Mercury Contamination Across Western North America
Mercury contamination is widespread, at various levels across western North America in air, soil, sediment, plants, fish and wildlife.
An international team of scientists led by the U.S. Geological Survey, recently documented widespread mercury contamination in air, soil, sediment, plants, fish, and wildlife at various levels across western North America. They evaluated potential risk from mercury to human, fish, and wildlife health, and examined resource management activities that influence this risk.
“Mercury is widespread in the environment, and under certain conditions poses a substantial threat to environmental health and natural resource conservation,” said Collin Eagles-Smith, USGS ecologist and team lead. “We gathered decades of mercury data and research from across the West to examine patterns of mercury and methylmercury in numerous components of the western landscape. This effort takes an integrated look at where mercury occurs in western North America, how it moves through the environment, and the processes that influence its movement and transfer to aquatic food chains.”
More than 80 percent of fish consumption advisories posted in the United States and Canada are wholly or partially because of mercury. Fish consumption provides many health benefits to people, but the presence of mercury at high concentrations in fish can reduce some of those benefits. Balancing the protection of human health from mercury while also communicating health benefits associated with fish consumption requires detailed information about the distribution of mercury among fish species and across various aquatic systems.
“The movement of mercury through the western landscape - traveling between the air, ground, and water to plants, animals, and ultimately humans, is extremely complex,” said Eagles-Smith. “This series of articles helps further our understanding of the processes associated with that complexity in western North America, highlights where knowledge gaps still exist, and provides information to resource managers that will help with making informed, science-based management and regulatory decisions.”
Effective management of environmental health risks associated with mercury goes beyond controlling the sources, and could be improved by development of tools to control the production of methylmercury and its bioaccumulation through the food web, ultimately affecting animals and humans.
”This effort provides critical information on mercury pathways to humans and wildlife that government regulators, lawmakers, and the public can use to make decisions,” adds David Evers, Executive Director of Biodiversity Research Institute and co-organizer of the effort. “It builds upon the Northeastern and Great Lakes regional efforts that collected and analyzed environmental mercury data that were often separated by sample type.”
Key findings of the report include:
- Methylmercury contamination in fish and birds is common in many areas throughout the West, and climate and land cover are some important factors influencing mercury contamination and availability to animals
- Fish and birds in many areas were found to have mercury concentrations above levels that have been associated with toxic effects
- Patterns of methylmercury exposure in fish and wildlife across the West differed from patterns of inorganic mercury on the landscape
- Some ecosystems and species are more sensitive to mercury contamination, and local environmental conditions are important factors influencing the creation and transfer of methylmercury through the food web
- Forest soils typically contain more inorganic mercury than soils in semi-arid environments, yet the highest levels of methylmercury in fish and wildlife occurred in semi-arid areas
- Vegetation patterns strongly influence the amount of mercury emitted to the atmosphere from soils
- Forested areas retain mercury from the atmosphere, whereas less vegetated areas tend to release mercury to the atmosphere
- Land disturbances, such as urban development, agriculture, and wildfires, are important factors in releasing inorganic 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
Mercury and Methylmercury
Mercury, also known by its chemical symbol Hg, is a naturally-occurring metal that can pose a threat to humans, fish, and wildlife when exposed to high levels of its most toxic form, methylmercury. Methylmercury is created from inorganic mercury in aquatic ecosystems by bacteria. This is a complex process that only occurs under the right conditions for the bacteria to thrive. Therefore, the movement of inorganic mercury from the atmosphere or land to the water does not always result in equivalent levels of methylmercury in fish and wildlife unless the environmental condition is favorable for methylmercury production.
Methylmercury is easily accumulated by fish, wildlife, and humans from their diet; primarily affecting the nervous and reproductive systems, and is particularly harmful during the developmental stages of life. It increases in concentration up the food chain, reaching its highest levels in predators and long-lived species. Because methylmercury readily accumulates through the food chain, exposure patterns in fish and wildlife reflect where local conditions favor the creation of methylmercury.
Sources, Storage, Transport, and Re-release
In the West, the distribution of mercury is a reflection of the diversity of sources combined with a landscape defined by extremes in climate, land cover, and habitat type. These characteristics of the western landscape influence mercury storage, chemical transformation, and buildup through the food chain.
Mercury enters the landscape from the atmosphere, natural geologic sources, historic mining activities, and re-released mercury stored in vegetation and soils. Atmospheric mercury sources are primarily direct natural emissions, such as volcanic eruptions; direct man-made emissions, such as fossil fuel emissions; and re-release from plants and soils. Mercury from the atmosphere makes its way back to earth through precipitation, dust particles, or direct uptake by plants through their leaves.
Densely forested areas, such as those found along the Pacific coastal mountain ranges, collect substantial amounts of mercury because they receive high amounts of precipitation. The deposited mercury easily binds to vegetation and rich forest soils. Soil mercury concentrations in these forests are on average 2.5 times higher than those in dry semi-arid environments. Similarly, waterbodies located in these forests have among the highest concentrations of inorganic mercury in their sediments.
Mercury Released from Soils
Soil-bound mercury can also move in the opposite direction, from land to the atmosphere. Much of the mercury emitted from the soil is re-release from previously deposited or “old” mercury. The amount of mercury released from soils varies across the region and is dependent upon vegetation patterns, which are important because these patterns affect both soil moisture and the amount of sunlight that reaches the soil – two factors associated with mercury release from soils.
In drier regions with less plant cover, the amount of mercury deposited from the atmosphere is similar to the amount released from soils, suggesting that these areas do not store mercury. In contrast, densely forested areas receive several times more mercury through atmospheric deposition than what is re-emitted to the atmosphere. As a result, western forests tend to provide long-term storage for inorganic mercury whereas much of the mercury deposited across the vast areas of sparsely vegetated semi-arid lands throughout the West either returns back into the atmosphere or becomes available for transport to aquatic ecosystems.
Mercury Released from Wildfires
Wildfire is one of the largest sources of re-released soil mercury to the atmosphere. The amount of mercury released during a wildfire depends on the size of the burned area, the amount of mercury stored in plants and soil, and the severity of burning. High severity fires, or fires that cause greater physical change in an area, release greater amounts of mercury than low severity fires because they burn more fuel and make the soil hotter. Although high severity fires release more stored mercury into the atmosphere, lower severity fires may leave behind mercury in soils in a form that can more easily be moved to aquatic ecosystems and converted to methylmercury. With the increasing rate and severity of wildfires in the West associated with a changing climate, there could be an increase in movement of mercury that has been stored for centuries.
Legacy Mining in the West
The West has rich geologic deposits of naturally occurring mercury, as well as gold and silver, where mercury was historically used to extract these valuable elements from rock formations. Historical mining and ore processing for these metals released extensive amounts of mercury into the environment, contaminating lake and river sediments downstream of mining operations. As a result, many of the highest levels of sediment mercury concentrations across the West are associated with legacy gold, silver, and mercury mines. However, the influence of mining on downstream mercury concentrations is most noticeable in small watersheds, because the amount of mercury from mining in larger watersheds is a fraction of what is contributed by other sources and processes such as atmospheric deposition, land disturbance, and erosion of less contaminated soils.
Land Use and Development
Agriculture and urban land development are more widespread across the West than mining, and those land uses have a large influence on the amount of mercury released from soils. As a result, lakes receiving runoff from agricultural or urbanized watersheds show higher rates of mercury accumulation in their sediments than lakes in undisturbed areas. The accumulation rate of mercury in lake sediments, calculated from sediment cores dated from to 1800-2010, showed the highest rate during the last decade (2000-2010) than at any time since the industrial revolution, and approximately five times higher than during pre-industrial times (1800 to 1850).
Landscape disturbance; such as wildfire, resource extraction, and land development, is a major component to the widespread movement of inorganic mercury to aquatic sediment throughout waterbodies of the West. However, mercury levels in fish and wildlife do not always match the levels of inorganic mercury because of the requirement for inorganic mercury to be converted to methylmercury before accumulating up the food chain.
“Methylmercury production is a complex microbial process that requires specific environmental conditions,” said Mark Marvin-DiPasquale, USGS microbiologist and co-organizer of the synthesis. “Only a small amount of the inorganic mercury is available to be made into methylmercury by bacteria, and under the right conditions even this small amount can result in methylmercury levels that pose a threat to fish, aquatic birds, and human health.”
As a result, sediment inorganic mercury concentrations alone often do not accurately indicate how much mercury makes its way into the animals living in the associated environment and ultimately, humans who may consume those animals.
Managing Mercury Risk to Wildlife and Humans
Western North America supports many fish and wildlife communities, several of which are threatened by habitat loss or other factors, including exposure to methylmercury. Fish are indicators of methylmercury contamination because they are an important link in the food chain for both wildlife and humans. Fish and wildlife also are indicators of methylmercury availability over many months to years in the food chain. Mercury contamination of fish and birds is widespread across the West, but the patterns of exposure do not fully match patterns of inorganic mercury distribution in soils and sediments. Although the highest levels of inorganic mercury in soil are found in forested areas, the highest levels of methylmercury in fish and wildlife tend to occur in more arid regions of the West such as the Great Basin. Many existing guidelines and regulations around mercury focus on inorganic mercury in soils and sediments. The combination of inorganic mercury movement, methylmercury creation, and how long mercury stays in the food chain are some of the challenges to managing methylmercury risk to animals and humans.
More than half of the land, lakes, rivers, streams, and wetlands in the West are publically owned or managed, much by the federal government. Natural resource management for both conservation and resource extraction can have a particularly strong influence on how mercury is transported over land, through water, and transferred to fish, wildlife, and humans.
Water and its management is a defining characteristic of the western landscape. It is among the continent’s most complex and widespread resource management challenges and has greatly influenced land use, development, and natural resource conservation. The need to store and transport water for shared ecological, agricultural, and human needs has resulted in complex networks of dams and man-made waterways that have transformed the western landscape and dramatically changed the physical, chemical, and biological characteristics of river systems, and in some cases influenced the movement of mercury through these systems.
Wetlands, lakes, and rivers can all promote the creation of methylmercury, and seasonal flow and flood patterns of the West result in numerous locations where methylmercury can be created. These habitats are also often important environments that are critical feeding areas for many fish and wildlife species. Management of water flows and storage throughout the West can influence methylmercury creation in these aquatic habitats and can have a strong impact on the degree of mercury exposure throughout local food webs.
"We found mercury contamination of birds was common in many areas throughout western North America, some at levels above what is considered toxic to birds,” said Josh Ackerman, USGS wildlife biologist and lead author of one of the articles on bird mercury exposure. “Certain ecological characteristics, such as the type of habitat the birds live in, and their diet were important factors influencing bird mercury concentrations and their risk to mercury toxicity."
This body of work was conducted as part of the Western North America Mercury Synthesis Working Group and supported by the USGS John Wesley Powell Center for Analysis and Synthesis. The Working Group is comprised of partners from other U.S. and Canadian federal, state, and provincial agencies, as well as academic institutions and non-governmental organizations. Primary funding support was provided by the USGS, National Park Service, and U.S. Environmental Protection Agency, with additional support from the individual authors’ organizations.
Findings are found in a 2016 special issue of Science of The Total Environment: Mercury in Western North America—Spatiotemporal Patterns, Biogeochemistry, Bioaccumulation, and Risks