Keywords: Birds, Fishes, Invertebrates, Marine habitat, Freshwater habitat, Toxic contaminants, Persistent contaminants, Stormwater, Monitoring, Salish Sea Currents magazine

High levels of mercury and other toxic chemicals are showing up in seemingly remote and pristine parts of the Puget Sound watershed, the result of atmospheric deposition. Scientists talk about a “dome” of pollution hanging over urban areas, leading to a never-ending cycle of persistent compounds working their way through the air, onto the land and into the water.

Common green darners (Anax junius). Photo: Judy Gallagher (CC BY 2.0)

Common green darners (Anax junius). Photo: Judy Gallagher (CC BY 2.0)


In the Olympic Mountains of Washington state, far from industrial sources of pollution, dragonflies are accumulating significant levels of toxic mercury, much of it apparently coming out of the sky.

The Dragonfly Mercury Project, a coordinated effort in national parks across the U.S., is helping researchers understand how methyl mercury — the most injurious form — can penetrate deep into our food webs, passing from aquatic insects to fish to mammals — including humans.  

A 2008 study of 20 national parks in the western U.S. and Alaska found the highest concentrations of mercury in fish caught in Olympic National Park, despite the park’s remote location.

Mercury is just one of many compounds used in industrial, commercial and agricultural operations that ride on air currents, most coming down in the vicinity of their release. Scientists in the Puget Sound region have spent years studying how a variety of airborne contaminants contribute to pollution in Puget Sound, Lake Washington and other waterways.

Pollutants in the atmosphere can come down directly on the waters of Puget Sound, but they also can come down on land and be washed into stormwater — the leading pathway for pollution. Among the airborne chemicals of greatest concern to human and ecological health are mercury, polychlorinated biphenyls (PCBs), polybrominated diphenyl ethers (PBDEs) and polycyclic aromatic hydrocarbons (PAHs).

The mercury dilemma

Mercury is among the most well-known of these airborne pollutants. It can affect brain development in infants and children and has serious neurological effects on adults as well. Tracing its source is of special interest to scientists and regulators.

Direct deposition of mercury from the atmosphere onto the surface of Puget Sound is roughly 44 pounds per year, according to a 2011 analysis conducted for the Washington Department of Ecology. Meanwhile, stormwater runoff delivers roughly 272 pounds of mercury a year, the study says.

Since most mercury pollution from human sources gets released into the air, the researchers conducting the study realized that much of the mercury contamination in Puget Sound must first come down on land and then get washed into streams, ultimately reaching the saltwater.

Stormwater pipe emptying onto beach. Photo: Beatrice Murch (CC BY-SA 2.0)

Stormwater outfall. Photo: Beatrice Murch (CC BY-SA 2.0)

“Stormwater is the largest contributor of pollution to Puget Sound, and stormwater contains atmospheric deposition from the land,” said Jenée Colton, water quality planner for King County, “How much of the stormwater pollution is coming from the air? That’s the million-dollar question.”

Overall, stormwater has been shown to be the primary route for pollution going into Puget Sound. Many state and federal rules have been developed to reduce and filter runoff at the sources, namely residential, commercial and industrial properties. But those measures may do little to address pollution that gets into stormwater via atmospheric deposition — a quantity that remains unknown for every chemical that washes into Puget Sound.

“Today’s air issue is tomorrow’s stormwater issue,” said Rachel McCrea, Northwest Region section manager for Ecology’s Water Quality Program, noting that many pollutants can move from air to soil to water to submerged sediments. To some extent, they can also reverse that course or follow other pathways, including a return to the air.

The fate of pollution from atmospheric deposition depends largely on the landing site. Pollutants that hit the ground may become bound to soil particles, their future tied to erosion. Pollutants that come down on trees and other plants may cling to or be absorbed into the vegetation, thus delaying their travel toward Puget Sound. Pollutants that land on impervious surfaces, such as roofs and roads, may be more swiftly carried into Puget Sound with stormwater runoff.

Studies of many airborne contaminants in the Puget Sound region have revealed that, in general, heavier compounds come down in the vicinity of their release, while lighter compounds may be carried farther on air currents. But mercury, in its elemental form such as that used in thermometers, can also travel high into the atmosphere to join up with a so-called “global mercury pool” forever circling the Earth.

Depending on atmospheric conditions, mercury can enter or exit the global pool in an uncertain cyclic pattern. Not unlike the water cycle, mercury may come down during rainfall (or even dry periods), then evaporate and return to the sky — especially when the weather is warm.

Mercury in dragonflies

The Dragonfly Mercury Project, which involves volunteers across the country, is one approach to better understanding what happens to all the mercury that gets released into the air.

Each summer, retiree Elmer Bartley of Spanaway joins other volunteers at Mount Rainier National Park as they hike to backwater ponds and marshes in search of dragonfly larvae.

“We use a net to scoop up the mud and look for the larvae,” Bartley explained. “It can be a lot of work, because you need 12 samples from each site. Sometimes you don’t find any.”

The larval samples are frozen and sent off to a lab, providing valuable scientific data for the nationwide research project, now in its 10th year. For the volunteers, who come from all age groups, the outings are a learning adventure.

Volunteers collecting dragonfly nymphs in Mount Rainier National Park. Photo: NPS

National Park Service ranger Annie Runde works with a volunteer to collect dragonfly nymphs (shown at right) at Mount Rainier National Park as part of the Dragonfly Mercury Project. Photo: Jenna Mason, National Park Service

“One time, we saw a lot of dragonflies out flying around,” Bartley said. “That was remarkable timing, because adult dragonflies don’t live very long.”

While dragonfly larvae may grow and accumulate mercury in the water for up to six years, the adults live only a few weeks before they mate, lay their eggs and die.

Because fish eat insects, the levels of mercury found in dragonfly larvae have proven to be a strong indicator for mercury levels in fish and the entire food web in the area, according to Rebecca Lofgren, aquatic ecologist at Mount Rainier National Park. Dragonfly larvae are good subjects because, compared to fish, they are easier to catch, handle and test for pollutants, she said. Results so far show that mercury levels vary greatly from place to place — even in the same national park.

Mercury permeates the food web

It turns out that most of the mercury found in fish in Washington state comes from atmospheric deposition, whether the fish are caught in the waters of Puget Sound, high-mountain lakes or reservoirs in Eastern Washington, researchers say. Much of the deposited mercury comes down from the global pool, including significant quantities from coal-fired power plants in China and other Asian countries.

Local sources of mercury in the air include fossil fuel combustion, waste incineration and improper disposal of household items, including mercury thermometers and fluorescent lights. In some areas of the world, small-scale gold mining is the major source of mercury releases to the atmosphere. In fact, gold mining — mainly in South America and Africa — is greatest contributor to worldwide mercury pollution, according to the United National Global Mercury Assessment.

When elemental mercury, such as that released from smokestacks, lands on the ground, it can be taken up by bacteria and converted to methyl mercury, an organic compound that bioaccumulates in animals. Its effects can be seen not only in fish but also in fish-eating predators, such as birds and mammals, including humans.

Belted kingfisher eating a fish. Photo: Andy Morffew (CC BY 2.0)

Belted kingfisher eating a fish. Photo: Andy Morffew (CC BY 2.0)

The rate of conversion from inorganic mercury to methyl mercury varies from place to place and can be especially pronounced in ponds and marshy areas, according to Collin Eagles-Smith, a research ecologist with the U.S. Geological Survey in Corvallis, Ore.

“You can have two locations with the same exact amount of inorganic mercury deposition, and there can be a magnitude of difference for what gets into the food web,” Eagles-Smith said.

A 2008 study of 20 national parks in the western U.S. and Alaska found the highest concentrations of mercury in fish caught in Olympic National Park, despite the park’s remote location. At both Olympic and Mount Rainier national parks, safe thresholds were exceeded for some fish eaters — including otter, mink and kingfishers — based on estimates of fish consumption.

“For the most part, we are about as close as we can get to global background levels of deposition,” said Bill Baccus, physical scientist for Olympic National Park. “We don’t have anything west of us except for the Pacific Ocean.”

The western half of North America accounts for about 20 percent of the continent’s mercury emissions, yet the Pacific Coast Range and the Olympic Peninsula receive some of the highest rates of deposition in the world —largely the result of trans-Pacific transport and high rainfall, according to a 2016 report by Eagles-Smith and other experts.

Dense forests help to collect mercury and other pollutants, which attach to leaves and needles as the wind blows through the trees, Eagles-Smith explained. The pollutants can then wash off the trees or otherwise end up on the ground as leaves and needles fall and decompose. The forests thus become vast reserves of inorganic mercury.

Health effects

Methyl mercury that gets into the body, mainly through food, can lead to neurological problems, with symptoms that can include tingling in the hands and feet, muscle weakness, loss of coordination and impaired speech and hearing. Effects are especially pronounced in infants and children, causing learning disabilities and impaired brain development. Pregnant women are advised to take extra precautions to diminish their exposure, and the World Health Organization ranks mercury among the top ten chemicals of public health concern.

Because of the wide dispersal of mercury across Washington state, a statewide advisory from the Washington State Department of Health warns against eating certain species of freshwater fish known to be high in mercury. For example, nobody should be eating northern pikeminnow, according to the statewide advisory, and people should eat no more than two meals per month of largemouth or smallmouth bass.

More specific health warnings have been issued for individual lakes and rivers throughout the state.

In Puget Sound, health officials have issued detailed advisories based on species of marine fish and where the fish live. In some areas, limitations have been placed on Chinook salmon, Pacific herring, English sole and rockfish. Such advisories are worded carefully to protect people from mercury while encouraging everyone to keep eating other fish as part of a healthy diet.

Health advisories for fish species in the Lower Duwamish River and Lake Washington. Images: Washington Department of Public Health

Health advisories for fish species in the Lower Duwamish River and Lake Washington. Images: Washington State Department of Health


When it comes to traveling long distances, mercury is in a class by itself, with the elemental form of mercury being the ultimate globetrotter. But other compounds also can be carried in the air, either in the form of a gas or as minute aerosol particles or even attached to dust particles.

PCBs, like mercury, have been found at low levels in the Olympic Mountains, probably arriving on prevailing winds from across the ocean. Depending on their initial use, PCBs may or may not become airborne. When they do get into the air, most come down within a few miles of the source, according to studies.

"PCBs are not simply historical contaminants but instead have numerous ongoing, diffuse urban sources."

King County DNRP

One study of pollutants in Lake Washington found that about 70 percent of the PCBs getting into the lake came from nearby drainages, including small creeks and local stormwater systems. Thornton Creek, which contributed the most PCBs, drains an area that had the most commercial and industrial development at the time the PCB ban took effect. May Creek, which contributed the least PCBs, had the least such development, according to the King County study led by Colton.

Direct atmospheric deposition to Lake Washington constituted about 14 percent of the total PCBs, according to the study. That does not count airborne PCBs that might have landed on the ground before being transported by stormwater into Lake Washington.

Researchers found the highest amounts of airborne PCBs coming down near those drainages that contributed the greatest PCB loads via stormwater. One explanation could be that most of the airborne PCBs were deposited close to the industrial and commercial sources of PCBs. Another explanation could be that the soils and vegetation in less developed areas were absorbing and retaining the pollutants. Both explanations could play a part.

“We have a pretty good sense of what the loadings are,” Colton said. But, as with mercury, it is difficult to know how much of the pollution is coming down on land before being picked up in the flow of water.

The study suggests that “an urban plume or dome of PCBs” might be lying over Seattle and Lake Washington, in much the same way that similar plumes of pollution have been described for San Francisco; Chicago; Camden, N.J.; and Toronto, Canada.

As the story is told, PCBs from old construction materials, paints, transformers and fluorescent light fixtures can volatilize into the air before attaching to surfaces or particles on or near the ground. From there, they can be washed away by rain or carried away by wind, the new airborne fraction becoming part of the lasting plume.

“All of these processes are part of the emerging understanding that PCBs are not simply historical contaminants but instead have numerous ongoing, diffuse urban sources,” states the Lake Washington report, which calls for further studies of this PCB cycle. Future investigations could look at factors that cause PCBs to become airborne — or to come back down— such as what happens when the compounds land on various types of surfaces under different temperatures, rainfall levels and wind conditions.

In a separate study of air deposition in the Lower Duwamish/Green River watershed near Seattle, researchers found the highest deposition rates of PCBs at sampling stations nearest the urban industrial center with lower rates at more distant stations. Similar findings were noted for a host of other airborne contaminants examined in the study.

While some pollution may disperse over a relatively wide area, it is important to realize that local sources make up the vast majority of pollutants, with most landing close to their source, noted Colton, the lead author of the study.


Like PCBs, a group of toxic chemicals used as flame retardants have persisted in the Puget Sound food web for years after a ban was imposed on the most toxic forms. Polybrominated diphenyl ethers, or PBDEs, can be found in organisms ranging from plankton to killer whales.

Ecology’s 2011 study suggests that direct air deposition adds far more PBDEs to Puget Sound than stormwater, something that cannot be said of any other pollutant examined in that study.

Unlike PCBs, primary uses of PBDEs have been indoors. Designed to reduce the risk of fire, PBDEs were incorporated into materials ranging from foam and fabrics in furniture to plastic housings for televisions and other electronic equipment. Significant levels of PBDEs have been associated with indoor air pollution in homes and office buildings — places where people can breathe the contaminated air or ingest these chemicals with food.

Studies suggest that PBDEs can mimic hormones in humans and animals, disrupting thyroid function, affecting behavior and learning ability, and altering reproductive systems.

As with PCBs, there is some uncertainty surrounding all the pathways that bring PBDEs into Puget Sound. Ecology’s 2011 study suggests that direct air deposition adds far more PBDEs to Puget Sound than stormwater, something that cannot be said of any other pollutant examined in that study.

Aside from air deposition, a major pathway for PBDEs begins when people wash fabrics and clothing either made with flame retardants or containing contaminated dust from the air. The chemicals are dislodged in washing machines and carried by water through sewage-treatment systems and into Puget Sound.

Ecology’s 2011 study of air deposition concluded that the total amount of PBDEs reaching Puget Sound through all major pathways was between 62 and 119 pounds per year, with direct air deposition contributing the most at 44-56 percent, followed by sewage-treatment plants at 25-38 percent and stormwater runoff at 18 percent. Other studies, based on computer models, suggest a lower ratio of airborne PBDEs.

“PBDEs deposited on land will also be mobilized during storm events and delivered to surface waters, but in quantities lower than for direct atmospheric deposition,” according to the report. “Some of the PBDEs deposited to land are also likely to be transported in storm sewers and delivered directly to Puget Sound or indirectly by way of (sewage-treatment plants).”

Evidence that indoor air pollution can contribute to outdoor air pollution comes from findings that PBDE deposition increases during warm months —presumably because more people open their windows to air out their homes and offices during spring and summer.

As with PCBs, the study found larger amounts of PBDEs coming from industrial and commercial areas, both in direct air deposition and in stormwater runoff.

PAHs and other pollutants

Polycyclic aromatic hydrocarbons are a large group of ring-based molecules formed during the incomplete combustion of fossil fuels and other organic compounds, such as wood. PAHs are a key component in creosote, a wood preservative. As with PCBs and PBDEs, toxicity can vary greatly from one PAH to another.

More than half of the PAHs released into the environment in the Puget Sound region are the result of combustion, with wood stoves and fireplaces the largest source, followed by residential trash burning, industrial emissions and automobile exhaust, according to Ecology reports.

In Puget Sound, PAHs have long been associated with liver disease in English sole, a bottom fish used as a primary indicator of localized pollution. Other effects include reproductive impairment and immune problems. In urban bays, elevated PAH concentrations in sediments may be 10 times higher than in more remote areas.

More than half of the PAHs released into the environment in the Puget Sound region are the result of combustion, with wood stoves and fireplaces the largest source, followed by residential trash burning, industrial emissions and automobile exhaust, according to Ecology reports. About a third of the PAHs released come from creosote-treated marine pilings, railroad ties and utility poles. Smaller amounts come from petroleum spills and leaks.

The Ecology study found that groundwater flows and stormwater runoff accounted for an estimated 70 to 82 percent of the PAHs reaching Puget Sound, while direct atmospheric deposition made up 15 to 26 percent. As with other toxics, it is difficult to know how much of the PAHs in stormwater first came from the air.

In urban areas, a higher rate of PAH deposition was found to be associated with copper and lead deposition, suggesting that vehicle traffic is a major source of PAHs in those areas, the report said.

Tracking PAH pollution from multiple sources into Puget Sound proved to be difficult. What is thought to go up does not seem to equate to what comes down. Based on measured deposition rates, one could make the extreme assumption that ALL of the PAHs deposited across the Puget Sound region are carried by stormwater into Puget Sound, but it still wouldn’t come close to the total estimate of PAHs released from combustion sources.

“One explanation for the difference is that PAH loads reported for surface runoff may underestimate actual loads to Puget Sound,” the report says, “while at the same time releases from combustion sources may be largely overestimated. However, there are no clear lines of evidence to support either supposition.”

Researchers say the mystery of the disappearing emissions can only be solved with more study.

Meanwhile, based on air-deposition measurements, the 2011 study estimated that 770 pounds of arsenic, 2,400 pounds of lead and 6,000 pounds of copper are deposited directly into Puget Sound each year. Those same chemicals are coming down on land, of course, with  variable quantities of each one washing into Puget Sound via stormwater. Since the study, the Washington Legislature has limited the amount of copper used in automobile brakes, believed to be a major source of copper in Puget Sound.

Climate change

Weather conditions, including temperature, precipitation and wind, help determine the fate of persistent pollutants — such as if they evaporate, wash out of the sky or are moved along by stormwater or air currents.

Climate change in the Puget Sound region is expected to bring higher temperatures and heavier rainfall events, according to climate-change models. As a result of the higher temperatures, more pollutants could become liberated to the air from uncontained sources in soils, sediments and man-made structures.

In addition, climate change could result in stronger and more persistent winds across the Pacific Ocean, bringing more pollutants to the West Coast, according to researchers at North Carolina State University and others. Those conditions could be exacerbated by increasing emissions in Asia as the population there grows, but some experts believe an international treaty on mercury emissions could curb those potential increases.

As winds have carried pollutants to the West Coast, they also have carried pollutants to the Arctic regions, where they have been sequestered in snow and ice since the beginning of the industrial revolution. It turns out that in cold environments organic pollutants tend to break down slowly and bioaccumulate more rapidly through Arctic food webs, according to a 2016 report by Chinese and Canadian researchers.

A growing body of evidence has revealed that Arctic warming – which is twice as fast as the worldwide average — is now unleashing that stored pollution, which not only enters the local food webs but becomes part of the worldwide pool of traveling contaminants.

Climate change also increases the risk of wildfires, with two different studies estimating that the annual burned area in Western Washington could more than double over the coming years.

Wildfires in California have been shown to unleash not only pollutants from the combustion of wood and vegetation, but also a variety of toxic pollutants stored in the vegetation and soils of the forests, which have been the recipients of decades of atmospheric deposition.

“The amount of mercury released from the landscape during wildfires depends upon the area burned, the amount of mercury stored in vegetation and soil, and the burn intensity or fire severity,” says the 2016 report by Eagles-Smith and others. “Overall, mercury released from the soil is the largest contributor to forest fire emissions, followed by mercury released from duff, foliage, woody fuel, litter and branches.”

Wildfires also leave behind massive amounts of mercury-laden ash, which can be washed away by rains from unprotected slopes, converted to the more toxic methyl mercury and contaminate the food web all over again.

A recent study led by Peijia Ku of the University of North Carolina at Greensboro found that black ash from low-intensity wildfires as well as white ash from high-intensive fires still contain significant amounts of mercury. White ash tends to release its mercury over time in water environments, where it can be converted to methyl mercury, whereas organic compounds in black ash bind its mercury more tightly.

Overall, the changing climate is expected to liberate and disperse more toxic chemicals throughout the environment. Such a change will further confound those who wish to reduce the amount of toxic chemicals affecting humans and all other species.

The answer, according to many experts, is to remove toxic compounds from soils and sediments where they have accumulated while also reducing the release of pollution at every source possible. In the process of reducing air emissions, people will be also address the primary causes of climate change.

About the author: Christopher Dunagan is a senior writer at the Puget Sound Institute.

Topic editor: Joel Baker

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