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People depend on the environment for certain ecosystem services, such as harvesting food from the environment. As a result, mercury pollution is a public health threat, because when mercury accumulates in the environment, it contaminates ecosystems, so human food sources (e.g., fish) that depend on these ecosystems for survival are contaminated as well. Accordingly, when we harvest these foods, we contaminate our bodies with these toxins.
According to the National Atmospheric Deposition Program, “mercury is in a class of chemicals called persistent bioaccumulative toxins.” Consequently, mercury “persists in the environment for long periods by cycling back and forth between the air, water, and soil, all the while changing chemical forms, [so] . . . mercury never is removed from the environment: it just moves to other locations and eventually ends up in soils and sediments.”
There are three forms of atmospheric mercury: (1) elemental mercury (which accounts for about 95% of the total Hg in the atmosphere), (2) reactive gaseous mercury, and (3) particulate mercury. Anthropogenic sources of mercury are delivered into the environment through atmospheric deposition, and coal-fired power plants are significant contributors of mercury deposition. Once deposited into aquatic environments, bacterial activity transforms mercury into methylmercury.
According to the United States Geological Survey (USGS), “methylmercury is an organic form of mercury that is produced largely as a byproduct of natural microbial processes, when inorganic mercury is present, [and] methylmercury is an extremely toxic form of mercury.” Methylmercury is extremely toxic, because it not only bioaccumulates or is magnified through the food chain, but it’s easily absorbed by living organisms. Furthermore, methylmercury is considered a neurotoxin. In humans, fish and “seafood consumption is the main way methylmercury is taken up by humans.”
Humans aren’t the only living things impacted by mercury pollution, because other animals are certainly impacted by anthropogenic sources of mercury as well. According to EnergyWisePA.org, “Effects of methylmercury exposure on wildlife can include mortality (death), reduced fertility, slower growth and development and abnormal behavior that affect survival, depending on the level of exposure. In addition, research indicates that the endocrine system of fish, which plays an important role in fish development and reproduction, may be altered by the levels of methylmercury found in the environment.”
A recent government study found that mercury contamination in freshwater ecosystems is widespread. Furthermore, another government study determined how mercury is transformed into methylmercury in oceanic environments. That study also projected that mercury contamination of oceanic environments will continue to increase. Likewise, mercury contamination of freshwater ecosystems will increase. The contamination of any type of aquatic ecosystem with anthropogenic sources of mercury negatively impacts public health and livelihoods, thus economic opportunities. Are coal-fired power plants and cheap energy worth suffering these impacts? More from the U.S. Department of the Interior:
WASHINGTON, D.C. – Scientists detected mercury contamination in every fish sampled in 291 streams across the country, according to a U.S. Geological Survey study released today.
About a quarter of these fish were found to contain mercury at levels exceeding the criterion for the protection of people who consume average amounts of fish, established by the U.S. Environmental Protection Agency. More than two-thirds of the fish exceeded the U.S. EPA level of concern for fish-eating mammals.
“This study shows just how widespread mercury pollution has become in our air, watersheds, and many of our fish in freshwater streams,” said Secretary of the Interior Ken Salazar. “This science sends a clear message that our country must continue to confront pollution, restore our nation’s waterways, and protect the public from potential health dangers.”
Some of the highest levels of mercury in fish were found in the tea-colored or “blackwater” streams in North and South Carolina, Georgia, Florida and Louisiana — areas associated with relatively undeveloped forested watersheds containing abundant wetlands compared to the rest of the country. High levels of mercury in fish also were found in relatively undeveloped watersheds in the Northeast and the Upper Midwest. Elevated levels are noted in areas of the Western United States affected by mining. Complete findings of the USGS report, as well as additional detailed studies in selected streams, are available online.
For a national listing of fish advisories from the Environmental Protection Agency, click here.
Mercury, a neurotoxin, is one of the most serious contaminants threatening our nation’s waters. The main source of mercury to natural waters is mercury that is emitted to the atmosphere and deposited onto watersheds by precipitation. However, atmospheric mercury alone does not explain contamination in fish in our nation’s streams. Naturally occurring watershed features, like wetlands and forests, can enhance the conversion of mercury to the toxic form, methylmercury. Methylmercury is readily taken up by aquatic organisms, resulting in contamination in fish.
“This study improves our understanding of where mercury ends up in fish in freshwater streams,” said USGS scientist Barbara Scudder. “The findings are critical for decision-makers to effectively manage mercury sources and to better anticipate concentrations of mercury and methylmercury in unstudied streams in comparable environmental settings.”
The USGS studied mercury contamination in fish, bed sediment and water from 291 streams across the nation, sampled from 1998 to 2005. Atmospheric mercury is the main source to most of these streams — coal-fired power plants are the largest source of mercury emissions in the United States — but 59 of the streams also were potentially affected by gold and mercury mining. Since USGS studies targeted specific sites and fish species, the findings may not be representative of mercury levels in all types of freshwater environments across the United States.
All 50 states have mercury monitoring programs, and 48 states issued fish-consumption advisories for mercury in 2006, the most recent year of national-scale reporting to the EPA. The EPA regulates mercury emissions to air, land and water. In February 2009, the EPA announced that it intends to control air emissions of mercury from coal-fired power plants by issuing a rule under the Clean Air Act.
For a podcast regarding today’s announcement, click here.
IMAGE: How mercury cycles through an aquatic ecosystem:
According to an abstract from the study:
Mercury (Hg) was examined in top-predator fish, bed sediment, and water from streams that spanned regional and national gradients of Hg source strength and other factors thought to influence methylmercury (MeHg) bioaccumulation. Sampled settings include stream basins that were agricultural, urbanized, undeveloped (forested, grassland, shrubland, and wetland land cover), and mined (for gold and Hg). Each site was sampled one time during seasonal low flow. Predator fish were targeted for collection, and composited samples of fish (primarily skin-off fillets) were analyzed for total Hg (THg), as most of the Hg found in fish tissue (95–99 percent) is MeHg. Samples of bed sediment and stream water were analyzed for THg, MeHg, and characteristics thought to affect Hg methylation, such as loss-on-ignition (LOI, a measure of organic matter content) and acid-volatile sulfide in bed sediment, and pH, dissolved organic carbon (DOC), and dissolved sulfate in water. Fish-Hg concentrations at 27 percent of sampled sites exceeded the U.S. Environmental Protection Agency human-health criterion of 0.3 micrograms per gram wet weight. Exceedances were geographically widespread, although the study design targeted specific sites and fish species and sizes, so results do not represent a true nationwide percentage of exceedances. The highest THg concentrations in fish were from blackwater coastal-plain streams draining forests or wetlands in the eastern and southeastern United States, as well as from streams draining gold- or Hg-mined basins in the western United States (1.80 and 1.95 micrograms THg per gram wet weight, respectively). For unmined basins, length-normalized Hg concentrations in largemouth bass were significantly higher in fish from predominantly undeveloped or mixed-land-use basins compared to urban basins. Hg concentrations in largemouth bass from unmined basins were correlated positively with basin percentages of evergreen forest and also woody wetland, especially with increasing proximity of these two land-cover types to the sampling site; this underscores the greater likelihood for Hg bioaccumulation to occur in these types of settings. Increasing concentrations of MeHg in unfiltered stream water, and of bed-sediment MeHg normalized by LOI, and decreasing pH and dissolved sulfate were also important in explaining increasing Hg concentrations in largemouth bass. MeHg concentrations in bed sediment correlated positively with THg, LOI, and acid-volatile sulfide. Concentrations of MeHg in water correlated positively with DOC, ultraviolet absorbance, and THg in water, the percentage of MeHg in bed sediment, and the percentage of wetland in the basin.
Information about the images used in this blog post:
- The image showing a sign warning of mercury contamination in fish is copyright 2009 Chuck Seggelin/Sagewood Studios: www.sagewoodstudios.com.
- The bioaccumulation of Hg image was found here.
- The image showing how mercury cycles through an aquatic ecosystem was found here.
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