Environmental Health - Toxic Substances Hydrology Program
Yellowstone is not as large a source of mercury to the atmosphere as once was thought. Although mercury occurs naturally in hot springs, its most toxic form, methylmercury, appears to be entering the food chain largely by accumulating in slimy microbial mats. Furthermore, the use of mercury isotopes holds promise for an increased understanding of the sources, pathways, and fate of geothermal mercury at Yellowstone. These are some of the findings of a group of U.S. Geological Survey (USGS) scientists and their colleagues who have been studying the dynamics of mercury in the thermal features of Yellowstone National Park, Wyoming.
For years, many scientists speculated that Yellowstone might have been one of the largest natural mercury emission sources on the planet. To test this assumption, scientists set up the USGS Mobile Atmospheric Mercury Laboratory to measure atmospheric mercury emissions around Yellowstone National Park. Instead, they found that wildfires burning outside of the park released appreciably more mercury to the atmosphere than the geothermal sources inside the park.
The first data on the chemical forms (speciation) of mercury in the geothermal springs of Yellowstone National Park confirmed that methylmercury occurs naturally in many, but not all, of the Park's hot springs. Methylmercury is a very toxic form of mercury that readily bioaccumulates in food webs, and can reach levels that pose health risks to people and animals that eat fish. The measured concentrations of total mercury and methylmercury were highly variable in both location and time.
The scientists also found that the heat-loving (thermophilic) bacteria that form the microbial mats that are found in many of Yellowstone's geothermal springs bioaccumulate methylmercury. The scientists also found that larvae feeding on the microbial mats had methylmercury concentrations two to five times higher than the microbial mat concentrations. This indicates that the microbial mats are an important source of methylmercury to Yellowstone's food web.
The scientists are using the ratio of naturally occurring mercury isotopes present in Yellowstone’s geothermal waters to trace or identify sources of mercury to the environment. In many cases, different sources of mercury will have different isotopic ratios. Measuring this ratio in an environmental sample, along with knowledge of the isotopic ratios that are characteristic of the different sources of mercury, has the potential to enable scientists to determine where mercury contamination came from.
The results of these studies have increased our understanding of the origins, transport, and fate of mercury from Yellowstone's geothermal areas. In addition, new insights have been gained on the relative contributions of natural versus human mercury sources and local versus regional mercury sources. Land managers, health professionals, and environmental regulators can use this understanding to develop sound policy regarding the potential exposure of animals and humans to methylmercury.
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