Comprehensive Assessment of Mercury in Streams Explains Major Sources, Cycling, and Effects

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A new USGS report, Mercury in the Nation's Streams—Levels, Trends, and Implications, presents a comprehensive assessment of mercury contamination in streams across the United States. It highlights the importance of environmental processes, monitoring, and control strategies for understanding and reducing stream mercury levels. This report summarizes selected stream studies conducted by the U.S. Geological Survey (USGS) since the late 1990s, while also drawing on scientific literature and datasets from other sources.

Burning coal for energy production contributes large amounts of mercury to the atmosphere

Burning coal for energy production contributes large amounts of mercury to the atmosphere. Photo Credit: Phillip J. Redman, USGS.

Worldwide, mercury inputs to aquatic ecosystems are primarily from atmospheric sources such as coal combustion. Mercury can travel long distances in the atmosphere and be deposited in watersheds in areas with no obvious source of mercury pollution.

Methylmercury (inorganic mercury that is converted to organic mercury within the ecosystem) poses a significant threat to wildlife because of its high bioavailability, its substantial bioaccumulation in food webs, and its extreme toxicity.

Methylmercury concentrations in predator fish exceeded the U.S. Environmental Protection Agency criterion for human health in about one in four of the nearly 300 U.S. streams sampled. Some of the highest fish mercury levels were found in coastal plain streams in the southeastern United States. Coastal plain streams typically drain forested watersheds that contain abundant wetlands.

Wetlands can increase the amount of inorganic mercury that is converted to methylmercury, the form that accumulates to harmful levels in fish. Elevated mercury levels also were noted in fish in areas of the western United States affected by historical gold and mercury mining. Fish mercury levels were lowest in urban streams, despite an abundance of sources of inorganic mercury. This occurs because urban streams lack conditions, such as wetlands, that are conducive to the production and bioaccumulation of methylmercury.

Eating fish is a source of mercury exposure for many people in the United States

Eating fish is a source of mercury exposure for many people in the United States. Photo Credit: Dennis A. Wentz, USGS.

Most mercury exposure in the U.S. population is from fish consumption. Fish consumption advisories exist in every State in the Nation. Methylmercury exposure from fish consumption has been associated with various adverse effects on human health, ranging from central nervous system toxicity (at extremely high levels of exposure) to diminished cardiovascular health and endocrine disruption (at lower exposure levels of exposure).

In contrast to other environmental contaminants, mercury emission reduction strategies need to consider global mercury sources in addition to domestic sources. In response to the widespread contamination of fish, mercury has been effectively removed from many products and product waste streams. Since 1990, mercury emissions in the United States have decreased by about 60 percent. However, to reduce mercury levels in fish to fully meet human health criteria, further reductions in mercury emissions are necessary. The development of a national monitoring approach will be critical to track the effectiveness of future management actions.

This research was funded by the USGS Ecosystems Mission Area’s Environmental Health Program (Contaminant Biology and Toxic Substances Hydrology) and the USGS National Water Quality Assessment Program.