The U.S. Geological Survey (USGS) released a report this week (entitled "Mercury in the Nation's Streams-Levels, Trends, and Implications"). The report summarizes stream studies conducted by USGS since the late 1990s, and also draws on scientific literature and datasets from a variety of other sources. The report assesses U.S. streams in what the agency calls a "comprehensive, multimedia" manner, meaning that it was the most extensive, exhaustive and complete study ever conducted on the topic of mercury in streams at the national scale, and included an assessment of mercury in multiple environmental media. The approach provides insights about the importance of watershed characteristics relative to mercury inputs to those streams. High mercury concentrations were found in Eastern U.S. soils, generally reflecting greater rates of atmospheric mercury deposition in the region. However, those relatively high soil mercury concentrations did not translate to high fish methylmercury concentrations, which were typically highest in streams draining forest or wetland-dominated basins in the Southeastern U.S. and in streams draining basins that had been mined for mercury or gold in the West.

The four major findings in the report include: (1) methylmercury concentrations in fish exceed the U.S. Environmental Protection Agency's criterion for the protection of human health in about one in four streams (0.3 mg/kg); (2) wetlands increase the amount of inorganic mercury converted to methylmercury (the form that bioaccumulates in fish); (3) existing mercury monitoring programs lack the design elements and data to link methylmercury levels in fish to causal mercury sources; and (4) mercury emission reduction strategies need to consider global mercury sources in addition to domestic sources. Click here to see the USGS report summary and overview, and to link to the report, itself.

For more information, contact Mark Gorman, Policy Analyst at the Northeast-Midwest Institute.