Methylmercury Found in 25 Percent of U.S. Waters Tested

A 2014 U.S. Geological Survey (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. The report summarizes selected stream studies conducted by the USGS since the late 1990s, while also drawing on scientific literature and datasets from other sources.

Copyright Michael R. Martin

Previous national mercury assessments by other agencies have focused largely on lakes. Although numerous studies of mercury in streams have been conducted at local and regional scales, recent USGS studies provide the most comprehensive assessment of streams across the United States and yield insights about the importance of watershed characteristics relative to mercury inputs. The report also summarizes information from other environments (e.g., lakes, wetlands, soil, atmosphere, glacial ice) to help readers understand how mercury varies in space and time.

Methylmercury is a toxic organic compound that bioaccumulates in the food web. The highest concentrations can be found in predator fish such as bass, mackerel, northern pike, shark, swordfish and tuna, and potentially in humans who consume large quantities of affected predator f ish. Methylmercury is created by combining inorganic mercury (released into the atmosphere by certain industrial activities) with natural processes and sources, particularly where mercury enters aquatic ecosystems and becomes methylated by anaerobic organisms in low-oxygen environments, such as wetlands.