National Water-Quality Assessment (NAWQA) Program
· Mercury contamination is widespread. Mercury was detected in all fish sampled from 291 streams across the U.S. Concentrations in about a quarter of the fish sampled exceeded the criterion for the protection of humans who consume average amounts of fish, established by the U.S. Environmental Protection Agency (0.3 parts per million, wet weight).
· Most rivers and streams across the U.S. receive mercury predominantly via atmospheric deposition. However, atmospheric deposition of mercury alone does not explain mercury contamination in fish in our Nation’s streams. Naturally occurring watershed features, like wetlands, forests, and organic-rich soils, can enhance the conversion of mercury to the most toxic form of mercury (methylmercury). Methylmercury is a form of mercury that is readily available for uptake by aquatic organisms, and biomagnifies to high concentrations in fish.
· Some of the highest levels of mercury in fish are from 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 Upper Midwest parts of the United States, in areas with abundant wetlands.
· Elevated mercury levels in fish also are found in streams of the western U.S. that are affected by mining of mercury or gold.
· Assessments in the USGS detailed studies on eight streams in Oregon, Wisconsin, and Florida showed large differences in concentrations of mercury. However, atmospheric inputs of mercury, the primary source of mercury to these streams, vary only about 4-fold, suggesting that some of these stream ecosystems are more sensitive to atmospheric deposition of mercury than others.
· Methylmercury, which is formed in wetlands and in seasonally inundated areas, is delivered by runoff to streams; methylmercury concentrations within streams increased with increased streamflow and dissolved organic carbon. Among streams, a high density of wetlands and forests, and high concentrations of dissolved organic carbon in water were associated with high methylmercury concentrations.
· An unexpected finding was that the production of methylmercury in channel sediment within the streams themselves appears to be relatively unimportant for controlling methylmercury in stream water.
· Stream water provides methylmercury to the base of the food chain, and it is the amount of methylmercury in the water that is the primary driver of how much mercury that accumulates in top predator fish. In general, concentrations in fish increased with increasing concentrations of methylmercury in water.
· Once in the food web, methylmercury biomagnifies at a fairly consistent rate from algae to invertebrates to fish—even among diverse stream ecosystems. In the ecosystems studied, foodweb characteristics have less impact on the amount of mercury in fish than do methylmercury levels in water.
· Findings from the USGS studies improve understanding of how mercury moves through the environment—where it comes from and how it ends up in commercially and recreationally important fish. 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.