USGS study shows that rain and wastewater discharges are sources
Mercury contamination in water and fish throughout Indiana has routinely exceeded levels recommended to protect people and wildlife, according to the U.S. Geological Survey (USGS). About 1 in 8 fish samples tested statewide had mercury that exceeded the recommended safety limit for human consumption. The causes include mercury in the rain and mercury going down the drain, according to a recently released federal study.
The most significant source of mercury to Indiana watersheds is fallout from the air. Much of the mercury in the air comes from human activity. In Indiana, coal-burning power plants emit more mercury to the air each year than any other human activity. In urban areas, wastewater discharge contributes a substantial portion of mercury to waterways.
These are among the key findings of a comprehensive study of mercury in the state’s watersheds during the past decade by the USGS in partnership with the Indiana Department of Environmental Management (IDEM).
“Indiana has been a national leader in understanding its mercury problems through a long-term statewide network of monitoring,” said USGS hydrologist Martin Risch, who led the study. “Actions by the IDEM provided data about mercury in fish and wastewater. Our understanding of mercury would not have been possible without their cooperation.”
During the study, scientists examined mercury in water, fish, precipitation, dry fallout and wastewater to determine the causes and effects of mercury moving through the environment. They also examined landscape characteristics, precipitation and streamflow for a total of more than 380,000 pieces of data that provide a snapshot of mercury in Indiana.
“The amount of mercury in precipitation was the main factor affecting mercury levels in the state’s watersheds,” said Risch. “But wastewater discharge can be a significant source of mercury. When wastewater is delivered to a stream from hundreds of discharge pipes, it increases mercury levels in watersheds more than was previously recognized.”
Mercury was detected in 96% of the wastewater discharge samples from public treatment facilities in this study. Mercury in wastewater samples typically exceeded criteria set to protect people and wildlife. Higher numbers of discharge pipes in a watershed were linked to higher levels of mercury in the streams.
Water draining from reservoirs in this study had significantly higher percentages of mercury converted to methylmercury than water from streams without dams. Dams can trap mercury transported by suspended particles in streams. Once the particulate mercury settles in the lake or reservoir behind the dam, natural processes change some of it to methylmercury, a toxin that accumulates in organisms throughout their lives. Methylmercury levels are amplified up the food chain and reach high levels in some sport fish and in fish that serve as food for wildlife.