Mercury contamination in water and fish throughout Indiana has routinely
exceeded levels recommended to protect people and wildlife, with about one
in eight fish samples tested statewide contaminated by amounts exceeding the
recommended safety limit for human consumption.
Causes include mercury in the rain and mercury going down the drain,
according to a recently released study conducted over the past decade by the
U.S. Geological Survey (USGS) in partnership with the Indiana Department of
Environmental Management (IDEM).
The most significant source of mercury to Indiana watersheds is fallout from
the air, the USGS said in a statement released this week. “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.”
“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.
“The amount of mercury in precipitation was the main factor affecting
mercury levels in the state’s watersheds,” Risch said. “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 percent of the wastewater discharge samples from
public treatment facilities in this study, the USGS said. “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.”
As a result, water from the White River near Indianapolis had some of the
highest mercury concentrations and carried some of the highest mercury
amounts found anywhere statewide. The White River and Fall Creek near
Indianapolis also had high percentages of fish with mercury levels above the
The Patoka River watershed in southern Indiana, meanwhile, had the highest
rate of mercury dry deposition. Mercury concentrations measured in air
samples led scientists to estimate that more mercury was dry deposited to
this watershed in an average year than was deposited by rain. This watershed
contains the most forest land, the USGS noted, and forest canopies act as a
trap for mercury in the air.
Water draining from reservoirs in this study had significantly higher
percentages of mercury converted to methylmercury than did water from
streams without dams, the USGS said. “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 life.
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.”
The report, “Mercury in Indiana Watersheds: Retrospective for 2001-2006,” is
available on line. Printed copies may be obtained by contacting the USGS
Indiana Water Science Center at (317) 290-3333.