Finding mercury in fish is a fairly easy process.
Figuring out where it came from is not.
Coal-fired power plants like those operating across the region are easy targets when it comes to fixing blame for mercury pollution, because they do release mercury in their emissions.
But to say that coal plants are the cause of mercury levels in fish, such as those detected in Newell Lake and other South Dakota waters, is oversimplifying a complicated problem.
“It’s no secret that coal plants contribute to the big mercury pool in the atmosphere,” said Patrick Snyder, a senior environmental scientist with the state Department of Environment and Natural Resources in Pierre.
“People’s perceptions on that are not wrong. You have a big smoke stack, gases come out, and things drop from the air. But there are a lot of mercury sources.”
Mercury exists naturally in the environment. It is also released from wood-burning stoves, forest fires, other industrial plants, such as those making cement, and even volcanic eruptions.
It also can be added to the environment through improper disposal of items containing mercury.
The source of the mercury is in most cases less pertinent in contaminating fish tissue than the organic process that occurs after the heavy metal ends up in lakes streams or rivers.
Certain conditions make certain waters more likely to change mercury into methyl mercury, the organic form of the metal that can be incorporated into the environment.
And in many cases, that process is initiated or magnified when areas of land are flooded, such as when creeks or rivers are impounded or when wetlands grow into lakes.
Bitter Lake in Day County of northeastern South Dakota is an example. Once a sprawling slough, Bitter Lake has grown over the past two decades of heavy moisture into a 19,000-acre lake. It also became the poster child for mercury advisories in South Dakota.
The expanding water flooded farmland, pastures, wetlands, shelter belts and other areas of once-dry ground.
It also apparently began the methyl mercury incorporation process that led to a fish advisory for walleyes and larger northern pike in 2000.
A similar situation occured a few counties south of Bitter Lake, in Kingsbury County, where a complex of wetlands along U.S. Highway 81 had grown into prime fishing water.
In 2003, tests revealed mercury levels exceeding 1 parts per million of mercury — the current South Dakota threshold for fish advisories — in walleyes 18 inches and longer and northern pike 19 inches and longer.
The state is considering setting the limit at 0.3 ppm.
The mercury advisories on Bitter Lake and the Highway 81 lake had people worrying about eating the fish and speculating about the possible source of the mercury. The Big Stone Power Plant near Milbank became a target of that speculation. But testing in lakes closer to the power plant didn’t show a mercury problem.
“We did some fish-tissue sampling in some lakes East River near Big Stone and did not see any elevated mercury,” said Shannon Minerich, a DENR environmental scientist.
Natural lakes with more consistent shorelines tend not to register the mercury levels of impoundments or expanded wetland lakes, Minerich said. Deeper lakes also tend not to be problems, she said.
Trevor Selch, a former South Dakota State University doctoral student now working as a biologist in Montana, did research on the mercury impacts in South Dakota from 2004 to 2007. He sampled about 1,000 fish for mercury during that time and saw strong evidence of “the reservoir effect.”
“This was already in the literature. Whenever they impound a river, all of the mercury that has been on that landscape for decades and centuries gets into the aquatic environment,” Selch said.
“That process methylates that inorganic mercury, and it gets into the food chain.”
There can be variations on given waters, but the overall principle is accepted, he said.
“That’s not to say point sources don’t have an impact. They certainly do,” Selch said.
“But it’s pretty much all atmospheric and hard to trace. It’s really the limnological (lake ecology) conditions that are causing it, rather than a point source.”
Contact Kevin Woster at 394-8413 or firstname.lastname@example.org.