Mercury mystery in state waters
Fish species aren't equally affected by the heavy metal. And some lakes feel it more.
By Mark Jaffe
The Denver Post
On an early spring morning, Robert Gong cast his line into Carter Lake even though he knew of the toxic danger beneath the surface.
The idyllic scene — calm, slate-blue waters fringed by evergreens and snow — belied the problem: a build-up of dangerous mercury in the reservoir's walleye.
The problem isn't limited to the Larimer County reservoir. About 20 percent of the Colorado lakes and reservoirs that have been tested by the state contain mercury-tainted fish.
"It seems it's everywhere," said Gong, a Loveland resident. "It scares some people, and some say they eat the fish anyway. Me, I catch and release. I'm not eating those fish."
The heavy metal, however, isn't found in fish in all lakes or all species in tainted lakes — a phenomenon in Colorado and in other parts of the country.
So scientists are now trying to unravel the mystery of why it pops up in Carter Lake walleye, but not those in Chatfield Reservoir.
"We've got some very hot fish in some, but not in all our reservoirs," said Nicole Vieira, a state Division of Wildlife aquatic toxicologist.
"If we can figure out what is at work, we might be able to manage the fish stocks to reduce mercury," she said.
At the same time, Colorado has issued regulations requiring mercury air emissions from power plants — a prime source of the pollutant — be cut 90 percent by 2018.
Advisories in every state
A dusting of mercury is falling into lakes and rivers all across the country — the Environmental Protection Agency estimates more than 112 tons of mercury emissions was generated in 2005.
Among the largest sources are power plants, cement kilns, refineries and commercial boilers, according to the EPA.
But the inorganic mercury coming out of those smokestacks would just sit on a lake bottom if not for bacteria that turn it into methylmercury — which animals along the food chain can absorb.
Every state has issued mercury health advisories on eating fish, according to the EPA.
Methylmercury poisoning can impair vision, walking, speech and hearing.
Children suffer neurological damage with just a tenth of the exposure it takes to harm adults.
A pregnant woman eating tainted fish can also can hurt her baby's growing brain and nervous system. Women of child-rearing age are also advised to limit consumption of mercury-tainted fish because it takes eight to nine months for the body to purge the toxic.
Colorado advisories to limit consumption are triggered when 0.5 parts per million of mercury or more is found in fish tissue.
"The more we learn, the more damaging mercury turns out to be to a child's brain," said Gina Solomon, a senior scientist with the Natural Resources Defense Council.
"Hot" fish vary lake to lake
While the sources of the mercury and the role bacteria play in changing it into a form that gets into the tissue of fish are known, what exactly is happening in Colorado reservoirs and lakes is still something of a mystery.
"It is a complicated process, and we are trying to break it down," said Steven Gunderson, director of water quality for the Colorado Department of Public Health and Environment.
Lake Pueblo, for example, is not far from a steel mill but has no fish advisories, Gunderson said. Brush Hollow Reservoir, 30 miles away in Penrose, has a mercury problem.
About 112 have now been tested, and 23 have fish with elevated mercury levels — from Totten Reservoir west of Durango to Horsetooth Reservoir near Fort Collins.
The "hot" fish species have varied from lake to lake and include walleye, lake trout, northern pike, largemouth and smallmouth bass, yellow perch, saugeye and wiper.
"These are all predators, top-of-the-food-chain fish, where the mercury gets concentrated," said Alisa Mast, a hydrologist with the U.S. Geological Survey.
Mast is studying the Pueblo and Brush Hollow reservoirs for the state health department, working on the theory that there may be a difference in the rate of methylmercury formation between the two.
Studies in Eastern and Midwestern lakes have indicated that water chemistry can play a role, as can water-level fluctuations — which enable terrestrial plants to grow on dried-out beds that are then flooded.
"In Colorado we are just beginning to study this problem," Mast said.
Brett Johnson, a CSU biologist, is conducting a similar study of four reservoirs for the Division of Wildlife.
Two of the reservoirs — Carter and Horsetooth — have mercury-tainted fish. Although they are physically similar and stocked with the same fish, the other two — Chatfield and Union — have no advisories.
Johnson is looking at the lakes' physical and chemical characteristics, but he is focusing on whether the nature of the food web is "driving mercury accumulation."
The CSU researchers have gathered samples of all the elements of the food web, from algae to midge larvae to crayfish to the shad and minnows that walleye prey upon.
Initial findings show that there is mercury in Chatfield and Union sediments, but not in its big fish.
The difference may be that those lakes are "more productive," generating a larger food base for the big fish, Johnson said.
"The faster the walleye grow, the less mercury they gather," Johnson said. "This may be a key."
A key is what the Division of Wildlife is looking for, division toxicologist Vieira said.
"If we can reduce mercury by the way we stock lakes, that would really help deal with the problem," she said.
That would come as welcome news to Gong, who often fishes at Carter Lake.
"I hope they can do something," he said. "Just look out there. It's a beautiful day, but knowing the fish are contaminated just ruins it."
Mark Jaffe: 303-954-1912 or email@example.com
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