The U.S. Geological Survey and Fish and Wildlife Service researchers conducted "reconnaissance-phase" studies of the lake during three years since the mid-1990s. They originally were gathering information on selenium in the lake, but decided also to test the samples for mercury.
"We thought we would find some high levels of methylmercury," said David Naftz, the USGS research hydrologist who is heading the Great Salt Lake project, "but not some of the highest [the USGS] has ever found."
Concentrations of methylmercury - the element's organic and most poisonous form - exceeded 25 nanograms per liter of Great Salt Lake water. Fish consumption warnings in the Florida Everglades were posted when water there was found to have 1 nanogram per liter.
There are no fish in the Great Salt Lake, and no evidence yet that mercury from the lake is getting into the human food chain. But the brine shrimp the project scientists have studied show evidence of mercury "bioaccumulation," that is, a buildup of the toxin, in levels harmful to the lake's migratory birds.
The bird they studied is the eared grebe, which eats brine shrimp from May to December. The researchers found mercury levels in the birds' livers more than doubled during their months on the lake.
The study's preliminary findings eventually may overturn the long-held idea that areas of the lake's deep brine layer, which has no oxygen, is a kind of disposal system where toxins sink to the lake bed and become inert. Instead, the USGS study suggests the lake's peculiar chemistry actually speeds the conversion of mercury to its more toxic organic form.
"It's not a disposal, it's a factory," Naftz said.
Mercury is a highly toxic element that occurs naturally in the environment but also has been introduced through human activity. Utah's industrial and mining past has exposed the lake to mercury pollution. Coal-burning power plants, however, are the largest human-caused source of mercury pollution in the world and continue to spread mercury through the atmosphere.
Mercury collects in water, plants and animals. The National Research Council, which is part of the National Academies of Science, has determined that the population at highest risk from methylmercury exposure are the children of women who eat lots of fish and seafood. The research council estimated more than 60,000 children each year are at risk for neurological problems due to in utero exposure to methylmercury.
Though the USGS studies have not found any evidence that mercury in the Great Salt Lake has entered the human food chain, ducks and geese that feed in the lake's wetlands could be subject to the same bioaccumulation found in the eared grebes, said Fish and Wildlife researcher Bruce Waddell.
People who eat the waterfowl might be exposed to mercury, agreed Steven Schwarzbach, a research manager for the USGS at the Western Ecological Research Center in Sacramento, Calif. The Great Salt Lake study hasn't yet tested that possibility, however.
"There are some mysteries out here that are just starting to be discovered, so it's not fair to make an assessment of what's happening," said Waddell, who is collaborating with Naftz and Madison, Wis.-based USGS scientist David Krabbenhoft.
Here's what they have found so far:
l All the water samples collected exceeded mercury standards for protection of aquatic life in marine environments.
l The median mercury concentrations in brine shrimp increased seasonally.
l Mercury concentrations in eared grebes' livers more than doubled from August through December, when the birds molt and their primary food source is brine shrimp. During May, the median concentration of mercury in the birds' livers was 1 part per million. By December, the concentrations rose to about 18 ppm.
Mercury is a difficult element to measure and is toxic in very small amounts. For example, Naftz said, most heavy metals are toxic between 1 and 10 parts per billion. Selenium is toxic at 2 ppb. But mercury is toxic at concentrations measured in the parts per trillion.
That causes problems for data collectors, who often wear special clothing and must observe strict protocols. Naftz said that researchers who have silver amalgam fillings in their teeth or who smoke can foul their collection samples merely by breathing on them.
Naftz emphasized that the data collected so far are preliminary. "I don't want to sound alarms," he said. What they've turned up "definitely concerns us. But it also warrants more data collection."
Environmentalists have questioned whether brine shrimp tainted by Great Salt Lake mercury could be getting into the human food chain via farm-raised fish and shellfish.
Naftz and Waddell said their preliminary analyses don't show any evidence that could be happening. Aquaculture companies raise the brine shrimp themselves, not in the lake, from freeze-dried eggs known as cysts. Tests of the cysts failed to detect any mercury, Waddell said, but that could be due to the limitations of the detection equipment.
Environmentalists also questioned whether humans could be exposed to mercury by eating salt or other minerals extracted from the lake that are sold as nutritional supplements. That's unlikely, said James Rytuba, USGS mercury commodity specialist based in Menlo Park, Calif., because the chemical compounds of the minerals can't accommodate mercury molecules. The extraction processes would leave any mercury behind, he said.
USGS studies elsewhere have shown mercury is affecting diving ducks in California, herons and egrets in Nevada and loons in the Northeast. The studies also have shown methylmercury can be more harmful to bird embryos when selenium is also in the diet.
Selenium, a trace element essential to human health, can be nontoxic. However, many of its compounds, particularly those associated with copper mining, are extremely toxic and also bioaccumulate in animal tissues. The state Department of Environmental Quality is working to establish selenium standards for the Great Salt Lake as part of a groundwater cleanup settlement with Kennecott Copper.