April 25 (UPI) -- Several years ago, mercury levels in Great Salt Lake waterfowl were high enough to warrant a human consumption advisory for ducks -- a rarity. Levels of toxic methylmercury were also high.
Today, mercury levels in wetlands and local wildlife remain elevated, but the methylmercury is mostly gone.
As explained in a study, researchers aren't exactly sure where the methylmercury went or what's to blame for elevated mercury levels in local wetlands.
The Great Salt Lake is dissected by a railway. The line divides the lake into two parts. The north arm of the lake has no natural inflow, and exchange of water between the north and south arms is limited to a pair of culverts built into the causeway.
Because the north arm is much saltier, water flowing into the south arm sinks to the bottom of the lake. The dense, briny water discourages mixing, limiting the amount of oxygen available at the bottom of the lake. Decaying plant matter eats up what little oxygen there is, forcing microorganisms to look for alternate sources of food.
Some microbes take advantage of resident sulfate. In addition to emitting sulfate, these microbes also convert mercury into methylmercury. In 2011, scientists measured an uptick in mercury and methylmercury levels at the bottom of the lake and among waterfowl living in nearby wetlands.
"That created this expectation that there's a link between these high concentrations of methylmercury down in the deep brine layer and the elevated concentrations in waterfowl in the wetlands adjacent to the lake," William Johnson, a geology and geophysics professor at the University of Utah, said in a news release.
In 2013, the railroad sealed off the culverts during a rehab construction project, isolating the lake's two halves. When researchers tested mercury levels in 2015, they found the abundance of the toxic metal had decreased 88 percent.
In their new paper -- published this week in the journal Environmental Science and Technology -- scientists hypothesize that without a constant inflow of dense, briny water, layers in the south arm of the lake finally began to mix. The influx of oxygen triggered a variety of chemical reactions which converted the methylmercury into other forms.
Mystery solved? Not exactly.
Duck carcasses tested before and after the culvert closures showed no change in mercury levels.
"If there's a direct connection between the environment at the bottom of the lake and the Hg in the ducks, you'd think you'd see a corresponding reduction of Hg in biota," Johnson says. "We didn't see that."