It’s a mystery that has some of the planet’s best minds stumped: Just where has the methylmercury in Utah’s Great Salt Lake disappeared to? After all, it was only seven years ago that levels in surrounding waterfowl were high enough to warrant a rare human consumption advisory for ducks. Yet by 2015, nearly 90 percent of the toxic organic compound appeared to go up in smoke, though not, as it would appear, as a result of any aggressive environmental remediation. A new study in the journal Environmental Science & Technology claims, however, that the methylmercury didn’t vanish; it just transmuted into other, more volatile forms.

Great Salt Lake, Utah, mercury, toxic chemicals

To understand the story, one must first grapple with the lake’s unique geography. There’s the Union Pacific railway line, for instance, which splits the waterway into a smaller north arm and a larger south arm.

Cut off from any major river inflow, the north arm is far brinier than its southern counterpart. It’s also denser, which means that any seepage through culverts in the railroad line plunges water from the north arm to the bottom of the south one, where any oxygen is quickly depleted by decaying organic matter on the lake bed.

Related: The Great Salt Lake is drying up and it could have a global impact

Because their difference in density prevents the deep and surface waters from intermingling, fresh oxygen has little chance of infiltrating the subterranean water layers. Any microorganisms who live there have to find something else to “breathe,” such as nitrate, according to William Johnson, a geology and geophysics professor at the University of Utah.

When the nitrate is gone, the bacteria may turn to iron, manganese, or sulfate as a lifeline. Finally, they may settle on elemental mercury and convert it into methylmercury, a neurotoxin that can accumulate up the food chain.

Great Salt Lake, Utah, mercury, toxic chemicals

In 2013, however, Union Pacific shut off the railway culverts for repair, cutting off the north-to-south flow of salty water. Two years later, when Johnson and his colleagues took samples of the deep brine layer and lake-bottom sediments, they found that methylmercury levels in both had plummeted by 88 percent.

“It seems clear that the deep brine layer was a cap,” Johnson said in a statement. Once the deep brine layer was able to mix with the shallower waters, the influx of oxygen possibly drove the methylmercury into forms such as elementary mercury, which would have dissipated into the atmosphere.

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Still, questions abound. Waterfowl carcasses collected both before and after the culverts closed revealed no significant difference 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 mercury in biota,” Johnson said. “We didn’t see that.”

And now that Union Pacific has opened a new breach in the railway line, reuniting the north and south arms of the Great Salt Lake, Johnson and company are waiting to see if another “methylmercury factory will lurk at the bottom of the lake.”

+ University of Utah

Photos by Emily Allen and Bruce Tuten