The Great Salt Lake in Utah is the largest inland body of salt water in the Western Hemisphere. In addition to heavy amounts of salt and minerals, the lake has high concentration of poisonous methylmercury — or at least that was the case until recently.

In 2010, the levels of methylmercury at the bottom of the lake and the surrounding wetlands were high enough to warrant an advisory against duck consumption. The lake was monitored over time by geoscientists and wildlife officials, and by 2015, they noticed a strange and puzzling change: The amount of methylmercury at the depths of the lake had decreased by nearly 90 percent.

While it would be nice to think that the reduction was due to tough efforts to clean up the environment, a recent study published in Environmental Science & Technology suggests that the decline could be the result of a happy accident involving the alteration of a Union Pacific railway line in 2013, reports Phys.org.

How the methylmercury showed up

A map of the Union Pacific Rail Road causeway splitting the upper half of the Great Salt Lake (at left) from the lower half. A map of the Union Pacific Rail Road causeway splitting the upper half of the Great Salt Lake (at left) from the lower half. (Photo: Beneathtimp/Wikimedia Commons)

In the 1950s Union Pacific built a railway that crosses through the Great Salt Lake. The railway divides the lake into a smaller north arm (Gunnison Bay) and a larger south arm (Gilbert Bay). The northern half is much saltier than the southern half because there's no major river inflow. This makes the northern half much denser, too.

Two culverts — tunnels that allow water to flow under structures such as railways — allowed the northern arm to flow into the southern arm. The higher density of the northern arm caused its salty water to sink to the bottom of the south arm, meaning the deep waters and shallow waters were unable to evenly mix.

Because the water layers were unable to properly mix, there wasn’t a way for fresh oxygen to reach the deeper layers of the lake. With a limited amount of oxygen available at the bottom and briny (salty) layer of the lake, the microorganisms that had lived there had to turn to different sources to help them breathe, so to speak.

In instances where microorganisms like bacteria need to find oxygen alternatives under deep water, they may look to feed off of nitrate, iron, manganese and, once all the options are exhausted, sulfate. The sulfate-breathing bacteria is what creates sulfide, the compound that creates the unpleasant odor of rotten eggs that arises from the lake.

Another side effect of a lack of oxygen (this is the really important one) is that its presence turns the elemental mercury that’s already in the lake into toxic methylmercury.

"Mercury’s really tricky," William Johnson, a geology and geophysics professor at Utah University and one of the authors of the study, told Phys.org. "It changes form."

Elemental mercury (what you would find in old thermometers) easily evaporates and attaches itself to dust particles in the air. When microorganisms in the water no longer have access to oxygen — such as the case with the Great Salt Lake — it converts mercury in the lake into methylmercury.

How it might have disappeared

In 2013, the railways culverts were closed down for repair. In 2015, when Johnson and his colleagues examined the sediment at the bottom of the lake and the deep brine layer, they found that the levels of methylmercury had dropped dramatically and had almost disappeared completely.

"It seems clear that the deep brine layer was a cap," says Johnson.

Johnson and his colleagues think the closing of the culverts allowed the deeper brine layer and the overlapping water on top to mix evenly. Now, without the heavy and salty water inflow of the north arm sinking into the south arm, oxygen reached the bottom of the lake.

Still a mystery

As far as the correlation between the methylmercury levels in the wetlands, the ducks and the exact ways in which the methylmercury disappeared — that’s still a mystery.

"If there's a direct connection between the environment at the bottom of the lake and the Hg [mercury] in the ducks, you'd think you'd see a corresponding reduction of Hg in biota [the animals that inhabit the surrounding area]," says Johnson. "We didn't see that."

In 2016, Union Pacific reopened the culvert. It’s going to take some more time and research to know if the culvert was the true culprit in the mystery of the disappearing mercury.