Wayne Wurtsbaugh | Utah State University Since 1959 Union Pacific Railroad’s rock causeway has isolated the Great Salt Lake’s hyper-saline North Arm, the reddish water on the left, from the rest of the lake to the south. Utah and federal regulators are now weighing the railroad’s plan to construct a 150-foot breach and bridge that would allow water to travel back and forth. Without such mixing, the lake’s overseers fear the South Arm could experience swings in its salt load to the detriment of industries and tiny organisms that depend on the lake.

The breach is giving researchers a rare opportunity to test their decade-old theory that the presence of methlymercury is linked to the causeway and its effects on sediments and bacteria in the lake’s murky depths.

"We kind of had this grand experiment going on the Great Salt Lake when the culverts were closed," said Cory Angeroth, associate director of the Utah Water Science Center and a researcher on the project.

Toxic mix • Mercury is a neurotoxin that becomes especially dangerous when it combines with carbon to form methlymercury, which is much more mobile in natural ecosystems.

Methylmercury forms when naturally occurring aquatic bacteria digest inorganic mercury present in their environment, whether natural or, more likely, deposited by human activities such as coal burning.

Anaerobic bacteria — microorganisms that live in environments that lack oxygen — are especially good at making methlymercury. And that’s where the Great Salt Lake is vulnerable, Johnson said.

The lake is known to feature what scientists call a deep brine layer. The water there is so salty that it can’t mix with the lake’s upper layers, blocking oxygen from penetrating to the bottom of the lake and creating a perfect environment for methylmercury-making bacteria to thrive.

Most living organisms have no means of disposing of methlymercury once it enters their bodies. So over time the toxin builds up in fish, wildlife and potentially humans who eat either, greatly increasing the likelihood of mercury poisoning.

The Great Salt Lake is, of course, devoid of fish. But birds that live around the lake have been found to contain elevated levels of mercury in their tissues, leading to warnings that hunters should not consume waterfowl caught within the lake’s vicinity.

Migratory birds that spend time at the Great Salt Lake also have been known to pick up mercury while there, Johnson said.

Open channel • Culverts underneath the Union Pacific Railroad causeway across the lake were filled in 2011 when they became unstable and began to sink, compromising the railroad tracks above them. In exchange for permission to fill the culverts, Union Pacific agreed to cut a new opening to allow water to flow to either side of the lake. That breach was opened last December, after several months of delay.

The Union Pacific causeway essentially cuts the Great Salt Lake into two halves. Fresh water only flows into the lake on the south end, so the north side of the lake tends to be shallower and saltier.

When water from the north arm flows back into the south, that salty water sinks to the bottom of the lake, creating the deep brine layer, Johnson said.

It wasn’t just the methylmercury that all but disappeared from the lake while the culverts were sealed. The deep brine layer also vanished — a result that Angeroth said scientists did not expect to see.

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Wayne Wurtsbaugh | Utah State University Since 1959 Union Pacific Railroad’s rock causeway has isolated the Great Salt Lake’s hyper-saline North Arm, the reddish water on the left, from the rest of the lake to the south. Utah and federal regulators are now weighing the railroad’s plan to construct a 150-foot breach and bridge that would allow water to travel back and forth. Without such mixing, the lake’s overseers fear the South Arm could experience swings in its salt load to the detriment of industries and tiny organisms that depend on the lake.

Environment » December opening of culverts tests theory on how mercury in the lake becomes more poisonous.

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"The deep brine layer is not natural," he said. "It only occurs when you’ve limited mixing between the two arms. … It’s definitely driven by the way that we’ve managed the lake. If we close the causeway or get rid of it, this won’t happen. The methylmercury will go away."

Or at least, that’s the prevailing theory.

Ripe for science • With the causeway breach re-opened, Angeroth said scientists are getting a glimpse of lake ecosystem dynamics denied to prior researchers. He said they will monitor multiple locations on the lake as they watch for the deep brine layer to reform.

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