In a period of just five years, the levels of toxic methylmercury in Utah’s Great Salt Lake dropped by almost 90%.Scientists are still trying to explain where it’s gone.
Although the disappearance might sound like good news, due to new environmental policies or local clean-up efforts it is not. The local wildlife is still carrying around just as much toxicity as before, which means something just doesn’t add up.
Mercury levels in the Great Salt Lake duck population were so high in 2010, local officials were forced to issue a rare advisory that hunters avoid eating them for fear of poisoning.
Mercury was also found in the surrounding wetlands. This led researchers to connect these elevated levels to the methylmercury, an organic form of the element which is produced by microbes on the lake bed as they absorb the mercury from the atmosphere.
The geologist William Johnson, from the University of Utah, says: “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.”
But almost 90% of that methylmercury had vanished by 2015, even as levels in the local wildlife and wetlands remained elevated.
“If there’s a direct connection between the environment at the bottom of the lake and the mercury in the ducks, you’d think you’d see a corresponding reduction of mercury in biota [animal and plant life],” said Johnson. “We didn’t see that.”
The difference between mercury and methylmercury is that elemental mercury, the kind which can be found in old thermometers, evaporates easily into the atmosphere. In contract, methylmercury can accumulate in the sediment and be passed up the food chain.
This was happening at the Great Salt Lake in unprecedented quantities. US Geological Survey tests revealed that it had some of the highest mercury readings ever recorded in a body of water anywhere in the country.
However, it is all down to a Union Pacific railway line that divides the Great Salt Lake into a smaller northern arm and a larger southern arm. This causes the separated waters to face with very different conditions.
The northern arm receives no major river inflow, so it become far saltier than the southern arm. A number of pipes allow this denser water to flow into the southern. There it sinks to the bottom, and creates a heavier bottom layer above the lake bed sediment.
This prevents fresh oxygen to reach the deepest waters of the Great Salt Lake, so microbes on the lake bed must find something else to ‘breathe’. They first depleted the nitrate, then the iron, manganese, and finally the sulphate.
These microbes have started processing sulphate because the byproduct – sulphide which is responsible for the strong rotten egg to smell that occasionally wafts from the lake.
Also, this process results in the conversion of any elemental mercury in the lake, deposited from the surrounding atmosphere, into toxic methylmercury.
“Mercury’s really tricky,”- Johnson says. “It changes form.”
The pipes connecting the two arms of the Great Salt Lake were closed off for repair in 2013, and two years later Johnson with his team found that methylmercury in both the water and the lake bed sediment had decreased by 88 percent.
Because this source of methylmercury had been temporarily cut off, it was expected we’d see an effect on the millions of birds that migrate to the lake every year, but carcasses collected both before and after the pipe closures haven’t shown significant change in mercury levels.
The source of methylmercury in the surrounding wetlands is unknown, as is the exact mechanism behind the almost complete disappearance of mercury in the deep lake layer since 2010, explains the team.
But the investigation is ongoing, and the researchers are keen to see if a new “methylmercury factory” will appear at the bottom of the lake.