Around 2010, the deep waters of Utah’s Great Salt Lake contained high levels of toxic methylmercury, measurements so outrageous they warranted a rare human consumption advisory for ducks. But by 2015, 90 percent of the deep mercury was gone in a large-scale unplanned chemistry experiment, published in Environmental Science & Technology.
A Union Pacific railway line crosses the lake, dividing it into a smaller north arm and a larger south arm, with the line drawn right at the base of the bunny-ear-like northern extensions of the lake. Because the north arm has no major river inflow, it’s much saltier than the south arm.
The difference in density between the deep and shallow waters prevented mixing, says geology and geophysics professor William Johnson, and kept fresh oxygen from infiltrating into the deeper water layers. Decaying organic matter on the lake floor sucked all the oxygen out of the briny layer, forcing microorganisms to find something else to “breathe.”
Without oxygen, some bacteria turn to fuel the chemical processes of life, transforming into iron, manganese, and finally sulfate. Residents of the Salt Lake Valley may have noticed a byproduct of the sulfate-breathing bacteria – sulfide, a stinky rotten egg smell emanating from the lake. In another side effect, the bacteria turn elemental mercury into toxic methylmercury.