At a second sample site further downstream, he found a high concentration of another type of mercury, monomethylmercury (MMHg), which accumulates in fish at toxic levels and can be detrimental to human health.
"In some cases, 100 percent of the mercury present at that location was MMHg," he said. This form of mercury also easily moves through the water and can accumulate in ponds and the ocean, where fish and other animals absorb it.
He quickly adds that the level of MMHg is not high enough to make the water dangerous to drink. However, the concentration would be even lower if wastewater had not been disposed into the ground in the first place.
"This should make us all think twice about what we dump into the ground. Adding more nitrogen into the ground through wastewater, and even fertilizers for our agricultural fields and golf courses, offers a potential for mercury to accumulate and move through the aquifer to our ponds, lakes, and the ocean. That's something I don't think people are really thinking about," he said.
Lamborg looked further into the chemical process occurring at the second site and found something even more surprising. He observed a chemical process where microbes use organic carbon with nitrate to break down organic matter, a process called denitrification. While previous studies have shown low levels of MMHg where denitrification occurs, this site exhibits high levels of MMHg occurring from denitrification.
"This kind of thing where you see denitrification resulting in the methylation of mercury has never been observed before," he said.
Lamborg is pursuing funding to support the next steps in this research to illuminate what is triggering these chemical changes in the mercury.
Accumulating out of sight
Curiously, there is even more mercury in the plume than was originall
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Woods Hole Oceanographic Institution