That finding, based on a two-year sampling of about 6,400 bulk containers of the deadly chemical cocktail, has spurred plans for a new furnace filtration system that Army officials say will ensure "maximum protection of the Utah environment."
Others aren't so sure.
Craig Williams, of the Kentucky-based Chemical Weapons Working Group, said he has repeatedly asked the Army to provide data proving that the proposed filters will eliminate mercury from stack emissions. So far, he said, the Army has not done so.
"We've asked them to bring forward evidence and data from operations such as what they are proposing to do, to demonstrate that this is a proven technology," Williams said. "But there's no data."
Deseret officials disagree. They say tests on the filters were conducted in the fall of 2007.
"A thousand hours of filter testing at the University of South Dakota energy and environmental research center demonstrated that mercury wouldn't even make it through the first of the four beds of the Pollution Abatement System Filtration System," said Deseret spokeswoman Alaine Griese.
Williams said he is further concerned that the sampling methodology was flawed. He said the tests were conducted on the liquid contents of the containers - not the semi-solid "heels" of coagulated agent at the bottom of many of the containers.
"You cannot compare the concentration of mercury in a semi-solid material to the concentration in the liquid," Williams said. "The molecular weight of the material is going to vary significantly."
In a statement, Deseret project manager Ted Ryba said the Army now has the data it needs to proceed.
"Everything we did was based on a need to know what was inside those bulk containers," Ryba said. "Now we know. It is information we need to help us complete our mission of safely eliminating the entire DCD chemical weapons stockpile."
It's unclear, however, how much the additional measures will cost or how far the new plan will set back the timeline for destruction.
The facility, along with several others like it across the country, is already well past a 2007 international Chemical Weapons Convention deadline for the complete destruction of its stockpile. Depot officials say they plan to be done with operations in Utah by 2011
But critics like Christopher Thomas, policy director for the Healthy Environment Alliance of Utah, say the military shouldn't risk the health of Utah residents for the sake of meeting a deadline that has already passed many times over.
For years, Thomas and others have been pushing the Army to destroy its mustard through a process of neutralization, which separates and eliminates dangerous chemicals using hot water. That process is proven, critics say - it was used in Maryland to successfully dispose of tons of mustard agent.
"The Army already has a proven technology to destroy the chemical agent without burning it," Thomas said.
He noted that Utah already "has some of the worst mercury contamination in the country and because no filter is 100 percent effective, burning this agent will only worsen the problem."
