STOCKTON, Tooele County — The chemical weapons incinerator that has been whittling away at the nation's largest stockpile of deadly chemical agents since 1996 is being modified to handle an additional challenge: mercury.

Environmental advocates, meantime, have been asking the Army to rethink the way it addresses the mercury challenge.

Testing in 2003 revealed the mercury problem, and since then mercury has been discovered in some of the depot's mustard-filled mortar shells and in 900 of the more than 3,000 one-ton mustard agent bulk containers.

Depot spokeswoman Alaine Grieser said Wednesday that technicians conducting tests on the stockpile have found no patterns to help explain why some of the weapons and bulk containers are tainted with mercury and others are not. Material found to contain mercury has been set aside until modifications to the plant, which began with site preparation last week, are completed.

The Army's plan is to add sulphur-impregnated filters to the incinerator's existing filtration system, which scrubs the incinerator's exhaust vapors before they are vented into the atmosphere. The mercury trapped in the filters would then be disposed of as a hazardous waste. The additional filters are expected to cost $25 million, Grieser said.

The plan is for the modified Tooele Chemical Agent Disposal Facility to begin incinerating the mercury-tainted weapons and bulk containers by 2010.

In the meantime, Healthy Environment Alliance of Utah and the Chemical Weapons Working Group are preparing a resolution they hope will persuade the Army to pause for a two-day discussion about alternative disposal methods.

Christopher Thomas, HEAL policy director, said he has prepared a resolution promoting that idea, which he plans to present to the Utah Citizens Advisory Commission on Chemical Weapons Demilitarization when it meets May 15. He hopes the commission will sign off on the resolution, that asks for "fact-finding dialogue with citizen groups to address questions about the feasibility of neutralization."

The resolution is similar to one presented to the advisory commission last year.

The Army uses a neutralization process as an alternative to incineration to destroy components of the chemical weapons stockpile in other parts of the country.

"To choose another technology that would be equally effective would cost more money," Grieser said.

Army estimates compiled at the request of the advisory commission in 2007 project changing the disposal technology would add five years to the overall program life at an additional cost of nearly $1.5 billion.

Grieser said the additional time a change in technology would require would certainly keep the plant from meeting international treaty deadlines that mandate all of the stockpile be destroyed by 2012.

Just how firm that deadline is to the international community "is a real political question," Grieser said. "We're serious about meeting the deadline."

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