The Energy Department decided in 2005 to move the 16 million tons of uranium tailings, a pasty toxic remnant of ore refining at the Atlas Corp. mill.

At the time, the department estimated the project could be complete as early as 2011.

Recently, department officials have said the project may not be complete until 2028 because there are other cleanup projects under way.

"I am concerned that the Department of Energy is unnecessarily falling behind schedule," Bennett wrote in a letter to Energy Secretary Samuel Bodman last week.

Bennett urged the secretary to hire a contractor for the first five years of the project and stick to a time frame of completing the job in seven to 10 years.

"Removing the tailings expeditiously is critical not only for my state but also for the more than 25 million downstream water users in the Colorado River Basin. I am committed to help secure the necessary funding and am anxious to see this project begin," wrote Bennett who is on the Senate panel that sets the Energy Department's budget.

The tailings pile now sits just outside Arches National Park on the banks of the Colorado River and studies have found that toxic chemicals such as ammonia are seeping into the groundwater, threatening four species of endangered fish.