The U.S. Department of Energy (search) has not made significant progress in treating contaminated groundwater at the Hanford nuclear reservation (search), a federal audit concluded.

The agency has estimated that 80 square miles of Hanford's groundwater were contaminated at levels exceeding state and federal drinking water standards during decades of plutonium production for the nation's nuclear weapons arsenal.

The study released Tuesday by the Energy Department's inspector general reviewed the effectiveness of the agency's methods for water treatment. Those so-called pump-and-treat systems siphon contaminated water out of the ground, run it through filters and re-inject it.

Those systems have been "largely ineffective," the audit concluded. The department has spent more than $85 million over the past eight years and will continue to spend about $8 million annually to operate the systems, the audit said.

More than $230 million is scheduled to be spent on the surface barriers.

For 40 years, the 586-square-mile reservation in south-central Washington made plutonium for the nation's nuclear weapons, beginning with the top-secret Manhattan Project (search) to build an atomic bomb.

Today, it is the nation's most contaminated nuclear site. Cleanup costs are expected to total $50 billion to $60 billion, with the work to be finished by 2035.