The Obama administration is set to announce on Monday that it will block new uranium mining on one million acres in northern Arizona near the Grand Canyon, lobbyists and Interior Department employees who had been informed about the decision said on Friday.

The department, which in 2009 imposed an interim ban that is soon due to expire, now plans a 20-year moratorium on new mines. Western environmental groups have long sought such a measure.

In proposing an extended moratorium last year, the interior secretary, Ken Salazar, cited the potential for pollution in waterways and harm to wildlife, desert vegetation and air quality.

There is perennial dispute over whether uranium contamination detected in the area is natural or caused by humans.