A "no trespassing" sign hanging on a fence surrounding part of the former Rocky Flats nuclear weapons plant near Denver, Aug. 11, 2017. Part of the site was designated a refuge after a $7 billion cleanup, but the area where plutonium was handled remains off-limits.

Dan Elliott/AP

Congressman Jared Polis is asking the federal government to keep the controversial Rocky Flats National Wildlife Refuge closed.

Fish and Wildlife said the refuge will still open Saturday.

Polis said in a letter that climate events such as major flooding in 2013 have changed the topography of the former Cold War-era nuclear weapons plant. Updated safety tests are needed before the cleaned-up refuge opens to the public, he wrote.

“Contaminant testing of the Rocky Flats National Wildlife Refuge property is more than a decade old and does not take into account flooding,” Polis said in a letter yesterday to the Department of the Interior.

The land, located 16 miles northwest of Denver, was used to produce plutonium triggers for nuclear bombs until the late 1980s. The FBI shut down the Rocky Flats plant and ordered its cleanup when it suspected the plant was breaking environmental laws. Exposure to uranium, plutonium and beryllium posed serious health risks to people in and around the plant.

Opponents don't trust that years of cleanup were enough to rid the land of contaminants.

Environmental groups have tried several legal measures to keep Rocky Flats from opening to the public. The refuge is still at the center of a lawsuit, but a federal judge rejected a request to delay the opening until the lawsuit is resolved.

Several Front Range school districts, including Denver Public Schools, have banned field trips to Rocky Flats.

The EPA, however, has ruled Rocky Flats to be safe.

“I will always choose the safety of my constituents above all else, and I think it’s best to err on the side of caution,” Polis said in the statement. “Opening the Refuge should happen after testing is completed and everything is determined to be safe.”

More than 10 miles of trails will be open to hiking, cycling and horseback riding. U.S. Fish and Wildlife say the refuge is now home to elk, deer, and the occasional bear.

Polis is running for governor of Colorado.