The radiation issue with phosphogypsum is mostly the Thorium.

Which isn't much of an issue really.

The environmental issues really come from the chemical properties of the material. As the quote goes; radiation is fleeting, arsenic is forever.

Not that phosphogypsum has elevated levels of arsenic, but it does have phosphates that cause Eutrophication. Which kills lakes and waterways.

Then there's the Thorium and Uranium. Those are radioactive, but are also heavy metals and cause heavy metal poisoning. Thorium doesn't have a common water soluble oxide, but Uranium sure does. So that's two things that leech out of the phosphogypsum...