From EPA website….
Stacks
The phosphogypsum, separated from the phosphoric acid, is in the form of a solid/water mixture (slurry) which is stored in open-air storage areas known as stacks. The stacks form as the slurry containing the by-product phosphogypsum is pumped onto a disposal site. Over time the solids in the slurry build up and a stack forms. The stacks are generally built on unused or mined out land on the processing site.
As the stack grows, the phosphogypsum slurry begins to form a small pond (gypsum pond) on top of the stack. Workers dredge gypsum from the pond to build up the dike around it and the pond gradually becomes a reservoir for storing and supplying process water. A total of 63 phosphogypsum stacks were identified nationwide in 1989. They were in 12 different states, but the majority, two-thirds, were in Florida, Texas, Illinois, and Louisiana.
The surface area covered by stacks ranges from about 5 to 740 acres. The height ranges from about 10 to 200 feet. In 1989, the total surface area covered by stacks was about 8,500 acres. More than half that acreage is in Florida.
The tops of operating phosphogypsum stacks (ones that are still receiving phosphogypsum) are covered by ponds and ditches containing process water. “Beaches,” saturated land masses, protrude into the ponds. These surface features may cover up to 75 percent of the top of the stack. Other surface features include areas of loose, dry materials; access roads; and thinly crusted stack sides. (The crust thickens and hardens when the stacks become inactive and no longer receive process slurry.)