A gypsum stack is a large, engineered storage mound built to hold phosphogypsum, a mildly radioactive waste product left over from manufacturing phosphoric acid. These stacks can rise hundreds of feet above the surrounding landscape and span thousands of acres, making them some of the largest industrial waste structures in the world. They are most common in central Florida, where the phosphate fertilizer industry is concentrated, but they exist wherever phosphate rock is processed into fertilizer.
How Phosphogypsum Is Created
Phosphoric acid is an essential ingredient in most commercial fertilizers. To produce it, manufacturers dissolve phosphate rock in sulfuric acid through what’s known as the “wet process.” For every ton of phosphoric acid produced, roughly five tons of a chalky, calcium-sulfate byproduct are left behind. That byproduct is phosphogypsum.
Chemically, phosphogypsum is mostly the same mineral found in drywall and plaster: calcium sulfate combined with water. But unlike mined gypsum, phosphogypsum carries along impurities from the original phosphate ore, including phosphorus, fluorine, organic matter, heavy metals like cadmium, and naturally occurring radioactive elements such as radium and uranium. Those impurities are what make it a regulated waste rather than a reusable building material.
What a Gypsum Stack Looks Like
The phosphogypsum is pumped onto the stack as a liquid slurry, similar to wet concrete. Over time, the water drains and the gypsum solidifies, and new layers of slurry are added on top. The result is a flat-topped hill that grows taller over years and decades of continuous operation. A large pond of acidic process water typically sits on top of the stack, contained by raised embankments of dried gypsum.
Newer stacks are built on top of engineered liners designed to keep the acidic water from seeping directly into the soil and groundwater below. Older stacks, some dating back several decades, may not have liners at all. Around the base and perimeter, systems collect runoff and seepage so it can be treated and tested before any discharge.
Why They Contain Radioactive Material
Phosphate rock naturally contains trace amounts of uranium. When that rock is dissolved in acid, the uranium-decay products, especially radium-226, concentrate in the leftover gypsum rather than in the phosphoric acid. This makes phosphogypsum measurably more radioactive than ordinary soil or rock. The radium steadily decays into radon, a colorless, odorless gas that can escape from the surface of the stack into the surrounding air.
Under the Clean Air Act, the EPA requires phosphogypsum to be managed in engineered stacks specifically to limit public exposure to radon emissions. Owners of inactive stacks must measure radon levels and demonstrate that emissions stay below the regulatory limit of 20 picocuries per square meter per second. This is the primary reason phosphogypsum cannot simply be spread on fields or used as fill material the way natural gypsum can.
Environmental Risks
The two biggest concerns with gypsum stacks are groundwater contamination and structural failure.
The process water sitting on and within the stack is highly acidic and contains elevated levels of heavy metals and radioactive isotopes. If that water reaches an underground aquifer, it can threaten drinking water supplies. Central Florida, where many of the largest stacks are located, sits on carbonate karst terrain, a type of geology where limestone slowly dissolves underground, creating voids and cavities. That makes the region prone to sinkholes.
In September 2016, a sinkhole 45 feet in diameter opened beneath a phosphogypsum stack in central Florida, tearing through the liner system at its base. An estimated 215 million gallons of contaminated waste fluid drained into the Floridan aquifer, the region’s primary source of drinking water. It was not the first time: a similar sinkhole had opened under a nearby stack in 1994. These events raise particular concern because the leaked fluids carry radionuclides that can migrate through the aquifer over time.
Even without a catastrophic failure, slow seepage from unlined older stacks is a persistent issue. Monitoring wells around active and inactive stacks track groundwater quality, but contamination plumes can take years to detect and decades to remediate.
How Stacks Are Closed
When a gypsum stack reaches the end of its useful life, closing it is a complex, multiyear engineering project. The process involves placing high-density polyethylene liners over the top of the stack as a cap. The cap prevents rainwater from soaking in, which in turn reduces acidic water from percolating out through the base.
Closure is not quick. Active closure activities alone can take several years, and long-term monitoring and maintenance continue well beyond that. The costs are substantial, and operators are expected to set aside financial assurance to cover them. According to EPA economic analyses, stacks are generally closed after reaching a certain total acreage, with planning timelines stretching decades into the future. Once capped, an inactive stack still requires ongoing radon monitoring to confirm emissions remain within legal limits.
Why the Waste Keeps Piling Up
The global phosphate fertilizer industry produces enormous quantities of phosphogypsum every year, and only a small fraction is reused. The impurities, particularly the radioactivity, make recycling difficult. In some countries, phosphogypsum is used in limited quantities for road base or soil amendment after treatment to reduce contaminant levels. In the United States, however, EPA regulations effectively restrict phosphogypsum to stack storage, with very narrow exceptions for approved research or low-radioactivity material.
The result is that gypsum stacks keep growing. Florida alone has more than a billion tons of phosphogypsum sitting in stacks, with millions of tons added each year. The stacks represent a long-term environmental liability: they require perpetual monitoring, they sit over vulnerable aquifers, and the radioactive material they contain will not decay to safe levels for centuries. For communities living near them, the stacks are a visible reminder that fertilizer production generates waste on a scale few other industries can match.