The sands and clays of the overburden sediments support a large phosphate mining and processing industry in west-central Florida. A gaping sinkhole formed abruptly on June 27, 1994, within a 400 acre, 220 foot high gypsum stack at a phosphate mine. The gypsum stack is a flattopped pile of accumulated phosphogypsum a byproduct of phosphateore chemical processing. The phosphogypsum precipitates when acidic mineralized water (about pH 1.5) used in processing the ore is circulated and evaporated from the top of the continually growing stack of waste gypsum.
The waste slurry of slightly radioactive phosphogypsum results from the manufacture of phosphoric acid, a key ingredient in several forms of fertilizer.

The sinkhole likely formed from the collapse of a preexisting dissolution cavity that had developed in limestone deposits beneath the stack. Its development may have been accelerated by the aggressive chemical properties of the acidic waste slurry. Infiltration of the applied waste slurry into the underlying earth materials was unimpeded because there was no natural or engineered physical barrier immediately beneath the stack. Enlargement of cavities by dissolution and erosion combined with the increasing weight of the stack would have facilitated the sinkhole collapse. This effect may have been exacerbated by the reduction of fluidpressure support for the overburden weight due to localized ground waterlevel declines; the phosphate industry withdraws ground water from the Upper Floridan aquifer to supply water to the ore refining plant.
Before the collapse, acidic water was ponded on top of the stack to evaporate, leaving gypsum as a precipitate.
Acidic water percolated into the stack and ground-water system, leaving gypsum as a precipitate. thus accelerating development of the sinkhole.
PREVENTING SINKHOLE COLLAPSE
There are approximately 20 gypsum stacks located within the sinkhole-prone region of west-central Florida and, with the exception of new construction, all of these stacks are unlined. Because of potential environmental impacts from the phosphate industry, the State of Florida created the Phosphogypsum Management Rule to manage all aspects of phosphate chemical facilities.

All new gypsum stacks are lined at their bases to impede the infiltration of process water and have specially designed water circulation systems to prevent the escape of waste slurry. Ground water quality and water-level monitoring are also required. Efforts are being made to close all unlined stacks and reduce impacts on the underlying ground water system. All new gypsum stacks must undergo an assessment of the susceptibility to subsidence activity and ground water contamination.

Geophysical surveys are used to locate potential zones of weakness so that any cavities or preexisting breaches can be plugged or avoided.
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