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The physical growth of Orlando, Florida, especially to the east and south, is apparent in these images. Builders in the area have to plan construction carefully because this land is karstic. Karst terrain is characterized by springs, caves, sinkholes, and a unique hydrogeology that results in aquifers that are highly productive but extremely vulnerable to contamination.

The term karstic comes from Karst, a region in the Balkans whose underlying rock is limestone, which slowly dissolves in the groundwater giving it a distinctive terrain and water cycle. Karstic lands comprise 5 to 10 percent of the Earth's land surface, where oceans have retreated as they did in Florida. Millions of years ago, Florida was under water; calcium crystals and seashells sank to this ocean floor and gradually compacted into hard limestone. As the ocean dropped, Florida became covered by plants and soil, and subject to rainwater.

Map of the featured area.

As rain falls through the sky it absorbs carbon dioxide, making it slightly acidic. All stone is subject to this acid, but limestone dissolves especially rapidly, and its cracked, fractured structure lets the water seep down through it. Over many years, elaborate networks of tunnels and caves form underground, often with a honeycomb of vertical “pipes” that drain the area underground, rather than through ordinary streams and rivers draining laterally to the ocean. Karstic areas have few streams. Other features of a karstic landscape are artesian springs, underground rivers, natural bridges, caves, quicksand, and especially sinkholes.

A sinkhole forms when the roof of an underground cave collapses and the rock and soil above it drop down to fill the void. Sinkholes occasionally make the news by swallowing buildings, roads, and trucks. Since water often lies just below the ground in Florida, these sinkholes often fill with water. This is why these Landsat images show so many lakes; Florida has over 7,000 lakes larger than 10 acres and many more smaller than that. Sometimes part of a lake floor will collapse—a sinkhole under a sinkhole—and the lake will drain down into the aquifer, like pulling the plug in a bathtub. You can see some good examples of karstic lakes in these images. They often have a round shape, steep sides, and no streams leading in or out.

Disney World and Epcot are labeled in these images. Epcot, located southeast of Disney World, is missing in the 1973 image and partly completed in 1986. The expansion of Epcot can be seen throughout the rest of the series of images, along with the growth of residential areas southwest of Orlando.

The bodies of water further south, east of Tampa, look completely different, because they have a much more human origin: phosphate mining.

South of Orlando lies the world's most productive source of phosphate, a critical nutrient for modern agriculture. In these images, plants look red and phosphate mines appear as a bright, high-contrast mix of white bare earth and blue-black ponds. The images show the phosphate region expanding, as more lands were put through the cycle of mining and reclamation.

Under just the right conditions, some ocean sediments (like those forming limestone) become rich in phosphorus. Ideally, an upwelling of cold, phosphorus-rich water to the shallow waters near shore stimulates all forms of sea life, from algae to animals. Their shells and bones, plus crystals of phosphorus, concentrate phosphorus on the ocean floor. Moving water—tides and currents underwater, streams and floods above sea level—sorts the heavy phosphate pebbles from the lighter sands, further concentrating the valuable nutrient.

The central phosphate region of Florida has been strip mined since 1888. By the 1980s, it accounted for almost 30% of worldwide production, and almost three quarters of U.S. production. 93% of Tampa Bay's exports are phosphates. Almost all mined phosphate goes into crop fertilizer. Modern phosphate mining involves complete removal of the land—plants, animals, soil, water, even bedrock—and then its approximate reconstruction minus the phosphate.

The steps are as follows:

  • The area to be mined is first stripped of vegetation and the water table is lowered, typically by digging a deep trench around the area.
  • An enormous crane-like machine, dragging a giant bucket, strips away the 20–50 feet of soil and stacks it nearby in an already-mined area. Then the dragline scoops the exposed phosphate ore, mixed with sand and clay, into a pit.
  • The ore in the pit is blasted by high-pressure water jets into a milkshake-like slurry.
  • This slurry is pumped by pipeline to a processing plant that separates the sand, clay, and phosphate ore. The phosphate ore is shipped to another plant that processes it into fertilizer.

This process creates several byproducts besides fertilizer. Topsoil lies stacked by the mine. Sand has been separated from the phosphate ore and pumped back from the processing plant to the mine where it is used to restore the site when mining is complete. Fine-grained clay has also been separated and is more troublesome since it stays mixed with the slurry water and swells to three times its original size. Finally, every pound of manufactured fertilizer also creates five pounds of phosphogypsum waste.

Since 1975, state law has required mining companies to reassemble these byproducts back into a reclaimed semblance of the pre-mined landscape. This means bulldozing the piles of topsoil and sand into gentle slopes and replanting them with vegetation sufficient to hold a 25-year downpour as well as the pre-mined land could. Sometimes part of the clay is mixed with this bulldozed sand, but most clay gets pumped into settlement ponds, where over several decades it consolidates to an acceptable density, though still more swollen than before mining. Many of the water bodies visible in the Landsat images are settlement ponds. These aboveground ponds are contained by earthen walls that have occasionally burst, releasing billions of gallons of waste water, threatening water quality and human lives.

A more stubborn problem is the phosphogypsum. This byproduct of fertilizer manufacture is too low-grade to be used like mined gypsum in products such as wallboard. It is also acidic and contains low levels of carcinogens like radon. It is kept out of reclamation and piled in massive "gypsum stacks" up to 200 feet high. Possible uses for the phosphogypsum such as roadbuilding have been stymied by toxicity concerns. Even establishing plant cover on the stacks has been challenging.

Meanwhile, the stacks grow rapidly. Recent regulations require liners under new stacks, but in 1994 an existing stack of 80 million tons was struck by that old karstic hazard, a sinkhole. Fifteen stories deep, it dumped millions of cubic feet of water and gypsum into the aquifer.

You can see in these images the expansion of the mined area, its southward shift, and the progress of individual mines through the mining process. Look for this progression: lush vegetation (red), then perhaps bare earth (bright), then ponds (black if deep and clear, brighter blue if shallow and/or full of sediment), and finally reclaimed vegetation (red if lush, pink if not).

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