These images show the wet, broken lands between Cape Canaveral and Tampa Bay, Florida. Orlando appears as a prominent bright area in the north of the images, east of large Lake Apopka. East of Tampa Bay, phosphate mines intersperse the red vegetation as a bright, high-contrast mix of white bare earth and blue-black ponds. Phosphorus is a critical nutrient for life, especially for modern industrial agriculture, and this circle of pits, ponds and stacks east of Tampa has been the world's most productive source. ¹

The physical growth of Orlando, especially to the east and south, is apparent in these images. This growth includes Epcot, southeast of Disney World, missing in 1973 and partly completed in 1986. Disney, like other builders in the area, has to plan its construction carefully, because this land is karstic. ²

Karst is a region in the Balkans whose underlying rock is limestone, slowly dissolving in the groundwater, giving it a distinctive terrain and water cycle. "Karstic" lands comprise five to ten percent of the Earth's land surface, where oceans have retreated as they did in Florida. Million of years ago Florida was under water; calcium crystals and seashells sank to this ocean floor and over millions of years compacted into hard limestone. As the ocean dropped Florida became covered by plants and soil, and subject to rainwater.

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" which 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. ³

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 a tub with its plug pulled. You can see some good examples of karstic lakes around Disney World. They often have no streams leading in or out of them, steep sides, and a round shape.

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

The phosphate mines

We first noticed these mines in a photorevised map, not in Landsat images. Photorevision uses aerial photographs but no ground checking; the old map gets a new purple layer showing new or changed features. This is not a full revision, but it makes for great change maps. Here is a fairly typical photorevised map from the edge of Oklahoma City-- you can see in purple which houses and roads were added between 1966 and 1975. Now look at the Homeland, Florida map from 1952, photorevised in 1986. It is almost all purple! Clearly, this landscape has been radically transformed.

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. ⁴

• 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 which separates the sand, clay, and phosphate ore. The phosphate ore is shipped to another plant which processes it into fertilizer. ⁵

Since 1975, state law has required mining companies to reassemble these byproducts back into a reclaimed semblance of the premined 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 premined 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 above-ground ponds are contained by earthen walls which have occasionally burst, releasing billions of gallons of waste water, threatening water quality and human lives. ⁵

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).

Footnotes

1. John M. Guilbert and Charles F. Park, Jr., The Geology of Ore Deposits (New York: W.H. Freeman, 1986), p. 716.

2. Leo D. Handfelt and others, "Exploration of Karst Conditions in Central Florida," in N. Sitar, ed., Geotechnical Aspects of Karst Terrains: Exploration, Foundation Design and Performance, and Remedial Measures (American Society of Civil Engineers, Geotechnical Special Publication No. 14, 1988).

3. Ed Lane, Karst in Florida (Tallahassee: Fla. Geological Survey, Special Publication no. 29, 1986).

4. Jack Klein, "Gypsum Finds Ecological Concerns Stacked Against It" Tampa Bay Business Journal 9 December 1996.

5. B.R. Lewelling and R.W. Wylie, Hydrology and Water Quality of Unmined and Reclaimed Basins in Phosphate-Mining Areas, West-Central Florida (Tallahassee: USGS Water Resources Investigations Report 93-4002, 1993)

6. Michael Stachell, "Sinkholes and Stacks: Neighbors Claim Florida's Phosphate Mines are a Hazard," U.S. News and World Report 118, no. 23 (12 June 1995), p. 53-56.

Other References

Peter W. Harben and Robert L. Bates, "Phosphate Rock" in Geology of the Nonmetallics (N.Y.: Metal Bulletin Inc., 1984).

S.J. Van Kauwenbergh and others, Phosphate Deposits of Florida (Reston, Va.? USGS Bulletin 1914, 1990).

Satellite images

The 1973, 1986, and 1992 images are made from two NALC triplicates, from the North American Landscape Characterization (NALC) project, which seeks to make Landsat data available to the widest possible user community. Contact the U.S. Geological Survey (USGS) Center for Earth Resources Observation and Science (EROS) Customer Services for more information.

LM1017040007310090 and LM1017041007311890 (Landsat 1 MSS, 10 April 1973 and 28 April 1973)

LM5016040008609290 and LM5016041008610890 (Landsat 5 MSS, 2 April 1986 and 18 April 1986)

LM5016040009207790 and LM5016041009207790 (Landsat 5 MSS, 17 March 1992)

L71016040_04020000127 and L71016041_04120000127 (Landsat 7 ETM+, 27 January 2000)

1-619-294-880406-162140-2-X (SPOT 1 multispectral, 6 April 1988)

Florida mosaic: sixteen Landsat MSS scenes, from April and May of 1983-1985, except path/row 17/41 from July 1979, and 16/39 from April 1981.

Aerial photographs

Disney World: NAPP (National Aerial Photography Program) 8681-102 (5 February 1995)

Epcot: NAPP (National Aerial Photography Program) 8681-168 (5 February 1995)

Maps

U.S. Geological Survey, 1973 (1967, rev. 1973), United States General Reference, scale 1:7,500,000.

U.S. Geological Survey, 1972 (1967, rev. 1972), National Atlas: Florida, scale 1:2,000,000, Albers Equal Area projection.

U.S. Geological Survey, 1972 (1955, rev. 1972), Orlando, scale 1:250,000.

U.S. Geological Survey, 1986 (1952, photorevised 1986), Homeland, Fla., scale 1:24,000.

U.S. Geological Survey, 1975 (1966, photorevised 1975), Bethany NE, Okla., scale 1:24,000.

Florida Department of Environmental Protection, Bureau of Mine Reclamation, 2000, Integrated Habitat Network, scale approx. 1:300,000. The superimposed area of phosphate deposits on the Florida map and Landsat mosaic came from this map.

This article was released on 12 January 2001.