Earth Science: Geology, the Environment, and the Universe

Chapter 23: The Paleozoic Era

Problem of the Week

The Salt of Earth

About 260 million years ago, in the Permian Period, much of the western United States was covered by shallow, tropical seas. Large-scale barrier reefs formed in these warm, shallow seas, in areas that are now New Mexico, Texas, and northern Mexico. Barrier reefs can restrict the flow of seawater that is of normal salinity. If the flow is restricted, evaporite deposits can form.

Evaporites are mineral deposits formed when water evaporates from a body of water, leaving behind the minerals that were dissolved in the water. When this process occurs over long periods of time, thick layers of evaporite minerals may be deposited. One of these minerals is halite or common table salt (NaCl). These thick evaporite deposits are valuable mineral resources and are mined for table salt, gypsum, potassium salts, and other minerals. Table salt and the other evaporite minerals have many uses, including:

  • food seasoning (halite)

  • food preservative (halite)

  • road safety in the winter months to melt snow and ice (halite)

  • fertilizer (potassium salts)

  • building materials (gypsum)

  • for use in water softeners (sodium and potassium salts)

  • for medicinal purposes (halite)

  • industrial chemicals (as sources of sodium, potassium, calcium, magnesium, and chlorine)

These ancient deposits are also being investigated for their usefulness as possible storage sites for nuclear wastes produced in nuclear power plants.

Think About This:

Nuclear power plants produce energy by harnessing the fission reaction of radioactive elements such as uranium. Once the fuel is used, it is highly radioactive and thermally hot. Currently, spent fuel is stored temporarily (for ten or more years) in cooling ponds, usually on site at the reactor plant. From there, the spent fuel is packed into strong storage casks and stored on concrete slabs, usually on land near the plant. This waste remains dangerously radioactive for many years. Both water storage and dry storage are temporary forms of storage. Most experts throughout the world agree that the most feasible and safe method for the long-term storage of highly radioactive materials is to place them deep underground.

The Problem:

You have covered many areas in your study of Earth. You have looked at Earth from many perspectives, and you have developed an understanding of the processes that form, shape, and change the rocks of the crust and mantle.

A major halite or evaporite deposit makes a good candidate for an underground nuclear waste repository because the layers of salt are highly impermeable. This means that liquids do not easily penetrate the rock layers. Impermeable rock layers will limit groundwater from entering the storage facility and they will also prevent radioactive fluids from leaking.

If you were on a committee responsible for choosing a site for underground storage of nuclear wastes, what other factors, besides the impermeability of the rock layers, would be important? The geologic setting of the potential site will be important. Investigate not only the type of rock in the area, but also the location of faults and other geological features. Consider the environment surrounding the potential storage facility, such as nearby population, transportation routes into and out of the area, security issues, and other factors.

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