Radioactivity was discovered near the turn of the twentieth century through the work of Wilhelm Röentgen (1895, discovers X-rays), Antoine Becquerel (1896, discovers radioactivity), Marie and Pierre Curie (1898, isolates polonium and radium), and Ernest Rutherford (1899 and following years, identifies alpha, beta, and gamma radiation). Since that time, use of radioactive materials has made significant impacts in such areas as biology, chemistry, medicine, energy production, and nuclear weapons production.
While use of radioactive materials has been essential to many recent advances in these fields, use of radioactive materials has also resulted in generation of various waste materials that have the potential for impact on human health and the environment. Indeed, one of the major concerns with the use of radioactive materials is their potential release to the natural environment. Radionuclides may be introduced to surface water and groundwater resources from both natural and human (anthropogenic) sources.
Radioactivity and Types of Radiation
The term "radioactive" means that certain isotopes of some chemical elements have an unstable nucleus that will spontaneously decay with the concurrent emission of ionizing radiation. The ionizing radiation, which can cause cellular damage in humans and other species, includes types called alpha, beta, and gamma radiation. These different types of radiation have vastly different penetrating power through matter.
With regard to human health impacts, alpha radiation is of most concern through internal (inhalation) exposure, whereas beta and gamma radiation may cause health impacts through either internal or external exposure. In particular, gamma radiation has significant penetrating powers, and one must shield gamma-radiating materials to reduce human exposure. Different radioactive materials emit different combinations of radiation types (that is, different ratios of alpha, beta, and gamma radiation).
Quantifying Radioactivity and Radiation.
The decay of radioactive materials follows a simple mathematical law. Observation has shown that the rate of decrease in the number of radiation emissions is proportional to the amount of radioactive material present. The amount of radiation of a certain type that is present in a sample of matter is called its "activity," and the standard unit for measuring activity is the "Becquerel" (often abbreviated Bq), which is 1 radiation emission per second. The frequency of radiation emission is characterized by the element's half-life, which is the time duration over which the amount of radioactive material decreases by a factor of two.
For radioactive materials, the half-life ranges from a fraction of a second to billions of years. The potential of the radiation to damage human health depends on both the amount of material present (its activity) and the type of radiation emitted.
Classifying Radioactive Materials
To help manage their potential impacts, a number of classes of radioactive materials have been defined for regulatory purposes. These classes include
The Connection Between NORM and Radon.
As its name implies, naturally occurring radioactive material (NORM) is naturally found in most soils, surface water, and groundwater throughout the world. Some NORM has existed since the origin of the Earth, while additional NORM is continually being generated within the atmosphere from cosmic radiation.
The primordial NORM category includes decay products of three primary decay chains that start with the isotopes 238 Uranium, 232 Thorium, and 235 Uranium. In evaluating the potential NORM impacts, particular attention has been paid to 226 Radium and 228 Radium, which are daughters of the 238 Uranium and 232 Thorium decay chains. Various technological and industrial processes may result in the material concentration of radium above natural background levels.
Radon, a natural decay product of radium, is a noble gas that will also dissolve in water. It exists worldwide in surface water and groundwater, and can be present in concentrations that are of concern for human health (see box on next page).
Anthropogenic Radioactive Waste.
High-level radioactive waste (HLRW) includes fuel rods and associated material from commercial nuclear power generation. Transuranic (TRU) waste consists of elements with atomic numbers greater than that of uranium, including plutonium, neptunium, and americium. These materials were generated as part of the nuclear weapons program.
Waste that is below regulatory concern (BRC) consists primarily of short-lived materials that are used in medicine and research. Low-level radioactive waste (LLRW) is a catch-all category and consists of everything that is not included in the other waste categories. Low-level waste does not necessarily mean low activity waste, though much of it is. Certain low-level waste streams may contain high levels of radioactivity, such as resins, filters, and certain pieces of equipment from the nuclear power industry.
Storing Radioactive Wastes Underground.
Within the United States, the first low-level radioactive waste disposal site was licensed in 1962 to help manage waste generated from the widespread use of radioactive material over the previous half century. During the remainder of the 1960s, five additional commercial sites were developed and opened throughout the country. All of these sites relied on shallow land burial technology in which the waste was placed in trenches and covered with earthen material. Experience from these facilities has highlighted the significance of local geology, hydrology , and site engineering in the performance of disposal facilities.
Licensing requirements for a shallow land burial facility are specified in Title 10, Part 61, of the Code of Federal Regulations. Among the provisions required in the license application are documentation that the site will be geologically stable, that the site will be safe from inadvertent intrusion, and that normal releases of radioactivity will not cause an unacceptable risk to the public.
Despite a number of licensing efforts, a combination of public concern and lack of political will has resulted in no licensing of additional facilities since the 1960s, while only two facilities remain open. In an attempt to address public concerns, policy appears to be turning toward the concept of "assured isolation" wherein radioactive waste would be placed in long-term storage. During the period of storage, most of the radioactivity would be
Disposal and Cleanup of Radioactive Wastes
The U.S. Department of Energy is investigating Yucca Mountain, Nevada as a site for subsurface disposal of high-level radioactive waste. The department has recently received a license for disposal of transuranic waste at the Waste Isolation Pilot Plant (WIPP) site near Carlsbad, New Mexico. Waste that is below regulatory concern is not regulated, and these materials may be disposed with normal municipal solid waste. Disposal of lowlevel radioactive waste is the responsibility of the states rather than the federal government.
Prime Candidates for Nuclear Waste Cleanup.
The most significant sources of concentrated radioactive materials in the environment have been facilities involved with nuclear weapons production. Cleaning up these installations in the United States and the former Soviet Union will be the most costly environmental restoration project in world history. In 1977, the U.S. Department of Energy was given stewardship of the nation's nuclear weapons arsenal, and it also inherited responsibility for cleaning up the legacy of environmental contamination associated with nuclear weapons production.
The processes that ultimately lead to soil and groundwater contamination include uranium mining and processing, isotope and chemical separations, component fabrication, and weapons testing. Like other industries, the U.S. Department of Energy and its predecessors frequently disposed of wastes in landfills, lagoons, or underground injection wells, and spills of byproduct materials were not uncommon.
The department is charged with cleanup of 113 installations in 30 states. However, the following five installations are expected to account for the majority (approximately two-thirds) of the costs for cleanup: the Rocky Flats Environmental Technology Site in Colorado; the Idaho National Engineering and Environmental Laboratory; the Savannah River Site in South Carolina; the Oak Ridge Reservation in Tennessee; and the Hanford Site in Washington.
While waste management programs have been greatly improved, cleanup of these legacy wastes remains a significant environmental and political challenge for the twenty-first century.
Eisenbud, Merril and Thomas F. Gesell, Environmental Radioactivity from Natural, Industrial, and Military Sources, 4th ed. San Francisco, CA: Morgan Kaufmann Publishers, 1997.
Glasstone, Samuel. Sourcebook on Atomic Energy. New York: Van Nostrand Reinbold,1979.
National Research Council. Groundwater and Soil Cleanup: Improving Management of Persistent Contaminants. Washington, D.C.: National Academy Press, 1999.
U.S. Department of Energy. Linking Legacies: Connecting the Cold War Nuclear Weapons Production Processes to their Environmental Consequences. Office of Environmental Management. Washington, D.C.: DOE/EM-0319 (1997).
RADIOACTIVITY IN DRINKING WATER
Radon is a naturally occurring noble gas which emits radiation and that, if present in sufficient concentrations, may be harmful to humans. Most of the radon found in a home comes from radioactive decay in the soil and geological materials beneath the house. Radon then enters the home, particularly via the basement or crawlspace. The greatest exposure risk to humans is through inhalation of radon via the indoor air. The U.S. Surgeon General has warned that breathing radon is the second leading cause of lung cancer.
Radon from tap water is a much smaller source (approximately 1 to 2 percent) of radon in the home than that derived from geologic materials beneath the house. Further, the risk applies only to groundwater-based water systems (wells and springs); the risk of radon in surface-water sources (streams, lakes, and reservoirs) is very low.
The U.S. Environmental Protection Agency has proposed a radon standard for drinking water that would incorporate the potential for radon contribution from indoor air as well as that from drinking water. The agency also has established maximum contaminant levels for beta particle and photon activity (4 mrem), gross alpha particle activity (15 pCi/liter), and combined 226 Ra and 228 Ra (5 pCi/liter).