Radionuclides

Science Center Objects

Many people might be surprised to learn that drinking-water sources, especially groundwater, can contain radioactive elements (radionuclides). Radionuclides in water can be a concern for human health because several are toxic or carcinogenic. Other radionuclides are useful tools for determining the age of groundwater in an aquifer or of sediment deposited at the bottom of a water body.

A radionuclide is an atom (element) with an unstable nucleus (core). The nucleus of the atom has excess energy that is released by different types of radioactive decay. Radionuclides in our environment are produced by minerals in the Earth’s crust, by cosmic rays hitting atoms in the Earth’s atmosphere, and by human activities.

 

RADIONUCLIDES IN GROUNDWATER

Radionuclides occur naturally in many rocks and minerals. Some radionuclides in rocks, such as uranium, have been present since the Earth formed. Others, such as radon and radium, are the product of the decay of those original radionuclides. As a result, these radionuclides frequently occur in groundwater. The overall amount of radioactivity in water, representing contributions from all of the radionuclides present, is measured as “gross alpha,” because naturally occurring radioactive elements emit alpha particles as they decay. For an overview of where radionuclides occur most commonly in groundwater, see Chapter 5 in USGS Circular 1360: Water Quality in Principal Aquifers of the United States, 1991–2010. You can also find information on radionuclides in groundwater in the fact sheets on U.S. Principal Aquifers featured here.

Uranium-238 is a common trace element in many rock types, but it is particularly enriched in crystalline rocks, such as granites, and in sediments derived from crystalline rocks. Uranium is only weakly carcinogenic, but is toxic and causes kidney damage at elevated concentrations when consumed in drinking water. Because uranium is highly soluble in its oxidized forms but only slightly soluble in its reduced forms, its concentrations in groundwater depend on redox conditions, as well as on pH and on concentrations of bicarbonate. 

Radon-222 is a daughter product of the decay of uranium. Radon is a dissolved gas that dissolves easily in water, and is present in most groundwater in the U.S. When water that contains radon is used in a home, most of the radon is released from the water into the air and can be inhaled. Inhalation of radon poses a risk of lung cancer. Although radon is produced by the decay of uranium, the amount of uranium in groundwater is affected by geochemical conditions such as redox, whereas the amount of radon in groundwater is not. Consequently, elevated concentrations of uranium and radon in groundwater don’t necessarily occur together.

Radium-226 and radium-228 are the two most common isotopes of radium and both are daughter products of the decay of uranium. Both isotopes are carcinogenic. Radium dissolved in drinking water is a human-health concern because it accumulates in bone and other tissues, increasing lifetime cancer risks. Like uranium, concentrations in groundwater depend on geochemical conditions in an aquifer, but the factors that enhance radium mobility are very different from those that favor the mobility of uranium, and high concentrations of uranium and radium rarely coincided in Principal Aquifers studied.

  ► Find drinking-water benchmarks for radionuclides here.

 

RADIONUCLIDES AS TRACERS

The presence of radionuclides can be very useful for determining the age of groundwater and of bed sediment. The age reflects when the water or sediment was last in contact with the atmosphere. For groundwater, this indicates how long the groundwater has been in the aquifer, or the time since recharge. Knowing groundwater age is useful because it can tell us what contaminants are most likely to occur in that water, and how long it might take those contaminants to be transported through the aquifer. For bed sediment, the age indicates when the sediment was deposited. Knowing the age of bed sediment is useful to help reconstruct contaminant histories recorded in sediment cores.

The radionuclides that are age tracers come from the atmosphere. Some are produced naturally when cosmic rays from stars (including our sun) strike the Earth’s atmosphere—these are called “cosmogenic”. Because the rate at which these radionuclides decay is known, their activity in water or sediment relative to their activity in the atmosphere can be used to deduce the amount of time since the water or sediment was in contact with the atmosphere. Cosmogenic radionuclides with very long half-lives (the amount of time required for one-half of the radionuclide to decay away), such as helium-4, can give us age information on very “old” groundwater. Very short-lived radionuclides, such as beryllium-7, can give us age information on very “young” water and sediment.

Other radionuclides are produced from human activities, such as nuclear weapons testing, nuclear facility releases, and radioactive waste. The presence of tritium (H-3), for example, was produced by nuclear detonations in the 1950s and 60s, and its presence in groundwater indicates that the groundwater is relatively young (< 60 or so years old). Similarly, cesium-137, also produced by nuclear tests, mostly adheres to sediments—its peak activity in a sediment core indicates the depth of sediment that corresponds to the early 1960s, when levels of cesium-137 in the atmosphere reached their peak.

 

RELATED USGS RESEARCH

 

ADDITIONAL RESOURCES

U.S. Environmental Protection Agency

Agency for Toxic Substances and Disease Registry (ATSDR)