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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: https://www.usgs.gov/science/mission-areas/water-resources/science/water-quality-benchmarks
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 radio- nuclides 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.
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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.
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