Link to USGS home page

Resources on Isotopes

Periodic Table--Radon

Radon-222 is a radioactive daughter isotope of Radium-226, which decays into Polonium-218 and then into other heavy-metal isotopes. 222Rn is the longest-lived (half-life is 3.82 days) and most-studied isotope of radon. Because radon is a gas, it will tend to seek a gaseous phase. Natural radon concentrations in the atmosphere are so low that natural waters in contact with the atmosphere will continually lose radon by volatilization. Hence, ground water has a higher concentration of 222Rn than surface water. Likewise, the saturated zone of a soil frequently has a higher radon content than the unsaturated zone due to diffusional losses to the atmosphere.

Radon is typically used in studies of ground water interaction with streams and rivers because a relatively short residence time in a stream or river channel will suffice for loss of most of the radon in a parcel of water. Any significant concentration of radon in a stream or river is a sensitive indicator of local inputs of ground water. Kraemer and Genereux (1997) provide a detailed discussion of 222Rn mixing models and the use of 222Rn to determine areas of ground water discharge to streams.

222Rn has also been used in combination with other isotopes for different applications. Leveque et al. (1971) took advantage of a relationship between 222Rn and 82Br to investigate zones of permeability. Sultankhodzev et al. (1971) related 222Rn to uranium decay and then used He/Rn and Xe/Rn ratios to date ground water.

Source of text: This review was assembled by Eric Caldwell and Dan Snyder, primarily from Faure (1986).

Ellins, K.K., Roman-Mas, A. and Lee, R. (1990). "Using 222Rn to examine groundwater/surface discharge interaction in the Rio Grade, DeManati, Puerto Rico." J. Hydrol., 115: 319.
Genereaux, D. P., and Hemond, H. F. (1990). "Naturally occurring radon 222 as a tracer for streamflow generation: steady state methodology and field example", Water Resour. Res., 26,, 12, pp. 3065-3075.
Genereaux, D. P., Hemond, H. F., and Mulholland, P. J. (1993). "Use of radon-222 and calcium as tracers in a three-end-member mixing model for streamflow generation on the wet fork of Walker Branch watershed", J. Hydrol., 142, pp.167-211.
Gudzenko, V. (1992). "Radon in subsurface water studies", in Isotopes of Noble Gases as Tracers in Environmental Studies; Proceedings of a Consultants Meeting, International Atomic Energy Agency, Vienna. pp. 249-261.
Kraemer, T.F. and Genereux, D.P. (1998). "Applications of Uranium- and Thorium-Series Radionuclides in Catchment Hydrology Studies." In: C. Kendall and J.J. McDonnell (Eds.), Isotope Tracers in Catchment Hydrology, Elsevier, Amsterdam, pp. 679-722.
Lee, R. and Hollyday, E.F. (1987). "Radon measurement in streams to determine location and magnitude of ground-water seepage." In: B. Graves (Ed.), Radon, radium, and other radioactivity in ground water. Chelsea, Mich., Lewis Publishers, Inc., pp. 241-249.
Lee, R. and Hollyday, E.F. (1991). "Use of radon measurements in Carters Creek, Maury County, Tennessee, to determine location and magnitude of ground-water seepage." In: L.C. Gundersen and R.B. Wanty (Eds.), Field studies of radon in rocks, soils and water. U.S. Geological Survey Bulletin, 1991, pp. 237-242.
Levêque, P. S., Maurin, C., Severac, I. (1971). "Le 222Rn traceur naturel complementaire en hydrologie souterranie." C. R. Hebd. Seances Acad. Sci., 272, 18, p. 2290.
Sultankhodzhaev, A. N., Spiridonov, A. I., Tyminsij, V. G. (1971). "Underground water's radiogenic and radioactive gas ratios (He/Rn and Xe/Rn) in groundwaters and their utilization for groundwater age estimation", Uzbek Geol. J. 5, p. 41.
Related Links
Periodic Table
Fundamentals of Stable Isotope Geochemistry
General References
Isotope Publications
Please contact Carol Kendall ( for questions and comments regarding this page.
This page was last changed in January 2004.
To the USGS Home Page
To the Water Resources Information Home Page