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Chlorine has two stable isotopes and one cosmogenic isotope. The cosmogenic isotope, 36Cl, has a long half-life, making it useful in age dating groundwaters up to 1 million years old. There is also limited variation in 37Cl. Most natural variation in 37Cl values in hydrologic systems are related to diffusion processes.


Cost of Analysis
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Accelerator Mass Spectrometry (AMS): $125-$500/sample

Approximately $200/sample

(See UA Laboratory of Isotope Geochemistry)
(See also Zymax Isotope Laboratory)

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Cl - Natural Production

36Cl is produced naturally in the atmosphere and within solid materials at the earth's surface.

36Cl - Atmospheric Production
Chlorine-36 is produced in the upper atmosphere through spallation reactions. High-energy cosmic ray particles collide with atoms in the earth's atmosphere producing protons and neutrons. After the emission of other particles to lower the energy state, the final result is either a stable element or a long-lived radioactive isotope. Roughly two thirds of atmospheric 36Cl is produced by the following spallation reaction:

40Ar + p ® 36Cl + n + a

The other third of the atmospheric 36Cl is produced by this spallation reaction:

36Ar + n ® 36Cl + p

where p is a proton, n is a neutron, and a is an alpha particle (or helium nucleus). These reactions result in an average atmospheric deposition rate of 12 to 20 atoms 36Cl per second per square meter.

36Cl - Earth Surface Production
Chlorine-36 can be produced in solid materials on the surface of the earth in three ways: spallation reactions, muon reactions and thermal neutron adsorption.

Spallation reactions also occur when gamma rays interact with minerals in the top several meters of the earth's surface. The following reactions can result:

35Cl + n ® 36Cl + y
39K + n ® 36Cl + n + a
40Ca + n ® 36Cl + p + a

where y is a gamma ray.

Chlorine-36 can also be produced through muon reactions. Muons are negatively charged, short-lived particles that are produced by high-energy cosmic ray reactions. When produced at the earth's surface, a muon can react with the nucleus of an atom. When a muon interacts with a calcium or potassium atom (both are commonly found in minerals at the earth's surface), 36Cl can be produced through the following reactions:

40Ca + µ- ® 36Cl + a
39K + µ- ® 36Cl + p + 2n

where µ- is a muon.

Finally, 36Cl is produced through thermal neutron absorption. The 35Cl isotope has a large neutron absorption cross-section, making a relatively large target for collisions with thermal neutrons. The following reaction results in the production of 36Cl from 35Cl in groundwater:

35Cl + n ® 36Cl + y

where the neutrons in the above reaction are produced in deep subsurface aquifers from the spontaneous radioactive decay of uranium and thorium. To a smaller extent, potassium can absorb neutrons from these same decay reactions and produce 36Cl in the following reaction:

39K + n ® 36Cl + n + a

Cl - Anthropogenic Source

36Cl was also produced during nuclear bomb testing in the middle of the 20th century. This thermonuclear testing produced many tons of neutrons which could readily react with 35Cl to form 36Cl:

35Cl + n ® 36Cl + y

Early tests were conducted on Pacific Ocean atolls. The neutrons from these earlier tests were mostly absorbed by rock. Later tests were conducted on barges in the Pacific Ocean, which were surrounded by a ready supply of 35Cl in seawater. The barge test explosions totaled over 60 megatons and were responsible for most of the stratospheric injection of 36Cl. Approximately 17% of the neutrons from these tests were absorbed by 35Cl in the ocean.
(See images of nuclear testing from the DOE Nevada Operations Office)

Fallout of 36Cl can be seen in ice cores in Greenland (see diagram below). The major fallout occurred between 1954 and 1958. 36Cl did not stay in the atmosphere for long; its residence time is approximately one week. The 36Cl comes down to the earth as either dry fallout or is cleaned out of the atmosphere by precipitation. Since 1980, levels of 36Cl have returned to near their natural atmospheric concentrations. However, 36Cl has been stored and recycled in the biosphere and therefore elevated levels can still be found in the hydrosphere as well.

Reprinted from Clark and Fritz 1997, p. 190. In addition to 36Cl fallout (data from Bentley et al. 1986) this also shows tritium measured in precipitation at Ottowa, Canada.

The ratio of 36Cl to stable 37Cl in the environment is ~700 * 10-15. Because of this the abundance of 36Cl for natural samples is most often reported as 36Cl/1015 Cl.

36Cl spontaneously decays in two ways: 98% of it decays through the emission of a beta particle, and the remaining 2% decays through electron capture.

Beta Emission
36Cl ® 36Ar + b-

Electron Capture
36Cl + e- ® 36S

Measurement Techniques - 36Cl (return to top)

Accelerator Mass Spectrometry (AMS): 36Cl is measured using AMS.

(See the University of Arizona's Accelerator Mass Spectrometry Lab for more information on AMS dating)

Chlorine-36 is currently measured at two facilities in the United States:

PRIME Lab - Purdue University

Center for Accelerator Mass Spectrometry - Lawrence Livermore Ntl. Lab/University of California

(See the New Mexico Bureau of Geology and Mineral Resources' Chemistry Laboratory for more information on the preparation for solid phase samples for 36Cl)

Gas Source Mass Spectrometry: measures d37Cl

Zymax Laboratory


Hydrological Applications -
36Cl (return to top)

36Cl is a long-lived radioactive isotope. Due to its long half-life (301,000 ± 4,000 years), it can be used to date groundwater that is up to a million years old. The chloride ion exists in most natural waters in varying concentrations due to the dissociation of sodium chloride. Most silicate surfaces onto which chloride could adsorb are negative. Due to their own negative charge, chloride ions do not adsorb onto these silicate surfaces and therefore move at approximatley the same rate as the groundwater.

(See Bentley et al., 1986b, for more information).

36Cl can also be used as an indicator of modern recharge. The testing of nuclear bombs in the 1950's put high levels of 36Cl into the stratosphere. High levels of 36Cl in a groundwater sample indicate that recharge occurred recently. Similarly, 36Cl can be used to identify and quantify salinity sources of a river system. 36Cl will be used to identify salinity sources to the Rio Grande in New Mexico. It is hoped that 36Cl will be able to separate agricultural solutes (which should be relatively young and have higher levels of 36Cl) from saline groundwater from deep groundwater flow in sedimentary basins (which would have lower levels of 36Cl).

(See the SAHRA Rio Grande page for more information on this research project)

36Cl can also be used to determine long-term average groundwater recharge rates in arid regions or more recent recharge rates using bomb pulse 36Cl


References and Further Reading

  • Bentley, H.W., F.M. Phillips, and S.N. Davis, Chlorine-36 in the terrestrial environment, in Handbook of Environmental Isotope Geochemistry, vol. 2, ed. by P.Fritz and J.-Ch. Fontes, pp. 427-480, Elsevier, Amsterdam, 1986a.

  • Bentley, H.W., F.M. Phillips, S.N. Davis, P.L. Airey, G.E. Calf, D Elmore, M.A. Habermehl, and T. Torgenson, Chlorine-36 dating of very old ground water: I. The Great Artesian Basin, Australia, Water Resour. Res. (22), 1991-2002, 1986b.

  • Clark, I., and P. Fritz, Environmental Isotopes in Hydrogeology, CRC Press, Boca Raton, 1997.

  • Phillips, F.M., Chlorine-36, in Environmental Tracers in Subsurface Hydrology, ed. by P.G. Cook and A.L. Herczeg, p. 299-348, Kluwer, Boston, 2000.

Internet Resources

USGS Periodic Table - Chlorine

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