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					Radioactive Isotopes
Radioactive isotopes have unstable nuclei that decay, emitting alpha, beta, and sometimes
gamma rays. Such isotopes eventually reach stability in the form of nonradioactive isotopes
of other chemical elements, termed radiogenic daughters. Decay of a radionuclide to a stable
radiogenic daughter is a function of time measured in units of half-lives. The decay constants
() and half-lives (t1/2) of radioactive isotopes that are frequently used as environmental
tracers in the field of hydrology are listed in Table 1.
Radioactive isotopes are useful indicators of the time that water has spent in the groundwater
system. For example, tritium (3H) is a well-known radioactive isotope of hydrogen that had
peak concentrations in precipitation in the mid-1960s as a result of above-ground nuclear
bomb testing conducted at that time.
Radon-222 (222Rn) is a radioactive daughter isotope of radium-226 that has a half-life of only
3.8 days. It is produced naturally in groundwater as a product of the radioactive decay of
226
   Ra in uranium-bearing rocks and sediments. Radon concentrations in groundwater depends
on the presence of these radioactive isotopes in the aquifer matrix, and can vary from <2 Bq/L
within clastic sediments to >200 Bq/L in igneous and metamorphic rocks (Lee & Hollyday,
1993).Several studies (Ellins et al, 1990; Crandall et al, 1999; Pritchard et al, 2000; Cook et
al, 2003) have demonstrated that radon can be used to identify locations of significant
groundwater input to a stream. Radon was also used in a study in France to determine stream
water loss to groundwater as a result of groundwater withdrawals (Bertin & Bourg, 1994).
Radon is a gas, and natural radon concentrations in the atmosphere are so low that natural
waters in contact with the atmosphere will continually lose radon by volatilization. Hence,
groundwater has a higher concentration of 222Rn than surface water. Any significant
concentration of radon in a stream or river is a sensitive indicator of local inputs of ground
water. Kraemer and Genereux (1998) provide a detailed discussion of 222Rn mixing models
and the use of 222Rn to determine areas of ground water discharge to streams.


Table 1: Decay constants and half-lives of selected radioactive isotopes with application to hydrology (adopted
from Browne and Firestone, 1999)

        Isotope                         Decay Constant                                  Half-life

                                 (Year-1)              (day-1)                 (year)                 (day)

Rubidium (87 Rb)        1.46 x 10-11            4.00 x 10-14           4.75 x 1010            1.73 x 1013

Uranium (238 U)         1.55 x 10-10            4.24 x 10-13           4.468 x 109            1.63 x 1012

Iodine (129 I)          4.41 x 10-8             1.21 x 10-10           1.57 x 107             5.73 x 109

Chlorine (36Cl)         2.3 x 10-6              6.30 x 10-9            3.01 x 105             1.10 x 108

Krypton (81 Kr)         3.03 x 10-6             9.03 x 10-9            2.29 x 105             8.36 x 107

Carbon (14C)            1.21 x 10-4             3.31 x 10-7            5730                   2.09 x 106

Radium (226 Ra)         4.33 x 10-4             1.19 x 10-6            1600                   5.84 x 105

Argon (39Ar)            2.58 x 10-3             7.06 x 10-6            269                    9.83 x 104

Silicon (32Si)          4.95 x 10-3             1.36 x 10-5            140                    5.11 x 104

Strontium (90Sr)        0.0241                  6.65 x 10-5            28.78                  1.05 x 104

Hydrogen (3 H)          0.0558                  1.53 x 10-4            12.43                  4540
Krypton (83 Kr)     0.0644             1.77 x 10-4         10.756              3929

Radium (228 Ra)     0.121              3.31 x 10-4         5.75                2100

Sulphur (35 S)      2.89               7.92 x 10-3         0.240               87.51

Argon (37Ar)        7.23               1.98 x 10-2         0.0959              35.04

Radon (222 Rn)      66.0               0.181               0.0105              3.8235



References
Bertin C, Bourg CM, 1994. Radon-222 and chloride as natural tracers of the infiltration of
river water into an alluvial aquifer in which there is significant river/groundwater mixing.
Environmental Science and Technology. 28, 794-798.
Browne E, and Firestone, RB 1999. Table of isotopes, 8 th Edition. Wiley and Sons, New
York.
Cook, PG, Favreau, G, Dighton, JC, Tickell, S. 2003. Determining natural groundwater influx
to a tropical river using radon, chlorofluorocarbons and ionic environmental tracers. Journal
of Hydrology 277, 74-88.
Crandall CA, Katz BG, Hirten, JJ, 1999. Hydrochemical evidence for mixing of river water
and groundwater during high-flow conditions, lower Suwannee River basin, Florida, USA.
Hydrogeology Journal 7, 454-467.
Ellins KK, Roman-Mas A, Lee R, 1990. Using 222Rn to examine groundwater/surface
discharge interaction in the Rio Grande De Manati, Puerto Rico. Journal of Hydrology
155:319-341.
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 RW, Hollyday EF, 1993. Use of radon measurements in Carters Creek, Maury County,
Tennessee, to determine location and magnitude of ground-water seepage. In: Gunderson
LCS and Wanty RB (eds), Field studies of radon in rocks, soils and water. CK Smoley, 237-
242.
Pritchard, J, Herczeg, A, Lamontagne, S. 2000. The use of environmental tracers for
estimating seasonal contributions of groundwater to stream flow. In CD Proceedings
International Groundwater Conference “Balancing the Groundwater Budget”, Darwin.
International Association of Hydrogeologists