Krypton-81 is the product of atmospheric reactions with the other naturally
occurring isotopes of krypton [78Kr (0.35%), 80Kr
(2.25%), 82Kr (11.6%), 83Kr (11.5%), 84Kr
(57.0%), 86Kr (17.3%)] and from decay of 238U. It
is radioactive with a half-life of 250,000 years. Little use of this isotope
has been made due to interference from 85Kr; however, it has
been used for age determination in old (50,000-800,000 year) groundwater
(Oeschger, 1978; Lehman et al.,1985).
Krypton-85 is an inert radioactive noble gas (half-life = 10.76 years)
that is produced by fission of uranium and plutonium. Sources have included
nuclear-bomb testing, nuclear reactors, and the release of 85Kr
during the reprocessing of fuel rods from nuclear reactors (Sittkus and
Stockburger, 1976). Concentrations of 85Kr in the lower atmosphere
show considerable spatial variability, primarily reflecting the locations
of the major sources (Solomon et al, 1998). A strong gradient also exists
between the northern and southern hemispheres where concentrations at the
North Pole are approximately 30% higher than the South Pole due to convective
mixing (Jacob et al., 1987; Zimmerman et al., 1989).
Although 85Kr has a half-life similar to that of 3H.
85Kr has the advantage of being an environmental tracer with
steadily increasing atmospheric input, whereas the 3H atmospheric
input is a more complex function of season and latitude and has declined
since cessation of atmospheric nuclear-bomb testing in the mid-1960s. Because
85Kr is a noble gas, it is not subject to microbial degradation
and other chemical interactions that can alter the concentrations of organic
environmental tracers. Krypton-85 enters ground water by equilibration
of the infiltration water with air in the unsaturated zone, assumed to
have a 85Kr activity similar to that of the atmosphere. If the
effects of hydrodynamic dispersion are small, the 85Kr specific
activity of groundwater defines the time since the infiltration water was
isolated from the atmosphere. The very large amounts of water required
for 85Kr analysis (at least 100 L) have prevented much use of
this potentially useful tracer.
Source of text: This review was assembled by Dan Snyder
from the references below.
||Fabryka-Martin, J. T. (1988) Production of Radionuclides in the Earth
and their Hydrogeologic Significance, with Emphasis on Chlorine-36 and
Iodine-129, University of Arizona, PhD. thesis. 400 pp.
||Florkowski, T. and Rózanski, K. (1986). "Radioactive Noble
Gases in the Terrestrial Environment." In: P. Fritz and J.-Ch. Fontes
(Eds.), Handbook of Environmental Geochemistry, Vol. 2b, Elsevier
Science, New York. pp. 481-506.
||Jacob, D.J., Prather, M.J., Wofsy, S.C., and McElroy, M.B. (1987). "Atmospheric
distribution of 85Kr simulated with a general circulation model."
Jour. Geophys. Res., 92: 6614-6626.
||Lehman, B.E., Oeschger, H., Loosli., H.H., Hurst, G.S., Allman, S.L.,
Chen, C.H., Kramer, S.D., Willis, R.D., and Thonnard, N. (1985). "Counting
81Kr atoms for analysis of groundwater". J. Geophy.
Res., 90, 11547-11551.
||Oeschger, H. (1978). Workshop on Dating Old Ground Water, University
of Arizona, March 16-18, Rep., Y/OWI/SUB-78/55412.
||Plummer, L.N., Michel, R.L., Thurman, E.M., and Glynn, P.D. (1993) "Environmental
tracers for age dating young ground water", In: W.M. Alley (Ed.),
Regional Ground-Water Quality, Van Nostrand Reinhold, New York,
||Solomon, D. K., Cook, P. G., and Sanford, W. E. (1998). "Dissolved
gases in subsurface hydrology." In: C. Kendall and J.J. McDonnell
(Eds.), Isotope Tracers in Catchment Hydrology. Elsevier, Amsterdam,
||Sittkus, A., and Stockburger, H. (1976). "Krypton-85 als indikator
des kernbrennstoffverbrauchs", in Naturwissenschaften, 63,
pp. 266- 72.
||Zimmerman, P.H., Feicher, J., Rath, H.K., Crutzer, P.J. and Weiss, W.
(1989). "A global three-dimensional source-receptor model investigation
using 85Kr." Atmos. Environ., 23: 25-35.