Chlorine has 9 isotopes with mass numbers ranging from 32 to 40. Only
three of these isotopes occur naturally: stable 35Cl (75.77%)and
37Cl (24.23%), and radioactive 36Cl. The ratio of
36Cl to stable Cl in the environment is about 700 x 10-15
: 1 (Bentley et. al., 1986). 36Cl is produced in the atmosphere
by spallation of 36Ar by interactions with cosmic ray protons.
In the subsurface environment, 36Cl is generated primarily as
a result of neutron capture by 35Cl or muon-capture by 40Ca
(Fabryka- Martin, 1988). 36Cl decays to 36S and to
36Ar, with a combined half-life of 308,000 years. The half-life
of this hydrophilic nonreactive isotope makes it suitable for dating in
the range of 60,000 to 1 million years. Additionally, large amounts of
36Cl were produced by irradiation of seawater during atmospheric
detonations of nuclear weapons between 1952 and 1958. The residence time
of 36Cl in the atmosphere is about 1 week. Thus, as an event
marker of 1950s water in soil and ground water, 36Cl is also
useful for dating waters less than 50 years BP. 36Cl has seen
use in other areas of the geological sciences, including dating ice and
sediments (Nishiizumi et al., 1983; Phillips et al.,1983).
The attractiveness of chlorine in hydrologic studies is that it is highly
soluble, exists in nature as a conservative non-sorbing anion, does not
participate in redox reactions, and has some quickly identifiable sources
(e.g., seawater). The abundance of 36Cl is usually reported
as the atomic ratio of 36Cl to total chloride in the sample.
The ratio is always quite small in natural waters, typical values ranging
from 10-15 to 10-11. The four orders of magnitude
range of 36Cl/Cl ratios is due to several factors. Over geologic
time an equilibrium will be established between the subsurface in-situ
production of 36Cl and its decay (similar to the secular equilibrium
of U isotopes). The equilibrium 36Cl/Cl value will depend on
the rate of production of 36Cl, which is a function of the U
and Th concentrations in the aquifer. Ratios higher than 10- 12
in natural waters indicate the presence of thermonuclear 36Cl;
maximum global values during sea-level atmospheric nuclear weapon tests
were on the order of 10-11. Thermonuclear 36Cl is
likely to become more frequently used as an indicator of young water as
the thermonuclear 3H in ground water decays to background levels
over the next couple decades.
36Cl has been used to date old ground water in confined aquifers
by consideration of the affect of radioactive decay on measured 36Cl/Cl
ratios (Bentley et al., 1986; Phillips et al., 1986; Nolte et al., 1991).
Potential complications include possible addition of stable Cl isotopes
to the water by reactions with rocks, ion filtration (Phillips et al.,
1986), or mixing with higher chloride waters. Such additions can substantially
change the 36Cl/Cl ratio. An age interpretation also requires
knowledge of the initial 36Cl/Cl ratio, which can be difficult
due to the wide range in possible precipitation input values (Andrews and
Fontes, 1993). In-situ production of 36Cl requires an adequate
assessment of the U and Th concentrations in the aquifer. Because precipitation
values are in the 20-500 x 10-15 range, in-situ production values
of about 50 x 10-15 can have a significant affect on observed
36Cl/Cl ratios (Nimz, 1998). The addition of Cl can be evaluated
by measurement of the Cl and other ionic concentrations in the water, in-situ
production can be estimated after a measurement of aquifer U and Th concentrations,
and precipitation input values can be estimated using present-day local
values. For the determination of water transit times between two sampling
locations instead of the absolute age of the water, the value measured
at the upgradient location is used as the input value.
Cl, like Li and B, has two stable isotopes (35Cl and 37Cl)
that are highly mobile in the hydrosphere; however, unlike Li and B, these
isotopes are not easily fractionated in nature. d37Cl
values (ratios of 37Cl to 35Cl) are reported in ‰
relative to the standard SMOC (Standard Mean Ocean Chloride). Small variations
in d37Cl values (< 2.1‰) were
reported by Kaufman et al. (1984) in several water types. Fractionation
can occur during diffusion-controlled processes in ground water (Desaulniers
et al., 1986), during high-temperature water-rock interactions (Eastoe
et al., 1989), and as a result of temperature variation in the ocean over
time (Kaufman et al., 1993). Therefore, there appears to be sufficient
isotopic variations in nature to make 37Cl useful as a hydrologic
tracer. Depending on local lithology, d37Cl
might be a useful tool for hydrograph separation analysis and determination
of mixing between regional and shallow ground water mixing (Nimz, 1998).
A multi-isotopic approach is almost always beneficial ("the more
isotopes the merrier!"). One recent example is the use the d13C
and d37Cl of chlorinated solvents
such as TCE and BTEX to identify the sources of the contaminants in ground
water, and the degradation reactions that may remediate the pollution plumes.
One such study found that chlorinated solvents (TCE, PCA, and TCE) supplied
by different manufacturers had distinctive C and Cl isotopic signatures
(Aravena et al., 1996). This appears to be a very promising avenue of research,
although the analysis of d37Cl is
still technologically challenging.
Further information can be found in Clark and Fritz (1997), Environmental
Isotopes in Hydrology (CRC Press):
Source of text: This review was assembled by Carol Kendall, Eric
Caldwell and Dan Snyder, primarily from a recent review by Nimz (1998)
and Bentley et al. (1986).
||Andrews, J.N. and Fontes, J-C., (1993). "Comment on 'Chlorine 36
dating of very old groundwater 3. Further studies in the Great Artesian
Basin, Australia'" by T. Torgersen et al.. Water Resour. Res.,
||Aravena, , R., Frape, S.K., Moore, B.J., Van Warmerdam, and Drimmie,
R.J., (1996). "Use of environmental isotopes in organic contaminats
research in groundwater systems," IAEA, Symposium on Isotopes in
Water Resources Management, Vienna, Austria, 20-24 March, 1995.
||Bentley, H. W., Phillips, F. M., and Davis, S. N. (1986). "36Cl
in the terrestrial environment", in: P. Fritz and J.-Ch. Fontes (Eds.),
Handbook of Environmental Geochemistry, Vol. 2b, Elsevier Science,
New York. pp. 422-475.
||Coplen, T. B. (1993). "Uses of Environmental Isotopes", in:
W. M. Alley (Ed.), Regional Ground-Water Quality , Van Nostrand
Reinhold, New York, pp. 227-254.
|| Desaulniers, D.E., Kaufman, R.S., Cherry, J.A. and Bentley, H.W., (1986).
"37Cl-35Cl variations in a diffusion- controlled
groundwater system". Geochim. et Cosmochim. Acta, 50:
||Eastoe, C.J., Guilbert, J.M. and Kaufman, R.S., (1989). "Preliminary
evidence for fractionation of stable chlorine isotopes in ore-forming systems".
Geology, 17: 285.
||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.
||Kaufman, R., Long, A., Bentley, H. and Davis,
S., (1984). "Natural
chlorine isotope variation". <i>Nature</i>,
||Kaufman, R., Frape, S.K., McNutt, R. and Eastoe, C., (1993). "Chlorine
stable isotope distribution of Michigan Basin formation waters". Appl.
Geochem., 8: 403.
||Nishiizumi, K., Arnold, J. R., Elmore, D., Ma, X., Newman, D., and Gove,
H. E. (1983). "36Cl and 53Mn in Antarctic meteorites and
10Be-36Cl dating of Antarctic ice", Earth
Planet. Sci. Lett., 62: 407- 417.
||Nimz, G. J., (1998). "Lithogenic and Cosmogenic Tracers in Catchment
Hydrology". In: C. Kendall and J.J. McDonnell (Eds.), Isotope Tracers
in Catchment Hydrology. Elsevier, Amsterdam, pp. 247-290.
||Nolte, E., Krauthan, P., Korschinek, G., Moloszewski, P., Fritz, P.
and Wolf, M., (1991). "Measurements and interpretations of 36Cl
in groundwater, Milk River aquifer", Alberta, Canada. Appl. Geochem.,
||Phillips, F. M., Smith, G. I., Bentley, H. W.,
Elmore, D., and Gove, H. E. (1983). "Chlorine-36
dating of saline sediments:preliminary results from Searles
Lake, California", Science, 222: 925-927.
||Phillips, F.M., Bentley, H.W., Davis, S.N., Elmore, D. and Swanick,
G., (1986). "Chlorine-36 dating of very old groundwater 2. Milk River
aquifer, Alberta, Canada". Water Resour. Res., 22: 2003.