An effective integrator of
hydrologic history, isotope hydrology is a key
to understanding fundamental physical, chemical,
biological, and climate forcing processes occurring
in a watershed.
of the concentrations of isotopes in groundwater
and surface water can be incorporated into models
to predict future responses of the watershed to
trends in land-use change, water resource management
decisions, and climate variability.
Isotope methods are useful in regions where more
traditional hydrologic tools such as geologic
mapping of aquifer material, piezometric data,
pump tests, hydraulic conductivity measurements,
major ion chemistry, and hydrologic models give
ambiguous results or insufficient information.
Isotopes can be used to efficiently unravel water
sources that have combined at the sampling location,
and they can accurately determine residence time
information, which has important implications
for water resources management. If a major urban
drinking water supply well from a Southwest basin
pumps thousand-yearold water, for example, then
it is mining the groundwater resource at a much
faster rate than natural recharge. Likewise, a
consultant might use isotope ages to prove that
owner A, who bought property in 1965, is responsible
for a contaminant leak rather than owner B who
bought the property in 1980.
This article serves as an introduction to isotopes
that are used to determine residence time, sources
for age-dating isotopes, and guides for assessing
which isotopes are appropriate with regard to
their age-range, sample volume size, and analytical
measurement. For more information on this subject,
see Clark and Fritz (1997) and Cook and Herczeg
What is an Isotope?
Isotopes of a particular element have the same
number of protons but a different number of neutrons
in the nucleus, resulting in a different atomic
mass. For example, the most common element in
the universe, hydrogen, by definition contains
one proton in its nucleus, but it can contain
none, one, or two neutrons. Some isotopes are
stable, meaning they do not decay to any other
form over time, and others are unstable, or radioactive,
meaning they spontaneously decay at a predictable
rate to form a new element. For example, hydrogen
with two neutrons is known as tritium, an unstable
element. Tritium decays by emitting a radioactive
beta particle and converting into a stable helium
Sources of Isotopes
Both anthropogenic and natural sources exist for
many isotopes. Anthropogenic sources are a result
of the nuclear age of weapons testing, nuclear
power generation, fuel rod reprocessing, and nuclear
medical waste. These activities have elevated
many isotopes to concentrations well above their
natural state. Most became elevated on a global
scale, particularly in the northern hemisphere,
Natural isotopic sources are divided into three
broad categories: 1) cosmogenic, 2) subsurface
production, and 3) uranium decay series.
Cosmogenic isotopes arise from highenergy cosmic
rays known as electrons and photons and lower-energy
cosmic rays such as protons and other light nuclei.
These cosmic particles assail elements in the
earth’s atmosphere, creating secondary particles,
such as neutrons, that subsequently bombard other
atmospheric nuclei and transform into other isotopes.
Subsurface production occurs through the by-products
of natural radioactive decay series. These isotopes
often must be accounted for when interpreting
cosmogenic isotope transit times. They can also
be directly used for agedating groundwater.
The uranium decay series (238U)
spawns many isotopes with long half-lives that
have been used in hydrologic studies and are listed
in Figure 1. Uranium-238 is referred
to as a primordial isotope because it was incorporated
into the Earth during its formation. The 238U
halflife is 4.47 billion years.
Understanding the various sources for each isotope
helps hydrologists determine which isotopes are
most appropriate for hydrologic problems. If the
focus is on recharge or vadose zone processes,
then cosmogenic isotopes are a good choice because
they are incorporated into rain, snow, or dry
deposition. On the other hand, if a hydrologist
plans to study processes that occurred after 1950,
anthropogenic isotopes are useful. For long-term
process studies within a groundwater system, cosmogenic,
subsurface production, and uranium decay-series
isotopes all may be appropriate.
Figure 1 lists age-dating isotopes
commonly used in hydrologic research, alongside
the associated sources of those isotopes. Many
of the same isotopes are listed under several
different categories; the multi-source potential
of these isotopes can complicate interpretation,
but can often be accounted for by multiple isotope
or chemical measurements.
Age Dating Fundamentals
Before we delve into practical aspects of age-dating
isotopes, it is worth mentioning a common misconception.
The term "age" sometimes creates the
impression that the number represents a simple
piston flow transit time of a small water parcel.
Despite the prevalent use of this term, isotope
hydrologists understand that the water sample
measured represents the integrated travel time
information; "age" and "mean residence
time" are used interchangeably.
Unstable isotopes periodically but predictably
emit a particle or break into two smaller nuclei.
The time period between emissions is known as
the halflife for radioactive decay, and forms
the basis for most age-dating methods. An ideal
age-dating isotope should behave conservatively
by not experiencing any losses or additions during
the transit time of the water. This is rarely
the case, but we will discuss the ideal case to
illustrate the straightforward age-dating technique.
We can calculate the time since groundwater recharged
(left the atmosphere) if we know the original
recharge concentration of a radioactive isotope
its associated half-life (T1/2),
and measure the number of atoms remaining in our
groundwater sample at the time of collection (Nt).
is related to the decay constant (l)
by l = ln2/T1/2.
Assuming a single flowpath without mixing, losses,
or additions of the isotope, we can calculate
the approximate time since recharge as t = -(1/l)•ln(Nt/N0).
For practical reasons, we might have to make
assumptions regarding the initial parent atom
concentration, which creates larger uncertainties.
Therefore, it is more direct to measure the parent
atom remaining (Nt)
and daughter produced (Dt)
at the sample collection time. This approach is
appropriate if the radioactive parent isotope
decays to a stable daughter product that remains
with the water parcel containing the parent isotope.
In this case the age calculation is t=(1/l)*ln(1+
Another method to determine residence time is
to compare measured concentrations with the time-varying
concentrations known as input source functions.
Careful historical measurements or reconstructions
of the time-varying global fluxes of anthropogenic
isotopes can be exploited to derive fairly informative
age determinations over the past several decades.
Precipitation measurements between 1950 and present
day record peak-shaped curves (such as for 3H,
– see page 22 – and 36Cl)
while others related to nuclear power facilities
or fuel rod reprocessing have either increased
steadily (such as 85Kr)
or remain elevated (such as 129I).
If only one anthropogenic isotope is measured,
then the interpretation may be limited to the
determination that the water was recharged within
the last 50 years.
Due to their varying concentrations through time,
if more than one anthropogenic isotope is measured,
then more precise age determinations may be possible.
Often the ratios of two isotopes (such as 85Kr/3H
and 36Cl/129I) can be combined with each separate
isotope concentration to create a unique time
when all three factors match the historical signals.
Tritium (3H), an anthropogenic isotope, has an
advantage because it decays to a stable daughter
product (3He), and both can be measured in the
water sample collected in the field. In this case,
one can use the second age equation mentioned
above as long as enough tritium and 3He remains
in the sample to be measured in the lab. Also
since 3H is part of the H2O molecule it directly
tracks the movement of the water.
Consideration of Age Ranges
The practical limit on the hydrology residence
time age range is a function of the half-life,
the laboratory detection limit, and the practical
constraints regarding local evidence for the different
source generation processes for each isotope.
Figure 2 depicts both the natural
and anthropogenic tracers and their respective
practical age-dating ranges. The y-axis on Figure
2 is logarithmic due to the immense range
between several days for the 222Rn
isotope to the potential 80 million-year maximum
age for 129I.
Practical Limits of Field Sample
The required sample volume must also be considered
when choosing an isotopic system. Figure
3 lists both the isotopes and their associated
general lab measurement category, which significantly
impacts the required field sample volume. The
y-axis on Figure 3 is also logarithmic
due to sample volumes that range between a few
milliliters to 3,000 liters. Few hydrologists
are willing to extract 81Kr
gas out of 3,000 liters of water as was done for
Cyclotron measurements of ancient Great Artesian
Basin water in Australia. In general, required
sample volumes have decreased with mass spectrometer
labs and especially accelerator mass spectrometer
(AMS) labs. However, the sample costs are higher
for AMS measurements. Thus, many larger volume
samples are collected to be measured in low-level
counting labs in order to lower the cost.
Lately, there is a trend toward more routine
use of isotope tools by hydrologists. The cost
of analyses is quite reasonable for many isotopes,
and a variety of commercial and research labs
are available to perform the analyses. One could
possibly spend a few thousand dollars on isotopic
analyses of water collected from existing wells
to produce a first order answer to a question
that alternatively could require several labor-intensive
pump tests, additional borehole installations,
and/or a groundwater model that relies upon extensive
water level data. As more hydrologists see the
power of the isotope techniques and learn to use
them effectively, we will ultimately gain an improved
understanding of our water resources and be better
equipped to manage them effectively.
For more information on this
article, contact Brenda Ekwurzel
Clark I. D.
and Fritz P. (1997)
Environmental Isotopes in Hydrogeology,
Lewis Publishers of CRC Press, New York, 328 pp.
Cook P. G., and Herczeg
A. L. (2000)
Environmental Tracers in Subsurface Hydrology,
Kluwer Academic Publishers, Boston, 529 pp.