Analysis Using Ingrowth Method (described
below): This is currently a research method only,
and is usually not available commercially.
Tracer Group, Lamont-Doherty Earth Observatory
at Columbia University)
Reston Chlorofluorocarbon Laboratory)
of Miami Rosenstiel School of Marine and Atmospheric
Science for more information)
Research Method: Cost not available
Helium has two stable isotopes, 3He
is radiogenic and abundant in the hydrosphere,
particularly in aquifers that contain appreciable
amounts of minerals rich in its parent isotopes.
It is generated within the earth's crust and mantle
by the decay of 238U,
is a direct product of U and Th a
decay, and of 6Li
by a-recoil induced
fission from neutrons emitted during the decay
of U and Th rocks, via:
+ n ® a
is also radiogenic but comparatively scarce (abundance
= .000137%). It is generated through decay of
by beta emission.
All helium atoms are eventually lost to space,
but first they reside within the earth for around
1 billion years, then reside on the surface for
another million years.
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and its parent 3H
can be measured using mass spectrometry, but other
dissolved gases (H2O,
etc.) must be removed first. Lower detection limits
are achieved by mass spectrometric measurements
produced by the "ingrowth method," where
the tritium sample is stripped of all gases and
then stored for a sufficient time for enough daughter
to be produced and measured. The mass spectrometer
method has the advantage over the liquid scintillation
method because both 3He
and the 3He
of a sample can be measured. This provides quantitative
age determinations discussed below.
the Mass Spectrometry
page for more information)
Berkeley National Laboratory site for more
information on the ingrowth method.
sampling techniques are outlined at the USGS
Reston Chlorofluorocarbon Laboratory.
The sampling protocol for 4He
is fairly straightforward. Samples are taken directly
from wellheads by means of a metal collection-tube
apparatus which is attached to the wellhead, and
which should be flushed with several volumes of
well water and checked for bubbles (which could
phase-fractionate samples) before being clamped
at both ends and removed.
In the lab, the helium is typically extracted
on a series of gas extraction lines which have
been calibrated to remove most chemically active
gases. Water vapor may be removed by a dry-ice
is then collected and measured on a Faraday collector.
A mass spectrometer is necessary for measuring
Groundwater can be aged quantitatively using 3H
Age is determined by:
where t is the time since isolation from contact
with the atmosphere after decay (estimated age),
is the concentration ratio of the two isotopes
expressed in tritium units (TUs). One tritium
unit is equal to one molecule of 3H
molecules of 1H
and has an activity of 0.118 Bq/kg (3.19 pCi/kg).
Aging by the 3H/3He
method also presents problems, since the total
in groundwater comes from a variety of sources:
the atmosphere, 3H
decay, subsurface nuclear reactions, and the earth's
mantle. The measured concentration of 3He
must be corrected for these other sources. 3He
is also not a routinely sampled or measured isotope.
Other concerns are the fractionation of 3He
if a gas phase is present and the fact that the
solubility of He is temperature dependent.
The use of 4He
in environmental hydrogeology has relevance to
both tracer and age studies. As the 4He
atom is essentially an a-particle
which has gained 2 electrons, this isotope may
be considered a viable research tool wherever
predominate, and where subsurface flow conditions
Groundwater ages are determined using 4He
by first determining the solid-to-liquid mass
transfer rate in the laboratory or by calibration
using groundwater age data obtained through other
methods. The age of the groundwater samples are
then determined by measuring total He and Ne to
correct for atmospheric helium, and then computing
Advantages of using 4He
1) Because 4He
is non-radioactive, it is relatively safe to use,
and adjustments for radioactive decay need not
be made. It is this linear property of activity
over time that allows it to serve as a more or
less direct indicator of age. A simple example
of this relationship is:
[He] = r
t ( (1.19*10-13
[U]) + (2.88*10-14
[Th] ) )
- [He] is the groundwater helium content in
cm3STP g-1 water
- r is the rock density in g cm-3
- f is the fractional porosity of the rock
- [U] and [Th] are the uranium and thorium contents
of the rock matrix in ppm (Clark and Fritz 1997,
after Andrews et al. 1982)
As such, activity may also change proportionally
to depth or along flow path (see Castro et al.
1998a and 1998b)
2) The relative importance and activities of
the major reservoirs of 4He
are known. In order of size, they are: the earth's
crust; the earth's mantle, and the atmosphere.
If the local crustal composition can be reasonably
approximated, the contribution from this source
can be tailored to an individual study (see Torgersen
and Clarke 1985, reference section). Often, surface
water (i.e., that which is in equilibrium with
the atmosphere) is used as a proxy for the average
atmospheric concentration (as in Castro et al.
1998), and is termed ASW.
Disadvantages of 4He
In using 4He
for groundwater analysis, the following problems
must also be considered:
1) many assumptions need to be made about the
aquifer of concern, including:
· distribution of isotope-producing rock
types within the aquifer (i.e., U, Th, and Li-bearing
assemblages) and, therefore, some idea of the
He production rate in the rock
· efficiency of isotope transfer from rock
· duration of rock-fluid contact
· subsurface fluid movement
2) The light nature of the atom makes diffusion
a serious, hard-to-quantify problem.
3) Vertical recharge can not be too low or He
will volatilize, which will lead to problems unless
ASW is correctly assumed or adjusted. Because
atmospheric helium often is contributed to an
aquifer during recharge, allowing for this contribution
is critical for sound research design.
4) Paradoxically, because of its abundance, 4He
often yields overestimations of groundwater age
since multiple sources and low diffusion velocities
may contribute to an accumulation of 4He
in an aquifer.
Despite the uncertainty of using 4He
for dating groundwater compared to 3H/3He,
or CFCs, it is effective in dating waters in the
range of 50 to 1000, where no other dating methods
- Andrews, J.N., I.S. Giles, R.I.F. Kay, D.J.
Lee, J.K. Osmond, J.B. Cowart, P. Fritz, J.F.
Barker, and J. Gale, Radioelements, radiogenic
helium and age relationships for groundwaters
from the granites at Stripa, Sweden, Geochimica
et Cosmochimica Acta, 46, 1533-1543, 1982.
- Castro, M.C., A. Jambon, G. de Marsily, and
P. Schlosser. Noble gases as natural tracers
of water circulation in the Paris Basin, 1.Measurements
and discussion of their origin and mechanisms
of vertical transport in the basin, Water
Resour. Res., 34(10), 2443-2466, 1998a.
- Castro, M.C., P. Goblet, E. Ledoux, S. Violette,
and G. de Marsily. Noble gases as natural tracers
of water circulation in the Paris Basin, 2.
Calibration of a groundwater flow model using
noble gas isotope data, Water Resour. Res.,
34(10), 2467-2482, 1998b.
- Clark, I., and P. Fritz. Environmental
Isotopes in Hydrogeology, Lewis Publishers,
Boca Raton, 1997.
- Torgersen, T., and W.B. Clarke. Helium accumulation
in groundwater, I: An evaluation of sources
and the continental flux of crustal 4He
in the Great Artesian basin, Australia, Geochimica
et Cosmochimica Acta, 49, 1211-1218, 1985.
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Periodic Table - Helium
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