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Helium

Helium has two isotopes, 3He and 4He. 4He (an alpha particle) is produced through radioactive decay and can therefore be used to determine groundwater age.

 


Cost of Analysis
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3H/3He Analysis Using Ingrowth Method (described below): This is currently a research method only, and is usually not available commercially.

(See Environmental Tracer Group, Lamont-Doherty Earth Observatory at Columbia University)

(See also USGS Reston Chlorofluorocarbon Laboratory)

(See also University of Miami Rosenstiel School of Marine and Atmospheric Science for more information)

3He/4He Research Method: Cost not available



Origin
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Helium has two stable isotopes, 3He and 4He.

4He 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, 235U and 232Th. 4He 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:

6Li + n ® a + 3He ® 4He + 3He + b-

3He is also radiogenic but comparatively scarce (abundance = .000137%). It is generated through decay of tritium (3H) 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.



Measurement Techniques (return to top)

Mass spectrometry
3He and its parent 3H can be measured using mass spectrometry, but other dissolved gases (H2O, CO2, O2, N2, etc.) must be removed first. Lower detection limits are achieved by mass spectrometric measurements of 3He produced by the "ingrowth method," where the tritium sample is stripped of all gases and then stored for a sufficient time for enough daughter product 3He to be produced and measured. The mass spectrometer method has the advantage over the liquid scintillation method because both 3He ingrowth (3H) and the 3He of a sample can be measured. This provides quantitative age determinations discussed below.

(See the Mass Spectrometry page for more information)

(See the Lawrence Berkeley National Laboratory site for more information on the ingrowth method.


Sampling techniques

3H/3He
3H/3He sampling techniques are outlined at the USGS Reston Chlorofluorocarbon Laboratory.

4He
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 trap. 4He is then collected and measured on a Faraday collector.

3He/4He
A mass spectrometer is necessary for measuring the 3He/4He ratio.



Hydrological Applications (return to top)

3He
Groundwater can be aged quantitatively using 3H and 3He. Age is determined by:

where t is the time since isolation from contact with the atmosphere after decay (estimated age), and 3Het/3Ht is the concentration ratio of the two isotopes expressed in tritium units (TUs). One tritium unit is equal to one molecule of 3H per 1018 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 3He 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.


4He
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 a-decay processes predominate, and where subsurface flow conditions are well-constrained.

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 the 4He radiogenic component.


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 f-1 t ( (1.19*10-13 [U]) + (2.88*10-14 [Th] ) )

where:

  • [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 to water
· duration of rock-fluid contact
· porosity
· 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, 85Kr, or CFCs, it is effective in dating waters in the range of 50 to 1000, where no other dating methods are practical.


References and further reading (return to top)

  • 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.

Internet resources (return to top)

USGS Periodic Table - Helium

WebElements.com



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