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
  
• Origin
  
• Measurement Techniques
  
• Hydrological Applications
  
• References and further reading
• Internet resources

Cost of Analysis (return to top)

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 (return to top)

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 ³He_t/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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