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Noble Gases

Introduction (return to top)

The noble gases are any of the chemically inert elements found on the far right of the periodic table in the helium group. The noble gas family consists of six different elements, five of which have stable isotopes (helium [He], neon [Ne], argon [Ar], krypton [Kr], and xenon [Xe]), and one that only has radioactive isotopes (radon [Ra]). Table 1 lists the stable isotopes in the noble gas family and their abundances per element. The radioactive isotopes of argon and krypton and the stable isotopes of helium are described separately on this website.

Source: WebElements.com

Cost of Analysis (return to top)

Gas Chromatography/Mass Spectrometry:

Price ranges for noble gas analysis are not currently posted. Many research labs perform this analysis but on an in-house basis only. For more information, visit web sites for individual labs such as:
Noble Gas Lab of Columbia University

Rosenstiel School of Marine and Atmospheric Science Noble Gas Isotope Lab

UA Center for Thermochronology and Noble Gas Studies

MIT Noble Gas Laboratories

Reston Chlorofluorocarbon Lab



Origin
(return to top)

Atmospheric Composition

Noble gases are naturally occurring and present in the earth's atmosphere.
Table 2 displays the abundances for each of these gases within the atmosphere:

Source: http://www.c-f-c.com/charts/atmosph.htm


Most of the noble gases have been in the earth's atmosphere since the earth formed. Two exceptions are radon and helium. Radon is continuously produced as a daughter product during uranium decay. It is radioactive and decays quickly to other elements. Due to its light weight, most helium originally present in the atmosphere has escaped to space. The helium currently present in the earth's atmosphere is largely the product of alpha decay of heavy isotopes (to 4He) and the beta decay of cosmogenic tritium (to 3He).



Measurement Techniques (return to top)

Sampling
Samples are typically taken from wells through a plastic hose into a copper tube (~1cm in diameter, <1m long). Water is run through the tube for several minutes before it is sealed using stainless steel clamps or special pliers. A regulator valve at one end of the copper tube is typically used to increase the pressure in order to minimize bubble formation. Detailed instructions for sampling are outlined at the USGS Reston Chlorofluorocarbon Laboratory FAQ page.


Measurement

Noble gas concentrations in groundwater samples can currently be measured in several ways: gas chromatography, mass spectrometry, or a combination of both.

The precision of mass spectrometry leads to a reproducibility (including all steps in the sampling and measurement procedures) of ± 1-2% for the abundance of each gas and ±0.1-1% for ratios of more abundant isotopes. Gas chromatography is slightly less precise.



Noble Gas Behavior (return to top)

The behavior of noble gases is governed by three factors: diffusion, partition between phases, and nuclear transformation.

Diffusion is the process by which the gases move from areas of higher concentration to areas of lower concentration until equilibrium is reached. Lighter isotopes are more mobile and thus more affected by diffusion than heavier isotopes. In gases, the ratio of diffusion coefficients is equal to the square root of their masses.

Partitioning between phases is an important process in bodies of water in contact with the atmosphere. Lakes, rivers, oceans, and groundwater at the water table are all generally assumed to be in equilibrium with the atmosphere with respect to their concentrations of noble gases. The atmosphere is the greatest source of noble gases for these bodies of water. The equilibrium relationship between the atmosphere and the body of water is described in Henry's Law:

pi = ki(T,S)xi

where pi is the partial pressure of the noble gas in the gas phase,
ki is a proportionality coefficient (Henry's coefficient, dependent on temperature [T] and salinity [S]),
and xi is the equilibrium concentration of the dissolved gas in water (or other liquid) as a mole fraction.
In other words, the equilibrium concentration of the noble gases depends on three parameters: temperature, salinity, and atmospheric pressure.

Nuclear transformation through radioactive decay may be important for some isosopes. For example, 4He [link] is a stable product of many U-series decay paths. In contrast, 222Rn gas is a radioactive intermediate.



Hydrological Applications (return to top)

Assuming that salinity and atmospheric pressure are known, water that is at equilibrium with the atmosphere at the time of recharge and is subsequently sealed off from the atmosphere may be used to reconstruct the temperature at the time of the recharge. Since noble gases tend to not react with other elements, the concentrations of noble gases in groundwater remain fairly constant once sealed off from the atmosphere.

Two additional parameters need to be considered in the reconstruction of these past or paleo-temperatures: amount of excess air (A) and degree of re-equilibration (R). Excess air is dissolved gas in the groundwater that does not originate with the initial equilibration with the atmosphere. One possible scenario for the formation of this excess air is as follows: the water table rises, quickly trapping air bubbles in the groundwater; the air bubbles partially dissolve in the water (with the heavier noble gases partially dissolving first, leading to fractionation); the gas slowly exchanges with the atmosphere (re-equilibration) until new recharge seals off the exchange (Stute and Schlosser 2000). The resulting gas component in the water is considered excess air. It can be recognized when the ratio of N2 to Ar in the sample is different from the equilibrium solubility ratio of N2 to Ar in water. High concentrations of excess air are caused by a rapid rise in the water table and are indicative of fractured rock aquifers and aquifers in semi-arid lands.

Because the solubility of gases varies as a function of temperature, noble gas concentrations can be used to determine recharge temperature of a groundwater sample. (Studies generally use stable neon, argon, krypton, and xenon gases in reconstructing the temperature of recharge from years to millions of years ago; due to high radiogenic production, helium is not often considered). This may provide a clue as to where the recharge occurred. Likewise, if combined with an age-dating isotope such as 3H, 36Cl, or 14C, a record of temperature can be reconstructed for the aquifer system. Such information is useful for determining change in groundwater recharge and the relationship to climate change. This technique deals with total gas concentration, not one particular isotope. Several noble gases must be measured for the best temperature determination. The solubilities of noble gases are highly temperature dependent but each isotope has a unique relationship to temperature. Therefore, by determining the ratios of several noble gas concentrations in groundwater samples, recharge temperature can be determined:


Variation in noble gas solubilities with temperature relative to their solubility at 0ºC (modified from Benson and Krause 1976 and Andrews 1992).

 

References and further reading (return to top)

  • Aeschbach-Hertig, W., et al, Interpretation of dissolved atmospheric noble gases in natural waters, Water Resour. Res. 35(9), 2779-2792, 1999.


  • Andrews, J.N., Mechanisms for noble gas dissolution by groundwaters, in Isotopes of Noble Gases as Tracers in Environmental Studies, pp. 87-110, International Atomic Energy Agency, Vienna, Austria, 1992.


  • Attendorn, H.G., and R.N.C. Bowen, Radioactive and Stable Isotope Geology, Chapman and Hall, New York, 1997.


  • Benson, B.B., and D. Krause, Empirical laws for dilute aqueous solutions of nonpolar gases, J. Chem. Phys. 64, 639-709, 1976.


  • Petrucci, R.H., Nuclear chemistry, in General Chemistry, 5th ed., pp. 936-941, Macmillan Publishing, New York, 1985.


  • Stute, M., and P. Schlosser, Atmospheric noble gases, in Environmental Tracers in Subsurface Hydrology, edited by P. Cook and A.L. Herczeg, pp. 298-348, Kluwer Academic Publishers, Norwell, MA, 2000.


Internet resources (return to top)

Chemical Elements.com

EAWAG (Swiss Federal Institute for Environmental Science and Technology, Environmental Isotopes Group, Interpretation of dissolved noble gases in groundwater.

Lawrence Berkeley National Laboratory, Chemistry and Material Science Division

Winter, Mark, WebElements Periodic table

 

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