Atmospheric noble gases 1

He, Ne, Ar, Kr, Xe, and Rn are called noble gases (or inerrt gases) because of their inactivity in chemical reactions. A nice summary of their discovery and history can be found here.

Abundance

Most aqueous systems are dominated by atmospheric noble gases:

atmospheric abundance of noble gases (Tab)
atmospheric isotope ratios (Tab)

noble gases concentrations in water are a function of temperature and salinity

they are often expressed as Bunsen coefficient (ccSTP of gas dissolved per cc of water at 1 atmosphere of pressure of the gas at a given temperature) (Fig)

Early studies of atmospheric noble gases in isotope hydrology

Oana (1957) studied the noble gas concentrations in rainwater, groundwater, and surface water and found that rainwater showed a seasonal variation, while groundwater did not (Fig)
Mazor (1972) suggested that paleoclimate information may be contained in aquifers
the first paleotemperature record was obtained by Andrews and Lee from an aquifer in the UK (Fig)

they found a difference between the noble gas temperature of Holocene and Glacial groundwater samples of about 6^oC and that there apparently was no recharge during the last glacial maximum

Heaton et al. (1981) measured N₂ and Ar on surface waters and groundwaters and discovered 'excess air' (fig)

Processes in the recharge area or in surface water bodies

Solubility equilibrium

the initial condition for the noble gases is set at the water table (fig): C = b (T,S) * P/P_o
b is the Bunsen coefficient as a function of temperature T and salinity S, P is the partial pressure of the noble gas at the water table, Po = 1013.25 mbar
noble gas concentrations as a function of T (absolute (Fig), relative (Fig) and salinity (Fig))
equations describing the concentrations of noble gases dissolved in water (in ccSTP/g) as a function of temperature often have the form (example for Ar, atmospheric pressure, 100% relative humidity):

C_Ar = 0.001 * EXP(-178.1725+251.8139*100/(273.15+T)+145.2337* LN((273.15+T)/100)-22.2046*(273.15+T)/100)

for equilibration at a different altitude, the concentrations need to be corrected by the equivalent drop in pressure.

for US Standard atmosphere:

C_Ar'(h) = C_Ar(0)*(P-P_H2O)/(P_o-P_H2O)

biological activity

partial pressures of O₂ and CO₂ can change but often do so in a complementary way

gravitational separation

heavier isotopes enriched at bottom of a column

thermal diffusion (due to temperature gradient)
water vapor flux fractionation (gradient in relative humidity near the surface) , Severinghaus et al., 1995)

fig,

Excess air formation

Xe vs Ne for two aquifers show fractionated and unfractionated excess aire relative to air (fig)
processes near the water table (fig)

dissolution of additional gas due to partial or total dissolution of bubbles
possibility for secondary gas exchange

solubility controlled (fig) or diffusion controlled (fig) secondary equilibration

Resources

Ozima, M and F. A. A. Podosek (2002) Noble gas geochemistry, Cambridge University Press
Aeschbach-Hertig, W., Peeters, F., Beyerle, U., and Kipfer, R. (1999). Interpretation of dissolved atmospheric noble gases in natural waters. Water Resources Research 35, 2779-2792.
Stute, M., and Schlosser, P. (2000). Atmospheric noble gases. In "Environmental tracers in subsurface hydrology" (P. G. Cook and A. L. Herczeg, eds.), pp. 349-377. Kluwer, Boston.