2.7 Noble Gases
Noble gases are a group of chemical elements that are stable (i.e. unreactive) because their outer shell of valence electrons is full. They are also colorless, odorless, tasteless and non-flammable. The six naturally occurring noble gases are the chemical elements helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn). Radon and helium have been discussed above in the context of radiogenic tracers. The common isotopes of the other noble gases are neither radiogenic nor radioactive, and therefore represent the most stable of all tracers. In environmental applications, concentrations of nitrogen are often interpreted together with noble gas concentrations, and so it is also discussed here. (Although nitrogen is more reactive than the noble gases, dissolved N2 is relatively stable and nitrogen concentrations in water are more easily measured than concentrations of many of the noble gases.)
The atmospheric abundances of the noble gases and nitrogen have been essentially constant throughout geologic time, and so for most of these gases differences in concentrations in water are principally due to differences in gas solubility in water, which is a function of temperature, pressure and salinity (Section 2.8). Solubility is the amount of a substance (the solute) that dissolves in a unit volume of a liquid (the solvent) to form a saturated solution at a specified temperature and pressure. The exceptions are He and Rn, which are produced by radioactive decay in quantities that sometimes overwhelm that resulting from equilibrium solubility with the atmosphere. For the other noble gases (Ne, Ar, Kr and Xe) and N2, concentrations in water provide information on the temperature and pressure at the time of recharge.
Measurements of noble gas concentrations in groundwater have shown that concentrations in water are often much larger than can be explained by equilibrium solubility. Such ‘excess’ gas concentrations are referred to as excess air and believed to be due to addition of air due to entrapment of air bubbles during recharge and their subsequent dissolution (Heaton and Vogel, 1981). If all the air bubbles completely dissolve, then excess air entrapment is not a function of solubility, and so gas ratios in excess air are different to those due to equilibrium solubility. Measurement of at least two noble gases typically allows both recharge temperature and excess air to be independently quantified (Figure 9). In mountainous terrain, gas concentrations may provide information on elevation (and hence location of recharge; Peters et al., 2018), whereas in large regional aquifers measurement of gas concentrations in conjunction with groundwater dating can provide information on paleo-temperatures (Aeschbach-Hertig and Solomon, 2013; Section 3.8). A few studies have also suggested that excess air might provide information on recharge processes (e.g., Massmann and Sültenfuss, 2008; Hall et al., 2012).
