EXCESS AIR IN GROUNDWATER: PROBLEMS AND OPPORTUNITIES

Dissolved conservative gases, especially noble gases, are powerful tools to study hydrogeological processes and environmental conditions during groundwater recharge. Such applications rely either on the atmospheric solubility equilibrium concentrations (noble gases, CFCs, SF₆) or on non-atmospheric excesses (tritiogenic ³He, radiogenic ⁴He). However, an additional atmosphere-derived component is ubiquitous in groundwater: the so-called “excess air”. This fact complicates the estimation of recharge temperatures or groundwater ages from dissolved gas data.

Excess air originates from the (partial) dissolution of air bubbles that are trapped in the quasi-saturated soil zone, i.e. the zone affected by periodic fluctuations of the groundwater table. Our current understanding of the formation and composition of excess air is in principle sufficient to enable a reliable determination of the atmospheric equilibrium and non-atmospheric excess components, if sufficient data are available. In many cases, simplifying assumptions (e.g. that the composition of excess air equals that of atmospheric air) have to be made, which can lead to systematic deviations of the derived ages or temperatures.

However, excess air is not merely an annoyance, it also discloses new opportunities to learn about the environmental conditions prevailing during groundwater recharge. Laboratory column experiments have confirmed that the size of the excess air component, often expressed by the relative Ne excess ΔNe, is primarily controlled by the hydrostatic pressure on the entrapped gas bubbles. The hydrostatic pressure in the quasi-saturated zone depends on the amplitude of water table fluctuations, which in turn reflects the intensity and variability of recharge. As a result, ΔNe is expected to be linked to the climate variables precipitation or humidity.

This link is particularly apparent in semi-arid climate zones, where precipitation exhibits large seasonal and inter-annual fluctuations. Systematic, climate-related variations of ΔNe have been observed in several noble gas field studies conducted in semi-arid regions, including two recent examples from Niger and Botswana. Very large excess air concentrations observed in such studies raise questions about the recharge mechanisms that lead to their formation.