GROUNDWATER OXYGENATION DUE TO EXCESS AIR FORMATION

Atmospheric gases, including O₂, N₂ and (most) noble gases, are delivered to groundwater not only by atmosphere / water equilibration, but also by excess air (EA) formation (EAF), i.e., the entrapment and subsequent (partial) dissolution of air bubbles in the quasi-saturated zone. This process injects atmospheric gases into groundwater. Thus, the concentrations of atmospheric gases in groundwater commonly exceed saturation equilibrium. The bio-geochemically inert noble gases are used to quantify EA.

EAF delivers O₂ to the groundwater, which might have an impact on the biochemical state, e.g. boosting aerobic microbial activity and constraining denitrification.

Up to now data on EAF during groundwater recharge are very scarce, as no adequate analytical methods are available to study air / water partitioning in natural water under natural conditions.

Common methods to determine noble gases are laboratory based allowing only a limited number of samples to be analysed. As EAF happens fast, e.g. during water-table changes, the available methods for noble gas analysis are not adequate to study groundwater aeration.

We therefore developed a membrane inlet mass spectrometric system enabling the quasi-continuous measurements of dissolved (He, Ar, Kr, N₂, O₂, and CO₂, every 3 min.) concentrations in natural waters under field conditions on site (precision ~ 1-3% [1,2]).

The method was employed in river-groundwater system to analyze the temporal dynamics and the spatial distribution of dissolved gases in groundwater during different hydraulic conditions. During high river discharge after a flood, significant amounts of EA were produced in the vicinity of the studied groundwater wells. EAF and the corresponding oxygen input triggered aerobic microbial activity and inhibited complete denitrifictation processes [3]. The observed concentration increase of dissolved N₂ was identified being entirely generated by EAF.

Hence, only the profound understanding of physics of gas exchange constraining noble gas concentrations in groundwater enables studying the fate of the biogeochemically active gases, such as O₂, N₂, CO₂ and CH₄ in groundwater in mechanistic and quantitative terms.

[1] Environ. Sci. Technol., 46, 7927-8522, 2012.

[2] Environ. Sci. Technol., 47, 7060-7066, 2013.

[3] WRR, 49, 1-10, 2013.