Experimental and numerical studies on excess‐air formation in quasi‐saturated porous media

Abstract

[1] The concentrations of conservative groundwater components of atmospheric origin often exceed the concentrations in equilibrium with the atmosphere. This phenomenon, called ''excess air'', is caused by gas exchange between entrapped gas and groundwater. We present experimental results from a horizontal quasi-saturated sand column, in which we have analyzed excess-air formation by noble-gas analysis and measurement of the total dissolved gas pressure. The experimental results agree with a numerical model based on kinetic dissolution of spherical gas bubbles. Parameter studies show that the dissolution of entrapped gas is controlled by gas-transfer kinetics only when the contact time between the water and gas phases is relatively short. In natural systems, this could be the case for gas exchange within the hyporheic zone of natural rivers. In the typical situation of excess-air formation upon regional groundwater recharge, it seems acceptable to apply gas-transfer models assuming local equilibrium. Additional parameter studies show that the direction of flow considerably affects the evolution of dissolution fronts. Our experimental and numerical results show that apparently unfractionated excess air is formed in the presence of a progressively dissolving gas phase, that is, the excess-air component has an elemental composition similar to that of free atmospheric air. This observation contradicts the common conceptual model that unfractionated excess air indicates the complete dissolution of entrapped air, as formulated in the unfractionated excess air (UA) model (Heaton and Vogel, 1981) which is also contained as a special case in the closed-system equilibrium (CE) model of Aeschbach-Hertig et al. (2000). We conjecture that the estimated fractionation factor, F, and amount of initial gas, A, determined by fitting the CE model to noble-gas concentrations, are biased when F is zero (apparently unfractionated excess air) and A corresponds to initial gas saturations smaller than one per cent.Citation: Klump, S., O. A. Cirpka, H. Surbeck, and R. Kipfer (2008), Experimental and numerical studies on excess-air formation in quasi-saturated porous media, Water Resour. Res., 44, W05402,

Impact of recharge variations on water quality as indicated by excess air in groundwater of the Kalahari, Botswana

“…This confirmed that excess air reflects the amplitude of water table fluctuations and hence the variability of recharge and precipitation, although the influence of periodicity of these fluctuations remains unknown. These findings were further confirmed by experimental and numerical studies on the formation of excess air (Holocher et al, , 2003Klump et al, 2007Klump et al, , 2008 which confirmed the dominance of total pressure on the formation of excess air.…”

“…If the hydrochemical and isotopic differences (EC, nitrate content, carbon isotopes, noble gas isotopes) between the different groundwater end-members are mainly related to temporal changes of recharge processes, the most dramatic effect on recharge may be expected for climate variations. Although the current understanding of excess-air formation is insufficient to unambiguously relate excess-air values to climatic conditions at the time of recharge (Klump et al, 2008), excess air may be used to distinguish between dry and wet conditions at the time of infiltration (Heaton et al, 1983;Stute and Talma, 1998;Beyerle et al, 2003). Information about paleoclimatic conditions in the Kalahari is sparse.…”

“…On the other hand, the estimated pressure factor q CE of 0.7 for samples with high excess Ne values suggests that recharge of groundwater EM2 is related to water table fluctuations of high amplitude. This shows that in the study area, excess Ne values can be used as a reasonable indicator of the hydrostatic pressure (at least for the end-members of the mixing line), although the pressure factor q CE provides a more robust proxy of the pressure on the entrapped gas (Klump et al, 2008). The presumed groundwater table fluctuations can be associated with the temporal variability of groundwater recharge .…”

A 40 ka record of temperature and permafrost conditions in northwestern Europe from noble gases in the Ledo‐Paniselian Aquifer (Belgium)

“…Experiments have shown that even if the real mechanisms of gas/water exchange are more complex than assumed by the CE model, the estimated NGTs are close to the actual equilibration temperatures (Klump et al , 2008). Another advantage of the CE model, which is particularly important for the present study, is that it can also describe degassing, as discussed by Aeschbach‐Hertig et al (2008).…”

Dating of ‘young’ groundwaters using environmental tracers: advantages, applications, and research needs

“…Two-phase flow and kinetic gas exchange models can shed light on the details of the processes for both excess air formation and degassing [31,55,56]. Both laboratory as well as small-scale field experiments have been performed to study the physical processes involved in the formation of excess air [35,56,57], but similar approaches to the problem of degassing are lacking so far.…”