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

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.