Ground water commonly dissolves an excess of
atmospheric gases during recharge infiltration. The nature of the processes responsible for this phenomenon - known as the excess air problem of ground water (Heaton and Vogel, 1981) - is poorly understood.
In the Toess Valley (Toesstal), Switzerland, the river Toess infiltrates continuously through the riverbed and feeds the groundwater in the aquifer beneath. Linsental is a branch of the Toess Valley located close to the city of Winterthur. Here, the Swiss Federal Institute of Environmental Science and Technology (EAWAG) has initiated an interdisciplinary research program focusing on the physical and chemical processes at an infiltration site where local river water enters the aquifer. To accomplish this, a suite of several boreholes has been drilled along the direction of supposed aquifer flow over a distance of about 150 m beginning at the river bed. Samples can be taken from each borehole at defined depths from 4 - 20 m.
The concentrations of various noble gases (He, Ne, Ar, Kr, Xe), N2 and freons (CFC11, CFC12) in water samples obtained from the boreholes during different seasons were determined.
Although inert gas concentrations in the river water are within a few percent of equilibrium with the atmosphere, all ground water samples analyzed were found to contain an excess of inert gases of up to 30%. Supersaturation levels for individual gases exhibit a high degree of mutual correlation.
In June, maximum supersaturation is found a few meters from the river. With increasing distance from the river, the gas excesses decrease continuously. At any given drill site, the degree of supersaturation generally increases with depth. All water samples (except one from the deep aquifer) do not have within the statistical uncertainties any tritiogenic 3He, implying that the residence time since infiltration is close to zero for samples near the river.
At the end of November the overall situation changed completely, the concentrations of inert gases within the aquifer generally becoming homogeneous. All deep samples near the river showed the same degree of supersaturation, which was of the same order as that of the maximum value found in June. All samples were found to have tritiogenic 3He, but the water ages are smaller to that of the deep aquifer, in which conditions seem to remain constant with time.
- The Linsental aquifer always exhibits an excess of atmospheric inert gases. Equilibrium with the atmosphere is never attained.
- In summer the aquifer is not sealed off from the atmosphere, and gas exchange occurs between the aquifer and the atmosphere. The largest gas excesses are found close to the infiltration site. The negligible tritiogenic 3He concentrations point to local infiltration of river water.
- Later in the year the aquifer becomes decoupled from the atmosphere. Inert gas concentrations within the aquifer become homogeneous and the corresponding excesses are large. Relatively high tritiogenic 3He concentrations indicate that local recharge decreases during winter.
- Further investigations concentrating on the elemental ratios of inert gases in supersaturated ground water and in the atmosphere will lead to a better understanding of the excess air problem.
Heaton, T. H. E. & Vogel, J. C., L. Hydrol. 50, 201-216 (1981).