The contamination of groundwater by geogenic arsenic is the cause of major health problems in south and southeast Asia. Various hypotheses proposing that As is mobilized by the reduction of iron (oxy)hydroxides are now under discussion. One important and controversial question concerns the possibility that As contamination might be related to the extraction of groundwater for irrigation purposes. If As were mobilized by the inflow of re-infiltrating irrigation water rich in labile organic carbon, As-contaminated groundwater would have been recharged after the introduction of groundwater irrigation 20-40 years ago. We used environmental tracer data and conceptual groundwater flow and transport modeling to study the effects of groundwater pumping and to assess the role of reinfiltrated irrigation water in the mobilization of As. Both the tracer data and the model results suggest that pumping induces convergent groundwater flow to the depth of extraction and causes shallow, young groundwater to mix with deep, old groundwater. The As concentrations are greatest at a depth of 30 m where these two groundwater bodies come into contact and mix. There, within the mixing zone, groundwater age significantly exceeds 30 years, indicating that recharge of most of the contaminated water occurred before groundwater irrigation became established in Bangladesh. Hence, at least at our study site, the results call into question the validity of the hypothesis that re-infiltrated irrigation water is the direct cause of As mobilization; however, the tracer data suggest that, at our site, hydraulic changes due to groundwater extraction for irrigation might be related to the mobilization of As.
[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,
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