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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.

1We developed a portable mass spectrometric system ("miniRuedi") for quantificaton of 2 the partial pressures of He, Ne (in dry gas), Ar, Kr, N 2 , O 2 , CO 2 and CH 4 in gaseous and 3 aqueous matrices in environmental systems with an analytical uncertainty of 1-3 %. The 4 miniRuedi does not require any purification or other preparation of the sampled gases and 5 therefore allows maintenance-free and autonomous operation. The apparatus is most suit-6 able for on-site gas analysis during field work and at remote locations due to its small size 7 (60 cm × 40 cm × 14 cm), low weight (13 kg), and low power consumption (50 W). The gases 8 are continuously sampled and transferred through a capillary pressure reduction system into 9 a vacuum chamber, where they are analysed using a quadrupole mass spectrometer with a 10 time resolution of 1 min. The low gas consumption rate (< 0.1 ml/min) minimises interfer-11 ence with the natural mass balance of gases in environmental systems, and allows the unbi-12 ased quantification of dissolved-gas concentrations in water by gas/water equilibration using 13 membrane contractors (gas-equilibrium membrane-inlet mass spectrometry, GE-MIMS). The 14 performance of the miniRuedi is demonstrated in laboratory and field tests, and its utility is 15 illustrated in field applications related to soil-gas formation, lake/atmosphere gas exchange, 16 and seafloor gas emanations.

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Abstract-The formation of an excess of dissolved gas ("excess air") in quasi-saturated media was studied by analyzing and interpreting dissolved noble gas concentrations in laboratory column experiments. Using quartz sand filled columns of 1 m length, two different experimental designs were realized. In the first, groundwater recharge was simulated by a unidirectional vertical water flow through the columns. In the second, groundwater level fluctuations in an aquifer zone without active infiltration were reproduced by cyclic water level fluctuations in the columns. The reproducible generation of excess air under these defined, near natural conditions was successful. Partial or complete dissolution of air bubbles entrapped in the quartz sand could be identified as the mechanism responsible for the generation of excess air. Depending on the experimental design, supersaturation of the dissolved atmospheric noble gases ranging between 1.4% ⌬Ne and 16.2% ⌬Ne was found. The measured noble gas patterns were interpreted using inverse modeling and conceptual gas exchange models and were compared to results of numerical simulations of gas bubble dissolution in water filled soil columns. The gas composition in most of the samples resembles either unfractionated pure atmospheric excess air or is fractionated in accordance with closed-system equilibration between entrapped air and surrounding water. In addition to the amount of entrapped air, the hydrostatic pressure exerted on the entrapped air bubbles is the dominating parameter responsible for the total amount of dissolved air. The composition of the excess air component is controlled by the water flow regime, the bubble size distribution, the initially dissolved gas concentrations and the initially entrapped gas composition.