Experiments on the formation of excess air in groundwater under various recharge regimes

Excess air is a contribution to the gases dissolved in groundwater in addition to the solubility equilibrium component, formed by partial or total dissolution of air trapped during water level rises in the unsaturated zone. The amount of excess air can be quite large, mainly if the water level increase is significant, for example in case of recharge from ephemeral streams in semi-arid regions or artificial recharge. The increased gas amount, especially of oxygen, can have an influence on biological activity, water treatment, and quality. In order to investigate the applicability of excess air as a tool to identify recharge mechanisms, we examine the formation of excess air under field and laboratory conditions using all five noble gases. In laboratory experiments with plexiglas columns filled with different types of sand, we investigate how the excess air amount and composition depend on the hydrostatic pressure as well as the size distribution of the sand and the entrapped air bubbles. In field experiments we study the relationship between excess air and water level fluctuations. Two study sites were selected where the groundwater level increased due to artificial recharge (Basel, Switzerland) and due to river floods (Danube River, Hungary). (author)