Excess air in groundwater
The experience from many measurements of dissolved conservative gases (mostly noble gases) in groundwater has shown that their concentrations lie almost always above the expected solubility equilibrium with the atmosphere. The composition of the gas excess relative to the solubility equilibrium proves its atmospheric origin, which lead to the common term "excess air" (Heaton and Vogel, 1981). The cause of the excess air are certainly small air bubbles that are trapped in the so-called quasi-saturated zone during a rise of the groundwater table (Faybishenko, 1995).
Originally it has been assumed that excess air in groundwater was due to complete dissolution of such entrapped air bubbles (Heaton and Vogel, 1981; Andrews et al., 1979) and therefore should have the exact same composition as air. The detailed investigation of noble gas data sets, especially with inverse numerical methods (Ballentine and Hall 1999; Aeschbach-Hertig et al. 1999) has however shown that this assumption is in many cases not correct. As a solution, more complex models of the formation of excess air have been developed (Stute et al. 1995, Aeschbach-Hertig et al. 2000), which include a fractionation of the composition of excess air with respect to air.
According to our experience, the model of formation and composition of excess air by equilibration between water and entrapped bubbles in a closed system has proven to be very reliable (Aeschbach-Hertig et al. 2000). This model also provides a theoretical basis for the interpretation of the excess air as an indicator of environmental conditions during groundwater recharge (Aeschbach-Hertig et al. 2002a). It shows that the hydrostatic pressure has a dominating influence on the size of excess air. Because the hydrostatic pressure is related to water table fluctuations, eventually a relationship between excess air and recharge or precipitation intensity can be expected (Aeschbach-Hertig et al. 2002a, Beyerle et al. 2003, Kulongoski et al. 2004). These relationships have essentially been corroborated by detailed investigations on excess air under controlled conditions (Holocher et al., 2002, 2003).
Because of the importance of the excess air correction for the 3H-3He and SF6-methods, we combine whenever possible complete noble gas analyses with these dating methods. Both the recharge temperature and the excess air component can then reliably be determined by the inverse modeling and are correctly accounted for (including a possible fractionation) in the age calculation.