Atmospheric Noble Gases

A large fraction of gases dissolved in surface and groundwater, mainly N₂, O₂ and the noble gases He, Ne, Ar, Kr and Xe, originate from the atmosphere. Whenever water comes into contact with the atmosphere, or other phases such as natural gas, oil and solid organic matter, gases are exchanged and gas concentrations of the individual phases record some characteristics of these processes. Even in the absence of a separate gas phase, dissolved gas concentrations in a water parcel may change as a result of molecular diffusion, and mixing on a variety of space and time scales. However, in many cases the impacts of most of these processes on the dissolved gas concentrations are small and the dominating processes may be reconstructed from the measured concentrations of the dissolved gases. This chapter deals with atmospheric noble gases dissolved in groundwater, which reliably record information on certain physical processes due to the lack of chemical reactions which affect them. As has been shown in studies performed over the past 40 years, atmospheric noble gases dissolved in groundwater yield valuable information on palaeoclimate, in particular temperature at the time of recharge, dynamics of groundwater flow, and denitrification and oxygen consumption rates.