ABSTRACT
Helium, neon, argon, krypton, and xenon occupy a special place in the periodic table-their outer shell of electrons is complete (2, 8, or 18) and therefore they do not interact with other atoms (section 5.4). This property gives these elements the name inert or noble. They are gases, and they are present in small quantities in air (Table 13.1); hence, they are also called rare gases. It has been pointed out that one of them, argon, is present in air in a concentration of nearly 1%, and therefore the term “rare” was challenged. In the last decade a few xenon and krypton compounds have been prepared under specific laboratory conditions, and the term “inert” has been challenged as well. In nature, however, their chemical nobility is well preserved, making them ideal tracers in hydrological systems. They have a number of advantages, including
• They are not involved in any chemical or biological activity. • They enter groundwater from two distinct major sources: by equilibration
with air during infiltration or from deep-seated
origins, which include flushing of radiogenic products from aquifer rocks and from mantle-derived gases. The origin of these different sources is identifiable by the isotopic composition.
