Fig. 1: Steady-state xenon isotopic fractionation in an unconfined aquifer.
From: Deglacial water-table decline in Southern California recorded by noble gas isotopes
Gravitational settling causes mass difference-normalized, heavy-to-light xenon isotope ratios (δ*Xe) in unsaturated zone (UZ) air to increase with depth below the land surface, relative to atmospheric air. At the water table, dissolved gases in the saturated zone (SZ) equilibrate with deep UZ air. Solubility fractionation causes δ*Xe to further increase in the dissolved phase relative to the gas phase. Note that nearly all water-vapor flux fractionation occurs between the surface and shallow wetting front (at one meter depth in this idealized illustration), below which water-vapor flux fractionation is nearly constant with depth and only changes due to its weak sensitivity to temperature12,13.
