Fig. 2: Dissolved Kr and Xe isotope ratios in groundwater.

Isotope ratio measurements in 58 groundwater samples collected from 36 wells illustrate the dominance of gravitational settling in driving isotopic departures from solubility equilibrium. Mojave Desert samples for which δ*Kr exceeds 200 per meg amu⁻¹ are shown both as original values (open circles) and corrected for fractionation due to oxygen consumption (Supplemental Note 5). Error bars indicate ±2-σ uncertainty. Inset: predicted fractionation associated with (1) two meters of gravitational settling; (2) thermal diffusion caused by a 2 °C difference between the surface and water table (WT); (3) a steady-state WT-to-atmosphere water-vapor flux driven by a 0.81% WT-atmosphere absolute humidity difference (equivalent to 20 °C, 65% relative humidity surface air); (4) complete dissolution of entrapped soil air bubbles (equivalent to 50% ΔNe); and (5) steady-state oxygen depletion leading to a 0.5% atmosphere-to-deep unsaturated zone difference in the sum of O₂ and CO₂. Note that the inset scale is magnified.