Steady state fractionation of heavy noble gas isotopes in a deep unsaturated zone

TitleSteady state fractionation of heavy noble gas isotopes in a deep unsaturated zone
Publication TypeJournal Article
Year of Publication2017
AuthorsSeltzer AM, Severinghaus JP, Andraski BJ, Stonestrom DA
JournalWater Resources Research
Volume53
Pagination2716-2732
Date Published2017/05
ISBN Number1944-7973
Keywords1009 Geochemical modeling; 1631 Land/atmosphere interactions; 1719 Hydrology; 1875 Vadose zone; 3346 Planetary meteorology; Groundwater; isotopic fractionation; noble gas; unsaturated zone
Abstract

To explore steady state fractionation processes in the unsaturated zone (UZ), we measured argon, krypton, and xenon isotope ratios throughout a ∼110 m deep UZ at the United States Geological Survey (USGS) Amargosa Desert Research Site (ADRS) in Nevada, USA. Prior work has suggested that gravitational settling should create a nearly linear increase in heavy-to-light isotope ratios toward the bottom of stagnant air columns in porous media. Our high-precision measurements revealed a binary mixture between (1) expected steady state isotopic compositions and (2) unfractionated atmospheric air. We hypothesize that the presence of an unsealed pipe connecting the surface to the water table allowed for direct inflow of surface air in response to extensive UZ gas sampling prior to our first (2015) measurements. Observed isotopic resettling in deep UZ samples collected a year later, after sealing the pipe, supports this interpretation. Data and modeling each suggest that the strong influence of gravitational settling and weaker influences of thermal diffusion and fluxes of CO2 and water vapor accurately describe steady state isotopic fractionation of argon, krypton, and xenon within the UZ. The data confirm that heavy noble gas isotopes are sensitive indicators of UZ depth. Based on this finding, we outline a potential inverse approach to quantify past water table depths from noble gas isotope measurements in paleogroundwater, after accounting for fractionation during dissolution of UZ air and bubbles.

DOI10.1002/2016WR019655
Short TitleWater Resour. Res.
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