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

Water Resources Research
By: , and 

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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.

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Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Steady state fractionation of heavy noble gas isotopes in a deep unsaturated zone
Series title Water Resources Research
DOI 10.1002/2016WR019655
Volume 53
Issue 4
Year Published 2017
Language English
Publisher American Geophysical Union
Contributing office(s) National Research Program - Western Branch, Nevada Water Science Center
Description 17 p.
First page 2716
Last page 2732
Country United States
State Nevada
County Nye County
Other Geospatial Amargosa Desert Research Site