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Title: Experimental Studies and Model Analysis of Noble Gas Fractionation in Porous Media

Abstract

The noble gases, which are chemically inert under normal terrestrial conditions but vary systematically across a wide range of atomic mass and diffusivity, offer a multicomponent approach to investigating gas dynamics in unsaturated soil horizons, including transfer of gas between saturated zones, unsaturated zones, and the atmosphere. To evaluate the degree to which fractionation of noble gases in the presence of an advective diffusive flux agrees with existing theory, a simple laboratory sand column experiment was conducted. Pure CO 2 was injected at the base of the column, providing a series of constant CO 2 fluxes through the column. At five fixed sampling depths within the system, samples were collected for CO 2 and noble gas analyses, and ambient pressures were measured. Both the advection–diffusion and dusty gas models were used to simulate the behavior of CO 2 and noble gases under the experimental conditions, and the simulations were compared with the measured depth-dependent concentration profiles of the gases. Given the relatively high permeability of the sand column (5 x 10 -11m 2), Knudsen diffusion terms were small, and both the dusty gas model and the advection diffusion model accurately predicted the concentration profiles of the CO 2 and atmosphericmore » noble gases across a range of CO 2 flux from ~700 to 10,000 g m -2d -1. In conclusion, the agreement between predicted and measured gas concentrations demonstrated that, when applied to natural systems, the multi-component capability provided by the noble gases can be exploited to constrain component and total gas fluxes of non-conserved (CO 2) and conserved (noble gas) species or attributes of the soil column relevant to gas transport, such as porosity, tortuosity, and gas saturation.« less

Authors:
 [1];  [1];  [2];  [2]
  1. Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States). Center for Isotope Geochemistry
  2. U.S. Geological Survey, Menlo Park, CA (United States)
Publication Date:
Research Org.:
Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22). Chemical Sciences, Geosciences & Biosciences Division
OSTI Identifier:
1480709
Grant/Contract Number:  
AC02-05CH11231
Resource Type:
Accepted Manuscript
Journal Name:
Vadose Zone Journal
Additional Journal Information:
Journal Volume: 15; Journal Issue: 2; Journal ID: ISSN 1539-1663
Publisher:
Soil Science Society of America
Country of Publication:
United States
Language:
English
Subject:
54 ENVIRONMENTAL SCIENCES

Citation Formats

Ding, Xin, Kennedy, B. Mack, Evans, William C., and Stonestrom, David A. Experimental Studies and Model Analysis of Noble Gas Fractionation in Porous Media. United States: N. p., 2016. Web. doi:10.2136/vzj2015.06.0095.
Ding, Xin, Kennedy, B. Mack, Evans, William C., & Stonestrom, David A. Experimental Studies and Model Analysis of Noble Gas Fractionation in Porous Media. United States. doi:10.2136/vzj2015.06.0095.
Ding, Xin, Kennedy, B. Mack, Evans, William C., and Stonestrom, David A. Fri . "Experimental Studies and Model Analysis of Noble Gas Fractionation in Porous Media". United States. doi:10.2136/vzj2015.06.0095. https://www.osti.gov/servlets/purl/1480709.
@article{osti_1480709,
title = {Experimental Studies and Model Analysis of Noble Gas Fractionation in Porous Media},
author = {Ding, Xin and Kennedy, B. Mack and Evans, William C. and Stonestrom, David A.},
abstractNote = {The noble gases, which are chemically inert under normal terrestrial conditions but vary systematically across a wide range of atomic mass and diffusivity, offer a multicomponent approach to investigating gas dynamics in unsaturated soil horizons, including transfer of gas between saturated zones, unsaturated zones, and the atmosphere. To evaluate the degree to which fractionation of noble gases in the presence of an advective diffusive flux agrees with existing theory, a simple laboratory sand column experiment was conducted. Pure CO2 was injected at the base of the column, providing a series of constant CO2 fluxes through the column. At five fixed sampling depths within the system, samples were collected for CO2 and noble gas analyses, and ambient pressures were measured. Both the advection–diffusion and dusty gas models were used to simulate the behavior of CO2 and noble gases under the experimental conditions, and the simulations were compared with the measured depth-dependent concentration profiles of the gases. Given the relatively high permeability of the sand column (5 x 10-11m2), Knudsen diffusion terms were small, and both the dusty gas model and the advection diffusion model accurately predicted the concentration profiles of the CO2 and atmospheric noble gases across a range of CO2 flux from ~700 to 10,000 g m-2d-1. In conclusion, the agreement between predicted and measured gas concentrations demonstrated that, when applied to natural systems, the multi-component capability provided by the noble gases can be exploited to constrain component and total gas fluxes of non-conserved (CO2) and conserved (noble gas) species or attributes of the soil column relevant to gas transport, such as porosity, tortuosity, and gas saturation.},
doi = {10.2136/vzj2015.06.0095},
journal = {Vadose Zone Journal},
number = 2,
volume = 15,
place = {United States},
year = {2016},
month = {2}
}

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