Experimental studies and model analysis of noble gas fractionation in porous media

Vadose Zone Journal
By: , and 

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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 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 ´ 10−11 m2), 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−2 d−1. 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.

Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Experimental studies and model analysis of noble gas fractionation in porous media
Series title Vadose Zone Journal
DOI 10.2136/vzj2015.06.0095
Volume 15
Issue 2
Year Published 2016
Language English
Publisher Soil Science Society of America
Contributing office(s) National Research Program - Western Branch, Toxic Substances Hydrology Program
Description 13 p.
First page 1
Last page 12
Online Only (Y/N) N
Additional Online Files (Y/N) N
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