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 ´ 10 −11 m 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 atmospheric noble gases across a range of CO 2 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 (CO 2 ) and conserved (noble gas) species or attributes of the soil column relevant to gas transport, such as porosity, tortuosity, and gas saturation.

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

“…Noble gases in the unsaturated zone are effective physical tracers of advection and diffusion in porous media, as confirmed by both field [ Weeks et al ., ] and laboratory [ Ding et al ., ] studies. In an idealized one‐dimensional UZ, atmospheric noble gases diffuse to reach steady state concentrations.…”

Gas source attribution techniques for assessing leakage at geologic CO2 storage sites: Evaluating a CO2 and CH4 soil gas anomaly at the Cranfield CO2-EOR site

“…468gas model. The dusty gas model is a mathematical description of viscous and diffusive 469 fluxes, coupled with the Knudsen diffusivity of the porous media(Ding et al, 2016). The 470 addition of CO 2 into previously stagnant gases results in differential diffusion caused by 471 mass differences between components(Evans et al, 2001;Ding et al, 2016).…”

“…The dusty gas model is a mathematical description of viscous and diffusive 469 fluxes, coupled with the Knudsen diffusivity of the porous media(Ding et al, 2016). The 470 addition of CO 2 into previously stagnant gases results in differential diffusion caused by 471 mass differences between components(Evans et al, 2001;Ding et al, 2016). This 472 fractionation effect has modeled experimental data using a dusty gas model that 473calculates changes in noble gas components on the basis of two end members:474 exogenous CO 2 and atmospheric noble gas compositions.…”

Biologically influenced gas fluxes revealed by high‐resolution monitoring of unsaturated soil columns

“…Evans et al (2001) assessed, using a dry sand column, the performance of accumulation chambers for volcanic or metamorphic CO 2 fluxes much higher than normally encountered in soil respiration studies. With simple laboratory sand column experiments, Ding, Kennedy, Evans, and Stonestrom (2016) and Sathaye, Larson, and Hesse (2016) evaluated the fractionation of noble gases as a tool to quantify gas dynamics in unsaturated porous media. These attempts used simplified abiotic systems and constant parameters.…”