Gas Component Transport Across the Soil‐Atmosphere Interface for Gases of Different Density: Experiments and Modeling

Abstract

We investigate the influence of near-surface wind conditions on subsurface gas transport and on soil-atmosphere gas exchange for gases of different density. Results of a sand tank experiment are supported by a numerical investigation with a fully coupled porous medium-free flow model, which accounts for wind turbulence. The experiment consists of a two-dimensional bench-scale soil tank containing homogeneous sand and an overlying wind tunnel. A point source was installed at the bottom of the tank. Gas concentrations were measured at multiple horizontal and vertical locations. Tested conditions include four wind velocities (0.2/1.0/2.0/2.7 m/s), three different gases (helium: light, nitrogen: neutral, and carbon dioxide: heavy), and two transport cases (1: steady-state gas supply from the point source; 2: transport under decreasing concentration gradient, subsequent to termination of gas supply). The model was used to assess flow patterns and gas fluxes across the soil surface. Results demonstrate that flow and transport in the vicinity of the surface are strongly coupled to the overlying wind field. An increase in wind velocity accelerates soil-atmosphere gas exchange. This is due to the effect of the wind profile on soil surface concentrations and due to wind-induced advection, which causes subsurface horizontal transport. The presence of gases with pronounced density difference to air adds additional complexity to the transport through the wind-affected soil layers. Wind impact differs between tested gases. Observed transport is multidimensional and shows that heavy as well as light gases cannot be treated as inert tracers, which applies to many gases in environmental studies.

Oxygen Propagation Fronts in Porous Media Under Evaporative Conditions at the Soil/Atmosphere Interface: Lab‐Scale Experiments and Model‐Based Interpretation

“…Future investigation should also address the processes investigated in this work, considering both chemical heterogeneity, such as the presence of reactive minerals in the solid matrix (Battistel et al., 2021; Erfani et al., 2021; Salehikhoo & Li, 2015; Tartakovsky et al., 2008) and physical heterogeneity, with spatially variable properties such as porosity and permeability (Heidari & Li, 2014; Muniruzzaman & Rolle, 2019; Ye et al., 2015). Finally, we also envision future research focusing on the exchange of gas components across the soil/atmosphere interface in the presence of variable dynamic forcing in the atmosphere by applying different wind speed in the atmospheric compartment (Bahlmann et al., 2020) and considering pressure fluctuations.…”

Small lateral air pressure gradients generated by a large chamber system have a strong effect onCO2transport in soil

“…Rajewski et al (2014) investigated the impact of pressure fields created by wind farms on surface fluxes and found enhanced CO 2 fluxes in the lee of the turbines. Bahlmann et al (2020) observed that increasing near‐surface wind velocity accelerates soil‐atmosphere gas exchange. Other studies have found that wind‐induced pressure pumping can increase transport rates in soil (Laemmel, Mohr, Longdoz, et al, 2019; Takle et al, 2004).…”

Study of methane migration in the shallow subsurface from a gas pipe leak

“…This may result in reverse influences on methane distribution. Moreover, there are many other factors influencing subsurface methane migration behaviors (e.g., Keskikuru et al, 2001;Poulsen et al, 2003;Patterson and Davis, 2009;Bahlmann et al, 2020), such as pipeline pressure, gas composition when pipelines carry heavier (C 2þ ) hydrocarbons, construction at/below ground, and soilatmosphere interactions. Despite these uncertainties, only a limited number of studies combine experimental and numerical approaches to investigate subsurface methane migration under different environmental conditions.…”