Assessing Vadose Zone Biodegradation by a Multicomponent Gas Transport Model

Abstract

This study calibrated a gas transport model that uses multicomponent approaches, i.e., the dusty gas model (DGM) and Stefan–Maxwell (SM) equations, to soil gas concentration depth profiles to determine aerobic biodegradation rates of hydrocarbons in unsaturated soils. I found that viscous advection induced by multicomponent gas transport in porous systems and Knudsen diffusion may be neglected for soil systems with intrinsic permeabilities >10−13 m2, and intrinsic permeability of low‐permeability layers, such as cemented limestone or concrete pavement on the ground surface, calibrated by the DGM model ranged from 10−15 to 10−18 m2. The SM model may be applicable for gas transport in soil systems with low‐permeability layers if low obstruction factor values are assigned for these layers. The obstruction factor defined in the DGM and SM equations accounts for impedance to gas diffusion in unsaturated soils and is similar to tortuosity for Fick's law of diffusion. Results further showed that flux rates of hydrocarbon vapors from sources in unsaturated soils need to be measured or assessed before models are fitted to hydrocarbon vapor depth profiles. If the flux rates are not assessed, and if the obstruction factor and intrinsic permeability in the low‐permeability layers are of uncertain magnitudes, models may produce nonunique results for biodegradation rates of hydrocarbons. Thus, I propose a two‐step procedure for determining aerobic biodegradation rates of hydrocarbons.

Methane transport in a soil column: experimental and modeling investigation

“…The DGM and Fickian-type diffusion approaches were assessed in this study. Two models were applied: one is a multidimensional, multicomponent DGM-based gas phase transport model (DGPT) developed by [9]; another is Michigan soil vapor extraction remediation (MISER), a Fickian-based model, developed in [10]. DGPT employs a set of coupled component molar balance equations (transport equations) and a total molar balance equation (flow equation) for the solution of the molar fraction of gas component i (I = 1,…, n-1) where n is the number of gas components and total molar concentration.…”

Numerical model for static chamber measurement of multi-component landfill gas emissions and its application

“…Current understanding of the relative error induced by using static chamber has been achieved mainly through extensive experimental studies in the last decade, while the developments in the theoretical investigation have been rather limited (Sahoo and Mayya 2010 ; Venterea 2013 ). In recent years, extensive efforts have been devoted to modeling multi-component gases transport in the cover systems (Fen 2014 ; Ng et al 2015 ; Feng et al 2020b ; Zuo et al 2020 ; Bian et al 2021 ). However, estimation models for landfill gases transport in the static chamber are limited to either empirical or single component models (Livingston et al 2005 ; Senevirathna et al 2007 ; Sahoo and Mayya, 2010 ; Parkin et al, 2012 ).…”

“…These issues can result in big errors by using Blanc’s model. Under these circumstances, the dusty-gas model (DGM), which considers the interactions between different component gas and the relationship between the gas concentration and the flux, would be more appropriate to investigate multicomponent gas emissions from the landfill cover (Fen 2014 ; Zuo et al 2020 ). Although DGM is widely used for investigating multi-component gases migration in soils (Hibi et al 2009 ; Fen, 2014 ; Zuo et al 2020 ), the application of DGM for evaluating gas emission from the landfill cover and transport in the static chamber system are relatively rare.…”

Transport of Trichloroethylene Vapor in a Dry Soil Column

“…Time-varied pressure differences between the column ends and vapor concentrations at the column ends were measured for 8 to 50 hours. Two gas phase transport models, Michigan Soil Vapor Extraction Remediation (MISER) (Abriola et al, 1997) and Dusty Gas Model-based Gas Phase Transport (DGPT) (Fen, 2014), were applied to simulate the transport scenarios of the experiment in order to assess their predictability.The results of three data sets measured from the transport experiment show that the upstream gas pressures (at the bottom of the column) are more than the downstream ones (at the top of the column) for 0.2~0.7 Pa. However, a total gas pressure difference is about 1.85 Pa for static equilibrium of gas between the ends of the vertical soil column.…”

“…Time-varied pressure differences between the column ends and vapor concentrations at the column ends were measured for 8 to 50 hours. Two gas phase transport models, Michigan Soil Vapor Extraction Remediation (MISER) (Abriola et al, 1997) and Dusty Gas Model-based Gas Phase Transport (DGPT) (Fen, 2014), were applied to simulate the transport scenarios of the experiment in order to assess their predictability.…”