NUMERICAL AND EXPERIMENTAL STUDIES OF MULTIPHASE, MULTICOMPONENT TRANSPORT FOR SUBSURFACE GAS MIGRATION AND REMOVAL IN THE VADOSE ZONE USING SOIL AERATION

Abstract

Subsurface methane (CH4) released from natural gas (NG) pipeline leaks can migrate through the vadose zone, accumulate in confined subsurface infrastructure, and pose flammability hazards. Isolating or repairing a leak source does not necessarily eliminate the subsurface hazard immediately. Residual CH4 can remain stored in the vadose zone, continue to migrate, and, in some cases, rebound after active mitigation stops. This creates a practical need for post-repair remediation of accumulated subsurface CH4. However, predicting plume evolution and mitigation of residual subsurface CH4 after leak isolation or repair requires a mechanistic understanding of coupled multiphase (gas and water) flow and multicomponent transport in variably saturated porous media under both variable atmospheric conditions and induced pressure gradients. Despite decades of vadose-zone research, predicting gas transport in variably saturated porous media remains limited by two unresolved challenges. First, transient leak behavior remains difficult to infer reliably from sparse near-surface CH₄ observations because the measured signals are influenced by transport in the vadose zone and the changes in soil and atmospheric conditions. Second, predictive understanding of soil aeration for remediation remains limited because the coupled effects of soil properties and design parameters on induced airflow pathways and diffusion-limited mass transfer during soil aeration are not quantitatively resolved. Therefore, this dissertation integrated field-scale experiments with numerical modeling to advance understanding of subsurface gas (i.e., CH4) migration and removal under both natural and induced pressure gradients and translate that understanding into a validated, field-relevant decision-making tool. The first contribution is to demonstrate that estimating transient subsurface gas migration and leak behavior requires integrating near-surface observations with subsurface transport properties. Controlled experiments demonstrate that belowground near-surface (BNS) measurements respond earlier and persist longer than surface measurements. Within a 4 m monitoring radius, BNS CH₄ concentrations are on average 20 to 486% higher than surface concentrations. Incorporating these observations into an inverse gas-migration model enables estimation of transient leak behavior with strong agreement with measurements (slope m = 0.99; R² = 0.77). These results support that transient source characterization benefits from a monitoring strategy aligned with vadose-zone transport physics rather than from short-duration surface observations alone. The second contribution is to use a modified multiphase multicomponent transport model to investigate the dominant mechanisms governing subsurface gas removal and to quantify how soil properties, water saturation, vacuum pressure, and bar-hole configuration interact to control removal performance. Integrating six controlled field-scale soil-aeration experiments with a transport model and 108 scenario simulations shows that subsurface gas removal follows a two-stage process. This process first includes rapid depletion dominated by advection during aeration, followed by diffusion-limited release from weakly aerated regions and less-connected pore space. Moderate vacuum pressure strengthens advective capture and accelerates cleanup, whereas increasing vacuum beyond this range can reduce efficiency by enhancing preferential airflow bypass and competition for removal. Low-permeability conditions are more effectively addressed by increasing bar-hole density and improving plume alignment than by increasing vacuum alone. Across tested conditions, optimized bar-hole number and layout achieve 80 to 98% subsurface CH₄ removal, while improper spacing reduces removal efficiency by 20 to 30%. Overall, these results provide a mechanistic basis for predicting subsurface CH₄ removal under the tested soil, moisture, and operating conditions, enabling the selection of bar-hole layouts and vacuum levels that maximize capture while minimizing bypass and diffusion-limited persistence. The third contribution is a reduced-order, physics-based decision-making framework that predicts subsurface CH₄ removal efficiency and aeration duration from measurable site conditions (soil type, moisture, and plume geometry) and controllable design parameters (vacuum level, aeration duration, and bar-hole layout). The framework shows good agreement with controlled experiments (R² = 0.74) and enables rapid screening of configurations that improve capture and reduce bypass under site-specific constraints. Therefore, this dissertation advances understanding of subsurface gas transport in the vadose zone and provides a field-validated pathway from detection and transient source estimation to mechanistically informed CH₄ mitigation design. Overall, this dissertation provides a foundation for a more practical approach to subsurface gas-leak response by integrating near-real-time detection, leak behavior quantification, and subsurface CH4 removal. The broader implication is that subsurface explosion hazards associated with CH₄ accumulation in the vadose zone can be more effectively evaluated and mitigated when monitoring data, transport physics, and mitigation design are integrated within a unified framework. Future research should extend and evaluate this framework under more complex site conditions, including layered and heterogeneous soils, surface sealing and subsurface infrastructure, and variable atmospheric conditions, while incorporating uncertainty quantification and real-time decision support. These next steps would not only improve scientific understanding of subsurface gas transport and removal but also optimize the design and operation of field subsurface gas-leak response strategies.