Subject Area

Civil Engineering, Physical Sciences


With the increased use of Natural Gas (NG) as a source of fuel, underground leak-related events are increasingly reported, and some are fatal. Subsurface migration and development up to flammable limits are highly dependent on leak characteristics, soil, atmospheric variables, and other surface and subsurface structural complexities. Although few studies have characterized the transient behavior of leaked NG in the shallow subsurface, the underlying mechanisms and their interactions are not well understood, which prevents prediction of the impact of environmental and operating conditions on gas migration. The study conducted a series of field scale-controlled release experiments and numerical simulations to systematically determine the influence of environmental and operational variables on the transient behavior of leaked NG. Field-scale experiments showed that any reduction in surface layer air-filled porosity (asphalt pavements, moist soil due to rain or snow/ice layers) result in CH4 concentrations extending 4 times further than the equivalent leak scenario under unpaved soil conditions. Subsurface complexities (soil disturbance and buried infrastructure) that increase air-pore connectivity result in increased lateral migrations 3 times as of undisturbed surrounding soil. Even after leak termination, gas continued to migrate laterally away from the leak source, extending the outermost boundary of the plume by 2-4%. Results further showed that the surface CH4 plume is not representative of the CH4 concentrations at deep soil layers and can be ≥4 times as of the surface plume under dry soil conditions and ≥16 times under moist near-surface soil conditions. Numerical simulations conducted to evaluate the performance of diffusivity parametric functions (DPFs) to accurately predict the soil-specific diffusivities showed that soil-type and structure-dependent DPFs should be used in simulating the diffusive transport of NG. DPF selection did not significantly influence NG migration simulations under dry soil conditions during the leak period. However, a Dp/Do specifically overpredicted and underpredicted CH4 changes after leak termination under dry soil conditions. Additionally, it showed a 10% deviation in CH4 concentration and a 2.5-day difference in reaching explosive limits under partially saturated soil conditions. Extended numerical simulations showed that the leaks associated with surface covers are advection-dominated, and leaks associated with subsurface complexities (disturbed soil, buried infrastructure, or fractures) are diffusion-dominated. Further, migration of CH4 concentrations ≤ 5% (v/v) are diffusion dominated and CH4 concentrations ≥ 5% (v/v) are advection dominated. The most critical soil surface and subsurface structural complexities increasing the rate and extent of belowground gas transport from leaking pipelines include (in priority order): 1) subsurface soil fractures or open utilities (e.g. an empty conduit) 2) surface covers (snow, rain, and pavement), 3) disturbed soil (e.g. trenched/backfilled soil). 4) Soil type and pipeline depths have very little influence, in comparison and 5) soil moisture has a negative impact. Combinations of conditions can increase the influence on both the rate and extent of gas transport. Under combinations of surface and subsurface complexities, gas migrates along and in the direction of subsurface soil structural disturbance due to reduced solid-induced tortuosity. The investigation highlights the interdependence of subsurface and surface structural properties, with the properties of the gas leak and with the duration after leak starts, which is a new contribution to the research field.

Degree Date

Spring 2024

Document Type


Degree Name



Civil and Environmental Engineering


Dr. Kathleen M Smits

Number of Pages




Creative Commons License

Creative Commons Attribution-Noncommercial 4.0 License
This work is licensed under a Creative Commons Attribution-Noncommercial 4.0 License

Available for download on Friday, May 02, 2025