Flow and Transport of Methane from Leaking Underground Pipelines: Effects of Soil Surface Conditions and Implications for Natural Gas Leak Classification

Reducing methane (CH₄) emissions from natural gas (NG) pipeline leaks is crucial to minimize global warming while also providing key safety benefits to communities. What is not well understood about pipeline leak scenarios is the impact of different soil surface conditions on the belowground leak transport behavior and subsequently the NG leak classification. In this study, we conducted a series of controlled leak experiments, varying based on the surface conditions including snow, moist soil layers, asphalt, and grass. Data indicated that temporary rain and snow surface cover conditions result in CH₄ concentrations extending 3 times further than the equivalent leak scenario under dry soil conditions, resulting in levels that pose heightened environmental and safety risks. Furthermore, after leak termination, CH₄ trapped under snow, moist soil, and asphalt surface conditions persisted for up to ∼12 days, with 5–15% CH₄ (v/v) conditions persisting underground for 7.5 days. Even after leak termination, NG continued to migrate laterally away from the leak source, extending the plume boundary by 2–4%. While efforts to study a wider range of environmental conditions are underway, the findings of this study provide crucial insight into identifying and prioritizing leaks from the perspectives of both safety and the environment.