Vadose Zone Gas Migration and Surface Effluxes after a Controlled Natural Gas Release into an Unconfined Shallow Aquifer

Core Ideas Subsurface gas migration results in localized surficial CH 4 releases. Surficial CH 4 emissions show pronounced temporal variations. Methane concentrations in soil gas exceed lower explosive limits at low leakage rates. Increasing CO 2 effluxes and stable C isotope signatures indicate vadose zone CH 4 oxidation. Instantaneous surficial effluxes do not indicate the magnitude of subsurface gas leakage rates. Shale gas development has led to concerns regarding fugitive CH 4 migration in the subsurface and emissions to the atmosphere. However, few studies have characterized CH 4 migration mechanisms and fate related to fugitive gas releases from oil or gas wells. This paper presents results from vadose zone gas and surface efflux monitoring during a natural gas release experiment at Canadian Forces Base Borden, Alliston, Ontario, Canada. Over 72 d, 51 m 3 of natural gas (>93% CH 4 ) was injected into a shallow, unconfined sand aquifer at depths of 4.5 and 9 m. Methane and CO 2 effluxes in combination with soil gas concentrations and stable C isotopic signatures were used to quantify the spatiotemporal migration and fate of injected gas. Preferential gas migration pathways led to vadose zone hot spots, with CH 4 concentrations exceeding the lower explosive limit (5% v/v). From these hot spots, episodic surface CH 4 effluxes (temporally exceeding 2500 μmol m −2 s −1 [3465 g m −2 d −1 ]) occurred during active injection. Higher injection rates led to increased average CH 4 effluxes and greater lateral migration, as evidenced by a growing emission area approaching 25 m 2 for the highest injection rate. Reactive transport modeling showed that high CH 4 fluxes resulted in advection‐dominated migration and limited CH 4 oxidation, whereas lower CH 4 effluxes were diffusion dominated with substantial CH 4 oxidation. These results and our interpretations allowed us to develop a conceptual model of fugitive CH 4 migration from the vadose zone to the ground surface.