Methane Transport during a Controlled Release in the Vadose Zone

Abstract

Shallow, small-rate releases of ethanol-blended fuels from underground storage tanks (USTs) may be quite common and result in subsurface CH 4 generation. However, vadose zone transport of CH 4 generated from these fuel releases is poorly understood, despite the potential to promote vapor intrusion or create explosion hazards. In this study, we simulated shallow CH 4 generation with a controlled subsurface CH 4 release from July 2014 to February 2015 to characterize subsurface CH 4 migration and surface emissions and to determine environmental controls on CH 4 fate and transport. July 2014 through November 2014 was an extended period of drought followed by precipitation during December 2014. Throughout the experiment, under varied CH 4 injection rates, CH 4 formed a radially symmetrical plume around the injection point. Surface efflux during the drought period of the experiment was relatively high and stable, with approximately 10 to 11 and 34 to 52% of injected CH 4 reaching the ground surface during the low-and high-rate injections, respectively. Following the period of precipitation and increased soil moisture, efflux dropped and stabilized at approximately 1% of injected CH 4 , even as soil moisture began to decrease again. Tracer and inhibitor experiments and estimates of soil diffusivity suggest that microbial CH 4 oxidation was responsible for the observed drop in efflux. The decrease in efflux only after soil moisture increased suggests a strong environmental control over the transport and oxidation of vadose zone CH 4 .

The Utility of Purge and Recovery Testing in Ground-Gas Flux Risk Assessment

“…The high concentrations of VOCs recorded after the third test could have been mobilized by the nitrogen gas injected into the borehole. Studies has also shown that the migration of CH 4 and CO 2 induces the migration of VOCs from contaminated sites (Katy et al, 2009;Felice et al, 2018) and therefore may be another reason.…”

Methane‐Oxidizing Bacteria Communities Shift to Attenuate a Controlled Vadose Zone Methane Release

“…Field moisture, efflux, and soil gas for the entire duration of the CH 4 injection period have been previously discussed in detail (Felice et al, 2018), but measurements from the final period of CH 4 injection are briefly summarized here to provide context for the microbial data that follow.…”

“…Field characterization and a detailed description of the CH 4 release and measurement of efflux, soil gas, and soil moisture were described in detail by Felice et al (2018). Briefly, a monitoring network consisting of soil gas sampling points installed at 0.5, 1.0, and 1.5 m (with additional points at 0.25 and 1.75 m in the center of the network) below ground surface (bgs); polyvinyl chloride collars for gas efflux measurement; and soil temperature sensors was established at the Putah Creek Riparian Reserve in Davis, CA (Fig.…”

“…We focused on the relationships among: (i) abundance of MOB groups and MOB community composition; (ii) lag period and steady‐state CH 4 oxidation rates; and (iii) soil moisture content. Physical transport processes were described in an earlier study (Felice et al, 2018). We hypothesized that CH 4 inputs would select for competitor MOB groups and that the increased competitor MOB abundance would coincide with increased CH 4 oxidation rates.…”

Methane transport in a soil column: experimental and modeling investigation

“…Methane, an important component of greenhouse gas, is often generated at municipal landfills and petroleum-hydrocarbon-contaminated sites. Methane transport in vadose zone soils has the potential to promote vapor intrusion or create explosion hazards if its generation in the subsurface environment is unnoticed or poorly managed [1]. Adequately assessing the migration of a methane gas plume in the subsurface is important for effective strategy design and management regarding methane releases.…”