9 research outputs found
Methane Bioattenuation and Implications for Explosion Risk Reduction along the Groundwater to Soil Surface Pathway above a Plume of Dissolved Ethanol
Fuel ethanol releases can stimulate methanogenesis in
impacted
aquifers, which could pose an explosion risk if methane migrates into
enclosed spaces where ignitable conditions exist. To assess this potential
risk, a flux chamber was emplaced on a pilot-scale aquifer exposed
to continuous release (21 months) of an ethanol solution (10% v:v)
that was introduced 22.5 cm below the water table. Despite methane
concentrations within the ethanol plume reaching saturated levels
(20–23 mg/L), the maximum methane concentration reaching the
chamber (21 ppm<sub>v</sub>) was far below the lower explosion limit
in air (50,000 ppm<sub>v</sub>). The low concentrations of methane
observed in the chamber are attributed to methanotrophic activity,
which was highest in the capillary fringe. This was indicated by methane
degradation assays in microcosms prepared with soil samples from different
depths, as well as by PCR measurements of <i>pmoA</i>, which
is a widely used functional gene biomarker for methanotrophs. Simulations
with the analytical vapor intrusion model “Biovapor”
corroborated the low explosion risk associated with ethanol fuel releases
under more generic conditions. Model simulations also indicated that
depending on site-specific conditions, methane oxidation in the unsaturated
zone could deplete the available oxygen and hinder aerobic benzene
biodegradation, thus increasing benzene vapor intrusion potential.
Overall, this study shows the importance of methanotrophic activity
near the water table to attenuate methane generated from dissolved
ethanol plumes and reduce its potential to migrate and accumulate
at the surface