Sand tank experiment of a large volume biodiesel spill

Abstract:

Although petroleum hydrocarbon releases in the subsurface have been well studied, the impacts of subsurface releases of highly degradable alternative fuels, including biodiesel, are not as well understood. One concern is the generation of CH_4­ which may lead to explosive conditions in underground structures. In addition, the biodegradation of biodiesel consumes O₂ that would otherwise be available for the degradation of petroleum hydrocarbons that may be present at a site. Until now, biodiesel biodegradation in the vadose zone has not been examined in detail, despite being critical to understanding the full impact of a release. This research involves a detailed study of a laboratory release of 80 L of biodiesel applied at surface into a large sandtank to examine the progress of biodegradation reactions. The experiment will monitor the onset and temporal evolution of CH₄ generation to provide guidance for site monitoring needs following a biodiesel release to the subsurface. Three CO₂ and CH₄ flux chambers have been deployed for long term monitoring of gas emissions. CO₂ fluxes have increased in all chambers over the 126 days since the start of the experiment. The highest CO₂ effluxes are found directly above the spill and have increased from < 0.5 µmol m^-2 s^-1 to ~3.8 µmol m^-2 s^-1, indicating an increase in microbial activity. There were no measurable CH₄ fluxes 126 days into the experiment. Sensors were emplaced to continuously measure O₂, CO₂, moisture content, matric potential, EC, and temperature. In response to the release, CO₂ levels have increased across all sensors, from an average value of 0.1% to 0.6% 126 days after the start of the experiment, indicating the rapid onset of biodegradation. The highest CO₂ values observed from samples taken in the gas ports were 2.5%. Average O₂ concentrations have decreased from 21% to 17% 126 days after the start of the experiment. O₂ levels in the bottom central region of the sandtank declined to approximately 12%.