Sand tank experiment of a large volume biodiesel spill

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 CH4­ 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%.