Natural attenuation of biodiesel in a sandtank experiment

The use of alternative fuels, including biodiesel, has increased steadily in the last two decades, increasing the risk of accidental spills. However, a comprehensive understanding of the fate of biodiesel in the subsurface is currently lacking. A large sandtank experiment was conducted over 18 months to evaluate the spatial and temporal evolution of biodiesel biodegradation with a focus on vadose zone impacts. 80 L of biodiesel was applied to the center of the sandtank. Monitoring and analysis focused on two zones: the saturated zone including the capillary fringe, and the unsaturated zone. Measured parameters included surficial CO2 and CH4 effluxes, gas concentrations and their isotopic composition in the vadose zone, moisture contents and temperature. In the saturated zone, groundwater chemistry was characterized based on dissolved cation and anion concentrations, volatile fatty acids (VFAs), pH and alkalinity. The experimental results displayed a rapid response to the biodiesel release, revealed by increases of surficial CO2 effluxes and CO2-concentrations in the vadose zone, while O2 concentrations remained close to atmospheric levels. In the saturated zone, elevated VFA concentrations were observed together with pronounced increases in cation concentrations, specifically Ca, Mg, Fe and Mn, indicating the rapid development of anaerobic conditions. The generation of acidity associated with aerobic and anaerobic degradation reactions led to a decline in pH, locally to values below 5, likely inhibiting the progress of biodegradation. The onset of CH4 generation was delayed and coincided with reaching maximum VFA concentrations in the saturated zone. CH4 effluxes at the ground surface were limited; however, stable isotope analysis indicated that CH4 oxidation in the vadose zone was weak, likely due to low-pH conditions. Increases in dissolved concentrations of Fe and Mn were attributed to the reductive dissolution of Mn- and Fe-oxides, with possible contributions from the dissolution of Fe- and Mn-bearing dolomite. Carbon balance estimates showed that the biodiesel was recalcitrant to degradation, and at 590 days less than 5% of the biodiesel had been transformed to VFAs, CO2 and CH4. The average biodiesel degradation rate derived from the carbon balance is 1.3 x 10-8 mol L-1 H2O s-1, comparable to literature values.Science, Faculty ofEarth, Ocean and Atmospheric Sciences, Department ofGraduat