Biodiesel Relative Risk: A Qualitative Approach to Determining the Environmental Fate of Animal Fat and Soy Biodiesels through a Direct Experimental Comparison with Ultra Low Sulfur Diesel and Screening Model Simulations using Hydrocarbon Spill Screening Model
As part of a multimedia relative risk assessment of biodiesel, small scale infiltration experiments, called “Ant Farm Experiments” were done to simulate and qualitatively evaluate the impacts of biodiesel fate and mobility in the subsurface compared directly to Ultra Low Sulfur Diesel (ULSD). ULSD is the current standard fuel in the state of California. It has been accepted for use statewide and nationally so it was used as a reference fuel for the relative comparison to determine if there were significant differences in the environmental fate of several organically derived biodiesels. For the purpose of the study, the two most feasible and readily available biodiesel feedstocks were used: Animal Fat and Soybean Oil. Experiments were run with a pure fuel (B100) and a blended fuel (B20) for both Animal Fat Biodiesel and Soy Biodiesel in a relative setting to qualitatively compare the differences in infiltration and lens formation.
The relative infiltration experiments were compared through several metrics. The first metric is the relative amount of spreading of the plumes at the water table. The second metric is the relative thickness of the lens at the water table. The third and fourth metrics are relative residuals (colorimetric, i.e. darker color implies higher residuals) in the vadose zone during and at the end of the experiment, respectively.
The experiments found that neither Soy B100 nor Soy B20 have noticeable differences compared to ULSD. The Animal Fat B20 does not appear to have any noticeable differences compared to ULSD either. However, the Animal Fat B100 appears to be much less mobile than the ULSD due to its higher viscosity at temperatures below 20 degrees Celsius. There was a noticeable difference in the amount of residual fuel along the vadose zone downward migration path that occurred in the Animal Fat B100 compared to the ULSD. In addition, the ULSD lens was much more developed than the Animal Fat B100 lens.
Further study was done using the Hydrocarbon Spill Screening Model (HSSM) to determine if there were any significant differences between the Soy B100 and the ULSD. The simulation was done for a 29000 gallon spill on three different soils: sand, silt, and clay. The results of the modeling simulations showed that there was very low mobility for the silt and the clay. Neither fuel was able to get to the water table four meters below the ground surface within a reasonable amount of time (2500 simulated days). The simulation with sand showed that the ULSD and the Soy B100 were very similar in the spreading and also in the lens thickness. While HSSM is a quasi-three-dimensional model (one-dimensional vertical infiltration and radial lens spreading) and the experiments were only two-dimensional, the similarities show that the Soy B100 and the ULSD are not noticeably different in their subsurface fate. HSSM model simulations with Animal Fat Biodiesel were not compared to the Animal Fat experiments since HSSM was only run at twenty degrees Celsius. At twenty degrees Celsius, the fuel properties of Animal Fat B100 are approximately the same as Soy B100. As a result, the biodiesel fuels tested in this research did not demonstrate any higher relative risk than that of ULSD with regards to the mobility and lens formation at the water table.
0543: Civil engineering