Numerical and experimental analysis of diesel and JP-5 transport in unsaturated soils.

摘要

Long-term and large-scale experiments were conducted to evaluate the migration of diesel and JP-5 fuels in sand and soil media. The experiments were conducted in a 3.66 x 4.27 x 3.05 meter (12 x 14 x 10 feet) test chamber with 56 monitoring wells installed to continuously monitor the hydrocarbon vapor concentration in three dimensions. Slower transport of diesel than JP-5 in a sand medium was observed in both horizontal and vertical directions. Fuel transport in soil was more limited than in sand. Accordingly, the concentrations measured in the contaminated soil were much higher than that in sand. In contrast to the concentration contour maps showing strong heterogeneity of fuel migration in sand, the diesel-wetted soil pictures demonstrated a much more homogeneous movement. Also discussed are the Time of Arrival and concentration versus contour radius for the experiments with diesel and JP-5 in sands.; An axial-symmetric three dimensional numerical model simulating both the liquid infiltration and the vapor diffusion in unsaturated soil was developed. The resulting partial differential governing equations were solved by the finite difference alternating direction implicit (ADI) method. The Picard iteration method was used to solve the nonlinear difference equations.; A new technique was developed to better describe the moving boundary at the liquid entry zone. The boundary condition near the liquid entry zone was treated as a discrete conical moving boundary, which was determined by a mass balance over the liquid entry zone that included the accumulation term. The liquid infiltration model was verified by applying it to the experimental data of Clothier and Scotter (1982) to simulate water infiltration in sand.; The complex method of Box was used to identify the model parameters. The parameter estimation technique was tested with a set of hypothetical data generated by the transport model with a given set of parameters. The five independent model parameters were successfully estimated within substantially wide limits.; Both the diesel vapor concentration profile and the liquid wetting front in the soil medium predicted by the model using the parameters estimated from the diesel vapor concentration data showed satisfactory match to the experimental observations (Test III). The numerical simulation of the transport of diesel fuel in sand (Test II) was also conducted and the results generally agreed with the experimental results. Numerical simulation of JP-5 transport experiment in sand (Test IV) was not successful. Possible reasons include the influences of chromatographic separation on the vapor diffusion, and the evaporation loss of the light components on the liquid migration.