Transport and biodegradation of volatile petroleum hydrocarbons in the unsaturated soil zone: Assessment of contaminant emission into groundwater

The fate and transport of fuel constituents in the unsaturated soil zone was investigated in laboratory tank experiments and in scenario-specific numerical modelling runs. Diffusive spreading in soil-gas generally resulted in transport of the fuel constituents in soil-gas up to great depths within the unsaturated zone, even for source zones penetrating only a few meters of the vadose zone. Percolating water was shown to have a neglible impact on the vertical transport of fuel constituents. Aerobic biodegradation limited the contaminant spreading of contamination depending on source zone geometries and soil properties as well as fuel composition. The laboratory tank experiments confirmed that mass fluxes of volatile compounds from soil-gas to groundwater across the capillary fringe are dominated by the transverse vertical dispersion, although the dispersivities measured were very small (< 0.5 mm).