Natural attenuation of volatile hydrocarbons in the unsaturated zone and shallow groundwater plumes: scenario-specific modelling and laboratory experiments

Numerical simulations were performed to assess the diffusive spreading of volatile fuel constituents with and without biodegradation and groundwat- er recharge, for unleaded gasoline and diesel fuel, various porous media (sand, silt, clay), and different boundary conditions (unsealed, partially/completely sealed soil surface, shallow and deep groundwater table, point and extended sources). Laboratory tank experiments were conducted with a medium to coarse sand to determine the mass transfer of volatile compounds from the soil gas into groundwater over a wide range of flow velocities and different boundary conditions. The experimenta- l and modelling results illustrate that with biodegradation the emission into groundwater depends mainly on the vapour pressure of the fuel constituents, the size and depth of the source, type of soil surface and the permeability of the porous medium. Sensitivity analysis showed that the biological degradation rate constant had nearly no effect above a value of about 1 day-1. Groundwater recharge is a significant contaminant pathway only in soils with high water content and for compounds with low Henry law constants (e.g. MTBE in silt or clay). The contaminant flux into the groundwater is dispersion controlled and very small compared to volatilization of source constituents to the atmosphere.