Numerical Analysis on the Effect of Soil Properties on the Generation of Volatilization Flux from Unsaturated Soil Contaminated by Volatile Chemical Substances

For the assessment of human health risks from soil contaminated by volatile chemical substances (VCSs), it is important to quantitatively estimate the volatilization fluxes that occur at the ground surface due to the upward transport of VCS components in unsaturated soil. The model constructed by the authors is based on the quantitative evaluation of advection–dispersion behavior associated with the volatilization of VCSs and allows detailed consideration of soil properties and the boundary conditions between the atmosphere-ground surface and unsaturated soil-aquifer compared to existing models. This study focuses on the evaluation of the effect of soil properties on the generation of volatilization flux through numerical analyses by changing the permeability characteristics of surface soil depending on the difference in soil particle size, porosity, and distribution coefficient between the water and soil phases, targeting benzene as a model substance of VCSs. A series of calculated results can be classified into cases dominated by either an increase of volatilization flux or transport to the aquifer, depending on soil properties, indicating the necessity of appropriate countermeasures for remediation and risk assessment. For the reduction of health risks derived from the generation of volatilization flux, removal of contaminants existing in the surface soil, including the ground surface, is essential. However, it is necessary to prevent the spread of contamination into the aquifer when the contaminants have high mobility in surface soil.