Contaminant migration in the unsaturated soil zone: The effect of rainfall and evapotranspiration

The potential effect of rainfall and evapotranspiration on contaminant migration in the unsaturated soil zone was studied using a deterministic dynamic modeling approach. The contaminant mass transfer equation was solved along with Richards' equation for water transport subject to dynamic surface boundary conditions that consider rainfall and evapotranspiration. As an illustration of the effect of rainfall and evapotranspiration on various transport mechanisms, test cases are presented for the transport and volatilization of benzene, 1,1,2,2-tetrachloroethane and methanol for a loam-type soil using self-consistent dynamic rainfall and evapotranspiration weather sequences. The present study suggests that transport mechanisms such as diffusion, dispersion and convection can all be significant, to varying degrees, during various parts of the year. The temporal variability of both the volatilization flux and the amount of chemical remaining in the unsaturated soil zone are significantly impacted by the dynamics of rainfall and the period of initial contamination.