Theme 3-7-Modelling pollutant movement in the unsaturated aquifer

Abstract

The most common approach used to model the transport of solutes in the subsurface is a mass balance partial differential equations, which combines two terms, viz., ( i ) solute displacement by convection with the mean pore flow velocity, and ( ii ) hydrodynamic dispersion. While the mean velocity convective term has a well-defined meaning, hydrodynamic dispersion in the unsaturated flow zone is still a subject of debate. The relationship found for saturated flow is adopted for unsaturated flow with values of dispersivity for one-dimensional flow taken from break-through-curves (BTC) measured through soil column experiments in the laboratory. The convective – dispersion equation describe the physical processes governing the movement, dispersion and transformation of a solute. An analytical solution describing the transport of solute in the unsaturated porous media with an asymptotic distance – independent dispersion relationship has been developed. The solution has a dispersion function, which is linear near the origin (i.e., for short travel distance), and approaches an asymptotic value as the travel distance becomes infinite. The results were compared with experimental results and with finite difference numerical solutions. The comparision indicates that the theory is reliable and can be used with confidence.