Last Updated July 17, 2017
When a contaminant enters the ground, the dissolved constituents will migrate due to the movement of the water. In addition, the contaminant disperses due to hydrodynamic mixing and molecular diffusion. The migration also may be retarded due to adsorption and radioactive decay. The advective-dispersive differential equation is derived by applying the principle of mass balance to an element of porous medium, with consideration of advection, dispersion, adsorption and decay.
CTRAN/W computes adsorption using the adsorption function and the computed nodal concentrations. It then computes the mass quantity in both the fluid phase and the solid phase.
CTRAN/W can also model contaminant movement by simply tracking particles from user-defined locations. For each time step, CTRAN/W moves the particles a distance based on the volumetric water content and the SEEP/W-computed water velocities.
One of the major components in a contaminant transport analysis is the velocity of the water, which can be obtained from a SEEP/W or VADOSE/W analysis. Once this velocity is known, it can be used in CTRAN/W to study the transport of contaminants.
In density dependent fluid flow, the velocity of the water is dependent on the solute concentration. The water velocity in turn influences the movement of the solute. The iterative transfer of water velocity from SEEP/W to CTRAN/W and the transfer of concentration from CTRAN/W to SEEP/W makes it possible to do density dependant fluid flow analyses.