Solute Transport Boundary Conditions

Specifies boundary conditions for solute transport. The following types of boundary conditions are available:

Solute flux versus solute concentration boundary conditions

The third-type (Cauchy or solute flux) boundary condition prescribes solute flux (not concentration) across the boundary. Then due to mixing with water (and solute) initially present in the profile, one will not get immediately c0 concentrations on the boundary. Since one prescribes solute flux, one has a complete control over the mass balance and how much solute enters into the transport domain.

If one uses the first-type (Dirichlet of concentration) boundary condition, one prescribes the concentration at the boundary (not the flux into the domain). Since solute flux consists of two components, i.e., convective and dispersive solute flux, both these fluxes are active on the boundary as well. Initially there is a large concentration gradient and thus the dispersive solute flux is large. In this case one has much larger flux into the domain than if one uses the third-type solute flux boundary condition.

We always recommend to use the third-type boundary condition since that is more physically realistic boundary condition. Dirichlet boundary condition is not physical boundary condition (and for above reasons it does not conserve mass) (e.g., van Genuchten and Parker, 1984; Leij et al., 1991). One could use it only if there is, for example, large reservoir of contaminant in contact with the transport domain, and thus one can assume that boundary concentration is fixed.

See also the "How to Edit Boundary Conditions" topic.