Third-Type Solute

A third-type (Cauchy, mixed, or solute flux) boundary condition is used when the solute flux along a boundary is specified. This type of boundary condition is mass conservative. When the water flux is zero or directed out of the region, third-type boundary conditions are automatically switched to second type (Neumann) boundary conditions. One exception to this rule occurs for atmospheric boundary conditions, since the program assumes that solutes cannot leave the flow region across this boundary (usually the soil surface).

We recommend users to always invoke third-type boundary conditions where possible since this condition is physically more realistic and preserves solute mass in the simulated system (e.g., van Genuchten and Parker, 1984; Leij et al., 1991).

When you apply the solute transport boundary condition on a boundary, a windows appear asking you for "Pointer to the vector of boundary conditions". This pointer selects the concentration value from the vector of boundary conditions in the Solute Reaction Parameters dialog window (e.g., cBnd1) for boundaries with time-invariable water flow boundary conditions (e.g., Constant Head BC, Constant Flux BC, etc), and from the Time-Variable Boundary Conditions dialog window (e.g., cValue1) for boundaries with time-variable water flow boundary conditions (e.g., Atmospheric BC, Time-Variable Head BC or Time-Variable Flux BC).

For boundaries with time-invariable water flow BCs, if the pointer is one, cBnd1 is selected, if the pointer is two, cBnd2 is selected, etc (up to 7).

For boundaries with time-variable water flow BCs, if the pointer is one, values from the column cVal1-1 (from the Time-Variable Boundary Conditions dialog window) are selected, if the pointer is two, values from the column cVal2-1 are selected, etc (up to 3).

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