Boundary Conditions for the Geospheric Transport Equation

A general formulation of the boundary conditions for the commonly used radionuclide transport equation in the geosphere is shown. To evaluate the accuracy of a widely used approximation of the source boundary condition, considering convective flux alone and neglecting dispersive flux, both solutions for an idealized one-dimensional case are derived and compared. It is demonstrated then that the simpler boundary condition gives a good approximation for all cases with weak dispersion. This criterion is fulfilled for a wide range of parametric values, but the applicability of the simpler boundary condition always has to be checked.