  We use the master equation approach to calculate the energy level shifts of
an atom in the presence of a general dielectric semi-infinite medium
characterized by a dielectric constant $\epsilon(\omega)$. Particularly, we
analyze the case of a non-dispersive medium for which we obtain a general
expression for the interaction as well as the asymptotic behaviors for $k_0 z
\ll 1$ (non-retarded regime) and $k_0 z \gg 1$ (retarded regime), where
$\omega_0 = k_0 c$ is the main transition frequency of the atom. The limiting
cases $\epsilon \simeq 1$ and $\epsilon \gg 1$ are discussed for both retarded
and non-retarded limits. For the retarded limit, we compute the non-additivity
contribution of van der Waals forces.
