  The presence of heavy elements in the atmospheres of the hottest H-rich DA
white dwarfs has been the subject of considerable interest. While theoretical
calculations can demonstrate that radiative forces, counteracting the effects
of gravitational settling, can explain the detections of individual species,
the predicted abundances do not accord well with observation. However, accurate
abundance measurements can only be based on a thorough understanding of the
physical structure of the white dwarf photospheres, which has proved elusive.
Recently, the availability of new non-LTE model atmospheres with improved
atomic data has allowed self-consistent analysis of the EUV, far UV and optical
spectra of the prototypical object G191$-$B2B. Even so, the predicted and
observed stellar fluxes remain in serious disagreement at the shortest
wavelengths (below $\approx 190$\AA), while the inferred abundances remain
largely unaltered. We show here that the complete spectrum of G191$-$B2B can be
explained by a model atmosphere where Fe is stratified, with increasing
abundance at greater depth. This abundance profile may explain the difficulties
in matching observed photospheric abundances, usually obtained by analyses
using homogeneous model atmospheres, to the detailed radiative levitation
predictions. Particularly as the latter are only strictly valid for regions
deeper than where the EUV/far UV lines and continua are formed. Furthermore,
the relative depletion of Fe in the outer layers of the atmosphere may be
evidence for radiatively driven mass loss in G191$-$B2B.
