  The influence of turbulent mixing on the chemistry of the interstellar medium
has so far received little attention. Previous studies of this effect have
suggested that it might play an important role in mixing the various phases of
the interstellar medium. In this paper we examine the potential effects of
turbulent diffusion on the deuterium chemistry within molecular clouds. We find
that such mixing acts to reduce the efficiency of deuteration in these clouds
by increasing the ionization fraction and reducing freeze-out of heavy
molecules. This leads to lower abundances for many deuterated species. We also
examine the influence of turbulent mixing on the transition from atomic
hydrogen to H2 and from atomic deuterium to HD near the cloud edge. We find
that including turbulent diffusion in our models serves to push these
transitions deeper into the cloud and helps maintain a higher atomic fraction
throughout the cloud envelope. Based on these findings, we propose a new
process to account for the significant scatter in the observed atomic D/H ratio
for galactic sightlines extending beyond the Local Bubble. Although several
mechanisms have been put forward to explain this scatter, they are unable to
fully account for the range in D/H values. We suggest a scenario in which
turbulent mixing of atomic and molecular gas at the edges of molecular clouds
causes the observed atomic D/H ratio to vary by a factor of ~2.
