  The D/H ratio in interstellar gas varies on scales of a few hundred pc in the
local Milky Way, with D/H values ranging from ~7 ppm to ~22 ppm. The reduction
in D/H relative to the primordial value of D/H (~ 26 ppm) is usually attributed
to "astration" -- conversion of D into other elements by nuclear fusion in
stars. However, it is shown here that astration has difficulty accounting for
the observations because the expected associated variations in O/H are not
seen.
  The lower D/H values are instead likely due to "depletion" of the D onto dust
grains. Polycyclic aromatic hydrocarbons (PAHs) are a possible repository for
the missing D, and it appears possible for gas-grain reactions to achieve
extreme deuteration of carbonaceous grain material. Grain destruction will
release D from the grains; the gas phase abundance of D should therefore be
positively correlated with the gas phase abundances of other elements that
exhibit strong depletions, such as Mg, Si, Ti, and Fe, which also will be
returned to the gas by grain destruction.
