  We have used a coupled dynamical and chemical model to examine the chemical
changes induced by the passage of an interstellar shock in well shielded
regions. Using this model we demonstrate that the formation of water in a shock
will be followed in the post--shock phase by depletion of the water molecules
onto the grain surfaces. To attempt to discriminate between the creation of
ices behind shocks and their production by means of grain surface chemistry, we
examine the deuterium chemistry of water before, during, and after a shock. We
show that chemical evolution in the post--shock gas can account for both the
deuterium fractionation and the abundance of carbon dioxide relative to water
observed in interstellar and cometary ices. Given the pervasiveness of shocks
and turbulent motions within molecular clouds, the model presented here offers
an alternate theory to grain surface chemistry for the creation of ices in the
interstellar medium, ices that may ultimately be incorporated into comets.
