  We have used a coupled time-dependent chemical and dynamical model to
investigate the lifetime of the chemical legacy left in the wake of C-type
shocks. We concentrate this study on the chemistry of H2O and O2, two molecules
which are predicted to have abundances that are significantly affected in
shock-heated gas. Two models are presented: (1) a three-stage model of
pre-shock, shocked, and post-shock gas; and (2) a Monte-Carlo cloud simulation
where we explore the effects of stochastic shock activity on molecular gas over
a cloud lifetime. In agreement with previous studies, we find that shock
velocities in excess of 10 km s^-1 are required to convert all of the oxygen
not locked in CO into H2O before the gas has an opportunity to cool. For pure
gas-phase models the lifetime of the high water abundances, or ``H2O legacy'',
in the post-shock gas is 4 - 7 x 10^5 years. Through the Monte Carlo cloud
simulation we demonstrate that the time-average abundance of H2O is a sensitive
function of the frequency of shocks. Thus we predict that the abundance of H2O
and other known outflow tracers can be used to trace the history of shock
activity in molecular gas. For gas-grain models we find that the abundance of
water-ice on grain surfaces can be quite large and is comparable to that
observed in molecular clouds. This offers a possible alternative method to
create water mantles without resorting to grain surface chemistry: gas heating
and chemical modification due to a C-type shock and subsequent depletion of the
gas-phase species onto grain mantles.
