  Detections of deuterium in high redshift Lyman limit absorption systems along
the line of sight to QSOs promise to reveal the primordial deuterium abundance.
At present, the deuterium abundances (D/H) derived from the very few systems
observed are significantly discordant. Assuming the validity of all the data,
if this discordance does not reflect intrinsic primordial inhomogeneity, then
it must arise from processes operating after the primordial nucleosynthesis
epoch. We consider processes which might lead to significant deuterium
production/destruction, yet allow the cloud to mimick a chemically unevolved
system. These processes include, for example, anomalous/stochastic chemical
evolution and D/{$^4$He} photo-destruction. In general, we find it unlikely
that these processes could have altered significantly (D/H) in Lyman limit
clouds. We argue that chemical evolution scenarios, unless very finely tuned,
cannot account for significant local deuterium depletion since they tend to
overproduce $^{12}$C, even when allowance is made for possible outflow.
Similarly, D/{$^4$He} photo-destruction schemes engineered to locally produce
or destroy deuterium founder on the necessity of requiring an improbably large
$\gamma$-ray source density. Future observations of (D/H) in Lyman limit
systems may provide important insight into the initial conditions for the
primordial nucleosynthesis process, early chemical evolution, and the galaxy
formation process.
