  Deuterium is the best indicator of the baryon density; however, only its
present abundance is known (and only locally) and its chemical evolution is
intertwined with that of $^3$He. Because galactic abundances are spatially
heterogeneous, mean chemical-evolution models are not well suited for
extrapolating the pre-solar D and $^3$He abundances to their primeval values.
We introduce a new approach which explicitly addresses heterogeneity, and show
that the decade-old big-bang nucleosynthesis concordance interval $\eta \approx
(2 -8)\times 10^{-10}$ based on D and $^3$He is robust.
