We discuss the correlation of light-element abundances with metallicity for metal-deficient dwarfs in the Galactic halo. We show that such stars can experience some metal enrichment after their formation because of accreting interstellar material during repeated encounters with molecular clouds in the Galactic plane. If light-element abundances are produced by the secondary spallation of preexisting heavy elements by cosmic rays, then this accretion affects the light-element abundances and metal abundances differently. We construct an analytic chemical evolution model for the halo and disk which reproduces the observed abundance correlations and the halo metallicity distribution. We show that the introduction of interstellar accretion leads to a low metallicity plateau in the correlation of light elements with metallicity which would mimic the formation of such elements in the big bang. We suggest here that the observation of a constant light-element abundance at low metallicity may not be a signature of primordial origin but rather a measure of the average accretion rate from metal-enriched gas in the Galactic plane.