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We critically examine a scenario for the enrichment of the interstellarmedium (ISM) in which supernova ejecta follow a long (10^8 yr) journey beforefalling back onto the galactic disk in the form of metal-rich ``droplets'',These droplets do not become fully mixed with the interstellar medium untilthey become photoionized in HII regions. We investigate the hypothesis that thephotoionization of these highly metallic droplets can explain the observed``abundance discrepancy factors'' (ADFs), which are found when comparingabundances derived from recombination lines and from collisionally excitedlines, both in Galactic and extragalactic HII regions. We derive bounds of10^{13}--10^{15} cm on the droplet sizes inside HII regions in order that (1)they should not have already been detected by direct imaging of nearby nebulae,and (2) they should not be too swiftly destroyed by diffusion in the ionizedgas. From photoionization modelling we find that, if this inhomogeneousenrichment scenario holds, then the recombination lines strongly overestimatethe metallicities of the fully mixed HII regions. The abundances derived fromcollisionally excited lines also suffer some bias, although to a much lesserextent. In the absence of any recipe for correcting these biases, we recommendthe discarding of all objects showing large ADFs from studies of galacticchemical evolution. These biases must also be kept in mind when comparing thegalactic abundance gradients for elements derived from recombination lines withthose derived from collisionally excited lines. Finally, we propose a set ofobservations that could be undertaken to test our scenario and improve ourunderstanding of element mixing in the ISM.

Enrichment of the ISM by metal-rich droplets and the abundance bias in HII regions