  Photoionization models of the giant HII region 30 Doradus are built and
confronted to available UV, optical, IR (ISO) and radio spectra, under
black-body or CoStar SEDs for the primary source and various density
distributions for the nebular gas. Chemically homogeneous models show very
small rms electron temperature fluctuations and fail to reproduce the heavy
element optical recombination line (ORL) spectrum of the nebula. Dual abundance
models incorporating small-scale chemical inhomogeneities in the form of
H-deficient inclusions which are in pressure balance with the normal
composition ambient gas, provide a better fit to the observed heavy element
ORLs and other nebular lines, while most spectral features are satisfactorily
accounted for. The inclusions, whose mass is ~2 per cent of the total gaseous
mass, are 2-3 times cooler and denser than the ambient nebula. Their O/H
abundance ratio is ~0.9 dex larger than in the normal composition gas and have
typical mass fractions of X = 0.687, Y = 0.273 and Z = 0.040. Helium is found
to be about as deficient as hydrogen in the inclusions, while elements heavier
than Ne, such as S and Ar, are quite possibly enhanced in proportions similar
to O. This suggests that the posited H-deficient inclusions may have arisen
from partial mixing of matter which was nucleosynthetically processed in a
supernova event with gas of normal LMC composition. Attention is drawn to a
bias in the determination of HII region helium abundances in the presence of
H-deficient inclusions. It is argued that these results provide evidence for
incomplete small-scale mixing of the ISM. The case for the existence of
abundance inhomogeneities in HII regions is examined in the light of current
theoretical considerations regarding the process of homogenization in the ISM.
