  Here we investigate how LyC-opaque systems present in the intergalactic
medium at $z\approx3$ can distort the spectral shape of a uniform UV background
(UVB) through radiative transfer (RT) effects. With this aim in mind, we
perform a multi-frequency RT simulation through a cosmic volume of
$10h^{-1}$~cMpc scale polluted by metals, and self-consistently derive the ions
of all the species. The UVB spatial fluctuations are traced by the ratio of
He$\, \rm \scriptstyle II\ $ and H$\, \rm \scriptstyle I\ $ column density,
$\eta$, and the ratio of C$\,{\rm {\scriptstyle IV\ }}$ and Si$\,{\rm
{\scriptstyle IV\ }}$ optical depths, $\zeta$. We find that: (i) $\eta$
spatially fluctuates through over-dense systems ($\Delta$) with statistically
significant deviations $\delta\eta >25$\% in 18\% of the volume ; (ii) same
fluctuations in $\zeta$ are also present in $34$\% of the enriched domain (only
8\% of the total volume) and derive from a combination of RT induced effects
and in-homogeneous metal enrichment, both effective in systems with $\Delta >
1.5$.
