  There is evidence that magnetized material along the line of sight to distant
quasars is detectable in the polarization properties of the background sources.
The polarization properties appear to be correlated with the presence of
intervening MgII absorption, which is thought to arise in outflowing material
from star forming galaxies. In order to investigate this further, we have
obtained high spectral resolution polarization measurements, with the VLA and
ATCA, of a set of 49 unresolved quasars for which we have high quality optical
spectra. These enable us to produce a Faraday Depth spectrum for each source,
using Rotation Measure Synthesis. Our new independent radio data confirms that
interveners are strongly associated with depolarization. We characterize the
complexity of the Faraday Depth spectrum using a number of parameters and show
how these are related, or not, to the depolarization and to the presence of
MgII absorption along the line of sight. We argue that complexity and structure
in the Faraday Depth distribution likely arise from both intervening material
and intrinsically to the background source and attempt to separate these. We
find that the strong radio depolarization effects associated with intervening
material at redshifts out to $z \approx 1$ arise from inhomogeneous Faraday
screens producing a dispersion in Rotation Measure across individual sources of
around 10~rad/m$^2$. This is likely produced by disordered fields with
strengths of at least $3\;\mu$G.
