  We analyze whether there is any residual foreground contamination in the
cleaned WMAP 7 years data for the differential assemblies, Q, V and W. We
calculate the correlation between the foreground map, from which long
wavelength correlations have been subtracted, and the foreground reduced map
for each differential assembly after applying the Galaxy and point sources
masks. We find positive correlations for all the differential assemblies, with
high statistical significance. For Q and V, we find that a large fraction of
the contamination comes from pixels where the foreground maps have positive
values larger than three times the rms values. These findings imply the
presence of residual contamination from Galactic emissions and unresolved point
sources. We redo the analysis after masking the extended point sources
cataloque of Scodeller et al. [7] and find a drop in the correlation and
corresponding significance values. To quantify the effect of the residual
contamination on the search for primordial non-Gaussianity in the CMB we add
estimated contaminant fraction to simulated Gaussian CMB maps and calculate the
characteristic non-Gaussian deviation shapes of Minkowski Functionals that
arise due to the contamination. We find remarkable agreement of these deviation
shapes with those measured from WMAP data, which imply that a major fraction of
the observed non-Gaussian deviation comes from residual foreground
contamination. We also compute non-Gaussian deviations of Minkowski Functionals
after applying the point sources mask of Scodeller et al. and find a decrease
in the overall amplitudes of the deviations which is consistent with a decrease
in the level of contamination.
