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The success of HI intensity mapping is largely dependent on how well 21cmforeground contamination can be controlled. In order to progress ourunderstanding further, we present a range of simulated foreground data fromfour different $\sim3000$\,deg$^2$ sky regions, with and without effects frompolarization leakage. Combining these with underlying cosmological HIsimulations creates a range of single-dish intensity mapping test cases thatrequire different foreground treatments. This allows us to conduct the mostgeneralized study to date into 21cm foregrounds and their cleaning techniquesfor the post-reionization era. We first provide a pedagogical review of themost commonly used blind foreground removal techniques (PCA/SVD, FASTICA,GMCA). We also trial a non-blind parametric fitting technique and discusspotential hybridization of methods. We highlight the similarities anddifferences in these techniques finding that the blind methods produce nearequivalent results, and we explain the fundamental reasons for this. Thesimulations allow an exact decomposition of the resulting cleaned data and weanalyse the contribution from foreground residuals. Our results demonstratethat polarized foreground residuals should be generally subdominant to HI onsmall scales ($k\gtrsim0.1\,h\,\text{Mpc}^{-1}$). However, on larger scales,results are more region dependent. In some cases, aggressive cleans severelydamp HI power but still leave dominant foreground residuals. We alsodemonstrate the gain from cross-correlations with optical galaxy surveys, whereextreme levels of residual foregrounds can be circumvented. However, theseresiduals still contribute to errors and we discuss the optimal balance betweenover- and under-cleaning.

21cm foregrounds and polarization leakage: a user's guide on cleaning and mitigation strategies