Evolution of the concentration of the passive contaminant, C(R,ϕ), defined in the text, with superimposed averaged streamlines in the midplane for disc models with parameters equal to those of Model 1. The contour levels are defined for each panel separately to highlight the region occupied by the contaminant at each time. The upper three panels are for a disc with no embedded planet. The middle three panels have an embedded planet with M_p = 3 × 10^-4 M_⋆. The lower panels are for an embedded planet with M_p = 10^-3 M_⋆. Times are given in units of the planet orbital period. Results are obtained with NIRVANA.