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We use a stochastic modelling framework to analyze the relative importance of preferential flow and diffusional mass transfer, compared to that of random spatial transmissivity variability, in predictions of expected contaminant transport by groundwater that is integrated with the unsaturated soil system. Besides potential existence of preferential flow paths and diffusional mass transfer between mobile and immobile water regions, we include in the analysis possible groundwater recharge through the unsaturated zone and the longitudinal extent of solute sources at the soil surface, which may both affect the expected temporal spreading of groundwater contaminants as observed at control planes located at different distances downstream of the source. The relative importance of preferential flow and immobile water for the quantification of temporal solute spreading increases with increasing immobile water content and decreasing rate (relative to the mean flow velocity) of diffusional mass transfer between mobile and immobile water. The contaminant spreading effects of random transmissivity variability and preferential flow are partly overlapping, such that, in some cases, neglect of either one of these factors, while accounting for the other, may yield small errors in estimates of the expected temporal contaminant spreading.