The source of the non-mare component in the lunar mare soil samples is still an open question. A spatially resolved numerical model tracing the diffusion of non-mare material was developed by means of the Monte Carlo method to study this issue in more detail. The vertically- and laterally-transported non-mare components are recorded separately to assess their concentration on the surface. We find a general higher efficiency of lateral transport than of vertical transport, but the opposite may occur within small zones of interest. The overall (background) distribution of non-mare component is estimated by averaging out regional variations of material composition caused by local impact mixing. We find that almost all the mare surface is mixed with non-mare material. If most of the mare regions have filled basaltic material about 500 m in thickness since the formation of basins, the average non-mare fraction in the top 5 m is about 0.1. The abundance of the non-mare component decreases with increasing distance from the mare-highland boundary, but the slope of the distance-dependence is shallower within ~100 km of the boundary than further away.