Monica,

A too coarse discretization would typically lead to oscillations
rather than high numerical dispersion. So please check whether your
dispersivity values are realistic and whether you have turned on an
upwind method (in Temporal&Control data). Upwind methods stabilize the
solution on too coarse meshes, but depending on the method used a high
amount of artificial dispersion might be added.

Hope this helps

Peter
FEFLOW Support Team