The word "hydrological" is meant to indicate that the model only includes a sort of bulk description of the water: where it is, how much is there, and where it flows. It generally only includes length and time scales of interest to the system in question. So, for ground water models, this may be tens of meters and longer in horizontal direction, and hours and longer in time.
On the other hand, "hydrodynamic" is generally understood to resolve the governing equations for water flow, i.e., the Navier-Stokes equations (possibly posed in a porous medium). This must then deal with the length and time scales of eddies, which are substantially smaller and shorter than the ones above, and consequently requires a lot more computational power.