At frequencies below radiation, ground currents must obey Kirchhoffs' current law and return to the source.
Say you have a Dyn1 distribution transformer that is serving multiple distribution feeders. When one of the feeders has a ground fault at a remote location, 100% of the ground current will return to the transformer neutral point when it is connected to ground (earthed). The return current may take many different paths depending on the configuration of the distribution system; however, Kirchhoffs' current law requires that 100% of the current will return to the source.
As the ground current flows through the soil, heat will be generated by I^2R loss just as in any element. The heat will dissipate in the soil according to the laws of thermodynamics for conductive heat flow. The current will return to the source (100%) by Kirchhoffs' current law.
If there is a return path, then the current must return to the source. If there is a neutral conductor, it will return through the conductor. If the neutral is periodically grounded, then a portion of the current will, in fact, return through the earth. Of course, the current will split between the neutral and the ground based upon the parallel impedance network and Ohm's Law. If there is no neutral conductor, the current must return through the ground. However, the ground will introduce a significant impedance which will limit the current severely. The neutral is important for detection of ground faults and relaying.
While the current may widely disperse from the point of the fault, it will have to converge at the source. This is the reason why, for safety sake, a perimeter ground should be run outside of a substation fence. Otherwise, should someone be standing outside the fence and leaning against it when a ground fault occurs, the concentrated current at the substation can cause a significant voltage differential between the fence and the ground only a meter outside the fence.
HOWEVER, there must be a path for the current to return. This is provided by a grounded Wye system or a corner grounded Delta. Ungrounded Wye and Delta systems do not provide a path for zero sequence currents and, hence, no path to single phase to ground faults.