How fast does groundwater contamination spread?

[ScienceNostalgia101]

I ask because I keep hearing of groundwater contamination, yet never hear of how quick the process is, vs. how gradual the process is.

[mistermack]

That really is like asking how long is a piece of string. It's different in every case. 

I vaguely remember reading that water from boreholes in Australia being a hundred thousand years old. But then, it's a big country. 

[zapatos]

I imagine it spreads as fast as the water flows.