Over billions or even tens of millions we would would expect much of the water lain minerals and sediments to be buried. This would effect the conclusions about how long water was present on the surface.
Members of the OMEGA infrared spectrometer team on Mars Express have argued water existed on the surface for a geologically short time because of the spotty presence of clays they have detected.
However, members of the MARSIS radar team on Mars Express found large amounts of buried water or ice on Mars. It is unknown if or how long this could have been at the surface before being buried.

Mars Express and the story of water on Mars
16 October 2006
http://www.esa.int/SPECIALS/Mars_Express/SEMADOV74TE_0.html

Bob Clark