does the water in the tidal basin ever become radiation free due to project purity or does it always have 1 or 2 rads?? and if it does go clean how long does it take?

In game? Never -- it'll always stay slightly irradiated. Though, over time it will be clean but we'll never actually be able to play Fallout 3 when it's like that.

Doesn't really feel like your choice has many conseqeunces, sadly, so maybe that's why it's better to have a definite ending.

You would've have thought that the water would become clean after a while and depending on how you dealt with Oasis I think greenery should spread from that location.

Well PC owners may have that ability, I however don't =(