Our colony has landed on a planet which is a complete barren desert wasteland, the surface as dead as the moon but also extremely cold, being quite far from the local star. They have settled at the only source of heat - a very deep crater with a hot geothermal spring at the bottom.
There is water in this spring, alas, the water is mixed with other toxic chemicals and so can't be drunk.
However, the heat from the spring causes a lot of the liquid to evaporate. As the vapour rises, (let's just hand-wave that they can breathe for now), it cools as it gets further and further from the heat at the bottom. Eventually, it condenses, and some of it lands on the sides of the crater, and as it comes in contact with the freezing surface, some of it freezes. This has been going on for thousands of years - or however long it needs to have
Now each component of the mixture has a different condensation/freezing point, and the temperature changes with altitude.
So this has the fortunate effect of separating out the components, and at a certain altitude, the walls will be covered in (relatively) pure ice water which can be chipped off, melted and drunk safely. The other chemicals will have condensed and frozen at either higher or lower levels.
Is this at all plausible?