Of course, it's readily obvious to anyone who observes the stuff and works with it long enough that at the very least it'll turn into white lead. Vitruvius comments:

Etiamque multo salubrior est ex tubulis aqua quam per fistulas, quod per plumbum videtur esse ideo vitiosum, quod ex eo cerussa nascitur; haec autem dicitur esse nocens corporibus humanis.

And water is much healthier drawn through clay pipes than lead, because what is drawn through lead seems to be unhealthy, because from it white lead is produced; moreover, this is said to be poisonous to the human body

That different sources of water had different properties and levels of purity was also pretty readily obvious--you really only need to look at two different rivers to see that the water flowing there is not the same stuff. Celsus ranks the purity of different sources of water in broad strokes:

Aqua levissima pluvialis est, deinde fontana, tum ex flumine, tum ex puteo, post haec ex nive aut glacie: gravior his ex lacu, gravissima ex palude.

The purest water is rainwater, then spring water, then water taken from a stream, then from a well, and after that water from snow and ice: heavier [i.e. less pure] than these is lake water, and the heaviest is water taken from a swamp.

Frontinus also complains about minerals in the water corroding pipes:

Haec duplici ex causa nascuntur: aut enim limo concrescente, qui interdum in crustam indurescit, iter aquae coartatur, aut tectoria corrumpuntur, unde fiunt manationes quibus necesse est latera rivorum et substructiones vitiari.

This [i.e. repairs to the aqueduct pipes] comes about for two reasons: for either with deposit collecting, which sometimes hardens into a crust, or the lining is broken, whence there are leaks, by which it must be that the sides of the channels and the superstructure are damaged.

Now, these aren't the most scientific descriptions, at least in terms of the works of natural philosophers (who discussed matters like this, but I'm generally not as familiar with the work of natural philosophers). But I think it's appropriate, given that we're talking specifically about the corrosion of piping and deposition of sediment and dissolved minerals. The Roman understood perfectly well that different water sources corroded pipes differently and also that different sources taste different. Hence their thorough searches for good water sources, and the disappointment that occurred when sources were found to be poor (as with the Aqua Alsietina, which was turned mostly into an agricultural supply).