Why was the water from some aqueducts that fed the water to ancient Roman fountains(nasoni) cold?
This drifts into a science Stack Exchange question somewhat, but consider the following:
Water mainly came via the Avens river Nar (that goes into Tiber) from lacus velinus near Reate. Its water came from mountains. Marius Curius Dentatus has drained the waters of the lake into the river Nar. In doing so, he created the marmore waterfalls:
The lake is now known as Lago Piè di luco
"Rivers and the Power of Ancient Rome" By J. B. Campbell
Dictionary of Greek and Roman Geography: Iabadius-Zymethus by Sir William Smith