Those Romans were pretty clever folk. Imagine being given the task of transporting water over a distance of 22 kms, that needed to fall from 680 metres above sea level, to just 50.
Well that's exactly what they did at Patara in Turkey, using thousands of individually carved stones that weighed about a ton each. We consider that building the Pyramids was a hefty task, but this aqueduct is certainly comparable.
I imagine there was some form of gasket (leather?) between the stones to act as a seal. Knowing the Romans, I doubt if there was hardly the tiniest of leaks.
There are many things in the world that make us gasp; this (for me) is one!