[nosse]

It doesn't matter much. Clean water tunnels have high pressure and good walls to keep anything out. Just plain soil getting into clean water tunnel can make loads of people really sick. If there is a clean water leak, they know it from pressure drop.

If there would be a leak in sewage, the biggest problem would be ground water getting into the sewage tunnel, and flooding the water treatment plant. There is probably larger pressure in ground water than in the sewage. Ground water pressure drop can also make foundations of buildings to get looser, so they try to monitor leaks to the sewage (little leaking is btw quite common in older ceramic/concrete pipes).

Sewage system works by gravity. That makes it sometimes really difficult for positioning the pipes. They must have either 2 - 7 % slope or they must be vertical. Uphill inside a sewage tube will gather all the shit and get stuck. With more than 7% slope the water runs too fast away from the pipe and leaves the shit behind, so again it will get stuck eventually.

PS. I used to study civil.