You talk about 1-2 Arduinos, with moisture sensors, to detect leaks. That should be very specific and predetermined locations, and sounds very much like monitoring a place and alarming when there are leaks, rather than searching for leaks along a pipe. Otherwise the person digging up the pipe could check for the leaks himself, no need to use an Arduino for that.
How big are those pipes?
How big do you expect the leaks to be?
How are you going to be sure that a good rainstorm is not mistaken for a leak?
Big leaks in big pipes (water mains: half a meter or more in diameter, many kilometres long) are searched for in a very different way: acoustic. Basically they take a probe, shove it in the pipe at one end, let it go with the flow and listen for leaks (leaks tend to make sounds in the water, especially bigger leaks). Then at the other end it's recovered, and the time when the leaks are heard give a good indication on where exactly the leak is. After that it's a matter of driving to that location and bringing out the diggers.
Much more efficient than digging up the whole pipe and placing sensors every half metre (you maybe can get away with a bit fewer but it's still a lot of work).