I've heard numbers of 30-50% for drinking water (that would be pressurised pipes, making leaks worse fast).
RS485 is a serial protocol designed for long (very long) lines, and to handle multiple drops on the same line - in contrast the better known RS232 is designed for peer to peer only. Not fast, around 1200 bps is normal, but that should be more than enough for your application.
You're underground so I'm afraid wired is the best way to get your data out.