The answers will depend a ...

The answers will depend a bit on the diameter, length and pressure of operation.

For short lengths and smaller diameters operating at a few bar (10s of psi) and above you may have success locating leaks using a modern leak noise correlator. In it's simplest form this requires access to the pipe at two points - one either side of the leak location - where sensors are attached to the pipe. It detects the noise generated by the leak and calculates how far it is from each sensor point to give the leak location.

For larger pipes you could use Sahara or Smartball technology (www.assessaddress.com) which are "in-pipe" acoustic techniques that use a sensor travelling through the pipe to detect and locate the leak noise. Both should detect leaks down to less than 1 bar. Sahara also has a "Conductivity" option that can find leaks in very low pressure systems.  

Repairing is a much bigger challenge - particularly if the pipes are carrying potable water. Again there are some questions about what is required: water quality requirements, working pressures (and variability), pipe size, likely leak size and whether you are looking for a long or short term fix.  

There was some work done in the UK to investigate a technique that used "platelets" (actually these were small blocks of elastomer) injected into the water flow. The platelets travelled with the water with the aim that they got sucked into the leak and blocked it. While this worked under ideal conditions it is much harder to make it work in the field - where there are branches, varying pressure, a range of leak sizes, water quality issues etc to contend with. I don't believe anyone is currently pursuing this option.

At WRc we have particular expertise in deploying sensors into pressurised water pipelines to utilise a range of different condition assessment techniques. We are also interested in techniques to carry out live repair.