Crazyfuzzi is on the right track with checking for leaks underground. It will be necessary to isolate the separate buildings or loops in question and pressure test these sections individually. As for leak checking the piping which is located underground will present some difficulties, however it is not that hard if it is metallic pipe. It will be first necessary to trace and locate the pipe, isolate areas in question, and the use of an ultrasonic sound detection system such as a Goldak 4360 along with the injection of air pressure in piping will create a high turbulence so that noise will be heard
Understand the symptoms and you will find a solution.
i have used many was to find water leak blue or green die altra sound turned off the sprinklers for a few days in arizona found the nice green spot .backflow preventor on closed loop every time cooling tower would start it drain the loop down some one capped the drain my favorite was when the water serfaced .the elevater rm around back of building was like mush .repiped all above ground ' coil leak s oh yes the water teatment person for got the cap on the pot feeder . so many ways fluid cooler tube bundle leaking ,pump seals .water hoses wrotted .I allways started with a water meter on the closed loop make up .