Perhaps continue and run the combined chlorine test, and also run the CYA test. There could have been something in the trucked in water that consumed all the chlorine.

According to my calculation, a gallon of 12.5% is only just over 5ppm FC. With no CYA it would only take 30 minutes or so for that to be all gone in sunlight; actually I don't think the concentration matters a lot when you have unstabilized chlorinated water exposed to direct sunlight. Here's some more details on how that works: http://www.poolforum.com/pf2/showthr...me-of-Chlorine