The FDA strictly regulates most of this stuff....most companies bottle water at high temps with ozone injected in them...which after 30min or so just breaks down to O2. Tap has Cl to kill bacteria along with cyanide and tons of other goodies....drink treated water that has been scrubbed with activated carbon and put through a RO filter for good water.

Was your experiment done in an aseptic environment? If not then the source could come from other than the water.

Id bet arrowhead is simply a better catalyst for growth...I dont believe there is a food that only E Coli can grow on...I bet there was a few different types of bacteria growing on there

Im not trying to be an ass, just wanted to point out some stuff.