the thing about brand name water is they test for the real bad stuff like lead, mercury, nitrates and the list go's on. now us brewers don't want any of that stuff in are beer but the stuff we do want like certain levels of all the salts, CA+2 ,MG-2, NA, CL, SO4 and the such, are not regulated to maintain pacific amounts in bottled water. they just have to keep the poisons out. So when you get bottled water tested or you here what someone's water report was, it is not set in stone that the water you get will be the same. the water could change with the seasons or the area it comes from. there is also the fact that a lot of bottlers of name brand water use reverses osmosis to purify the water then add some salts back to it to make it taste better. Now we get to the cost. that water is not worth what it sells for.. the best thing to do is find and buy RO water and add the salts yourself. its cheaper, more accurate, and more versatile for us brewers who may want to do a pilsner one day and a porter the next.
Please remember that this is the way I see it and by all means I'm not the master of anything but my beer is good, most of the time and the most I pay for water is 39 cent a gal for RO + salts I add, witch cost next to nothing .
just don't want to see home brewers get ripped off paying for fancy RO water with a big name on it. spend the cash on hops instead.