I’ve only been able to find news reports of this study (originally published by the UK Daily Mail), no scientific publication as of yet, as far as I can tell. It appears that the main reason that tap water quality is superior is its ability to prevent bacterial infections due to the presence of residual chlorine. While tap water is tested daily at its treatment point, bottled water is tested much less frequently, and only at its source. It can sit in storage for months, with no disinfection additives. And once opened, it won’t remain sterile.

A quarter of people who drink bottled water in the UK do so because they believe it is better for them than tap water. But basically we’re paying 100 to 1000 times more for water that is no better, and possibly worse, than what we can get from our taps.