tl;dr, water treatment authorities accidentally dumped a bunch of aluminum in the water, but told all their customers it was safe to drink before they knew what it was, but scientists and doctors are apparently still arguing over what effects aluminum exposure can have (see: “Aluminum and Alzheimer’s disease: after a century of controversy, is there a plausible link?”)

Btw, this all got swept under the rug when it was happening in part because there was a massive privatization scheme against all the water systems in England and Wales going on at the time, and the authorities didn’t want any scandals and investigations slowing down their giant business deals.