"A potentially carcinogenic form of the element chromium is in Boulder’s drinking water and there are no plans to remove it anytime soon.
Hexavalent chromium, or chromium-6, has been linked to cancers of the liver, stomach, small intestine and more when it is consumed through water sources. It is unclear just how much chromium-6 must be consumed — and for how long — before any carcinogenic effects occur; research to date has been inconclusive and incomplete.
Because of the lack of data, there is no standard for chromium-6 levels in drinking water in Boulder, the state of Colorado and the rest of the country, besides California. What does exist is a standard for total chromium, (which includes chromium-6 and the benign trivalent chromium, chromium-3), a nascent wave of research and a lot of unanswered questions."