Recently, PBS NewsHour ran a two-part investigative story that must have seemed to have all the elements of a compelling piece. There was a relatively unknown but apparently widespread carcinogen and a great hook—it was tied to the story of contaminated water behind the film Erin Brockovich (and the book A Civil Action). Issues relating to public health threats demand careful and thorough reporting. Unfortunately, those qualities were at times absent from the PBS story.
The story focused on hexavalent chromium (Cr6+) in drinking water. Most chromium is trivalent, which isn’t very soluble in water. When oxidized to hexavalent chromium, however, it becomes mobile. Unlike the trivalent variety, hexavalent chromium is carcinogenic. It's much worse if inhaled, but there’s evidence that it is dangerous when ingested as well. As the NewsHour story noted, recent sampling has shown that hexavalent chromium is present in drinking water across the country. Does that mean we have a public health crisis on our hands, caused by shockingly widespread contamination? Let’s slow down and get some context.
Meet the metal
Hexavalent chromium occurs naturally. That’s a rather important fact never mentioned in the NewsHour story, which describes the water as “tainted” by industrial chemicals. Chromium, like many elements, is present in Earth’s crust. Some types of rock have more than others, but it’s actually a little bit more common than copper or zinc—the average concentration in the upper crust is something like 90 parts per million. Oxidize some of that chromium to the hexavalent state, and it can be mobilized into groundwater. If hexavalent chromium is showing up in wells nearly everywhere we look, it might be because it’s naturally present rather than a ubiquitous, human-introduced contaminant.