Long before the tainted water of Flint, Mich., became a national issue, some cities had to shut off their water because of contamination. (LLOYD FOX/BALTIMORE SUN)

The water crisis in Flint, Mich., officially began on April 25, 2014. That’s the date the city’s water supply was switched from Lake Huron to the Flint River. What was supposed to be a cost-saving measure soon turned into a nightmare for residents. But Virginia Tech professor Marc Edwards can trace the story back a decade earlier, and to a different city: Washington.

A coverup of lead problems in Washington’s water helped promote the notion that children couldn’t be seriously injured by what was in their sippy cups. (One government analysis in 2004 reported there was no evidence that high levels of lead had harmed health, even among children, an error that was not corrected until 2010. Later studies found that an increase in miscarriages had coincided with the high lead levels.) “That’s the danger of bad science,” and it’s what allowed Flint to happen, Edwards explains in “Nova: Poisoned Water,” which premieres Wednesday on PBS. The hour-long documentary lays out the process of how water comes to flow through our taps, and it highlights the importance of understanding what we’re drinking, using the recent events in Michigan as a cautionary tale.

It’s a fascinating, if depressing, science lesson. Basically, lead pipes are all over the place, which should be a disaster for human health. The reason it’s not — not usually, at least — is that chemical reactions between pipes and properly treated water help build a protective coating called a scale. Although this substance is made up almost entirely of lead, it effectively prevents lead from getting into the water.

But you can’t take this coating for granted, which is what happened both in Washington and in Flint. The experts in charge didn’t add the necessary ­corrosion-control chemicals, so the scale flaked away, exposing the pipes and drastically changing the makeup of the water.

“Poisoned Water” highlights what that meant in Flint for stay-at-home mother LeeAnne Walters. She could tell there was a connection between the brown stuff pouring into her kitchen sink and her family’s various health woes, all of which seemed to be related to water use. After requesting documentation from the city, she is the one who first identified that Flint wasn’t doing corrosion control.

That finding is what spurred action by Edwards and other researchers, which helped Walters and her neighbors finally get taken seriously.

And that is the promise of good science.