Lead pipes and lead solder in a city’s water distribution system can mean dangerous water — like in the case of Flint, Michigan — but it doesn’t always mean a city’s water is bad. Many cities in the U.S., particularly older cities in the Northeast and Midwest have lead infrastructure, but lead in the water is minimal, meeting standards established by the Environmental Protection Agency. 

Why is that?

Because the water’s properties can affect how it interacts with the piping material and what eventually comes out at the faucet.

In Flint, the water’s lead levels rose after the city changed water sources. The city had been purchasing its water from Detroit, which drew water from Lake Huron. To save money it started drawing water out of the Flint River. But the water from the Flint River, despite going through a water treatment facility, had different properties. It was more corrosive. As the water from the Flint River filled Flint’s pipes, some of which were made of lead, it started corroding the interiors, wearing away years’ worth of build-up that separated the actual lead from the water that eventually entered people’s homes. 

That build-up —made up of biofilms and mineral coating  — consists of microorganisms, and organic and inorganic materials. It’s basically a lining between the water and the lead pipe.   

The Flint River water first corroded away the film and then started to pick up lead from the exposed pipe.

Many other cities with corrosive water sources add anti-corrosive chemicals to the water before it gets distributed to households. In fact, this is mandated by the EPA’s Lead and Copper Rule, which requires utilities to develop and use a corrosion control plan. If tests reveal elevated levels of lead in the water, utilities have to tweak the corrosion controls in place until water quality improves (or take other steps to decrease lead levels in the water).

The controls can include various adjustments to the water, like changing its alkalinity and pH, hardness, and/or adding phosphate or silicate-based “corrosion inhibitors.”

In some cases, corrosion controls don’t work or have side-effects unacceptable to a utility, and the utility has to seek alternative ways to reduce lead levels. That’s what happened in Madison, Wisconsin.

But in Flint, the water utility simply did not have a corrosion control plan in place, despite statements to the contrary by some state regulators.

In January, Pittsburgh tweaked its corrosion control plan, switching from adding caustic soda to soda ash to make the water less corrosive. The city’s latest reported water tests, which included samples from 50 homes in 2013, showed lead levels just a hair shy of triggering action as mandated by the Lead and Copper Rule. Ten percent of tested taps — that's five homes — had lead levels above EPA's action level, which is 15 parts per billion. The lead level at the 90th percentile — that's the statistical measure EPA uses to gauge compliance and means that 90 percent of samples were below that number — was 14.7 parts per billion. The utility is scheduled to retest 50 taps this summer, though there's no guarantee it will be the same homes from 2013.