You’ve likely heard of the terrible water crisis currently afflicting the city of Flint, Michigan, in the United States. The city’s water supply contains very high levels of lead, which is well-known to cause serious health issues. This lead is coming from the pipes that bring water to the city from the Flint River, but how is it getting into the water that the residents are drinking? Here’s a few quick explanations to some of the chemistry-related questions surrounding the story.

Why is lead leaching into the water?

Flint made the switch to getting its water from the Flint River back in April 2014, as a money-saving exercise. Previously it got its water from Detroit, which cost the city millions of dollars. The issue is that the water from the Flint River is naturally quite corrosive; it contains relatively high levels of dissolved chloride ions (about 8 times more than the Detroit water) which can cause metals such as iron and lead to leach into the water. The high chloride levels are largely due to road salt which runs into the river.

The problem was exacerbated not long after the switch to the Flint River water supply. In the August following the switch, E. coli was found in the water, and to combat this extra chlorine was added as a disinfectant to remove it. However, this higher level of chlorine generated unsafe levels of trihalomethanes, compounds which are byproducts of the chlorine reacting with organic matter in the water. To combat this, ferric chloride was added.

Ferric chloride, FeCl 3 , acts a coagulant, allowing for the removal of organic matter from the water. However, it also helps to increase the chloride concentration still further, making the water even more corrosive, and causing the concentration of lead in the water to increase. The corrosiveness of the Detroit water can be compared to that of the Flint River using their chloride to sulfate mass ratios (CSMR). For the Detroit water before the switch, this had a value of 0.45, indicating low corrosion. After the switch to Flint River water, this increased to 1.60, a value denoting very high corrosion.

Why isn’t this a problem in other cities where lead pipes are used?

This isn’t usually an issue because in areas where lead pipes are present, corrosion inhibitors can be used to prevent the lead getting into the water. A common corrosion inhibitor is orthophosphate; this is simply phosphoric acid, or salts of phosphoric acid. Orthophosphates form low-solubility complexes with the lead in the pipes, forming a layer inside the pipe and preventing lead getting into the water. These compounds were used in the Detroit water supply before the switch, even though the water had a comparatively low corrosiveness.

In Flint, orthophosphates weren’t used; nor were any other corrosion inhibitors. This meant that there was nothing preventing the lead getting into the water supply. It also led to the unpleasant discolouration present in the Flint River water coming out of residents’ taps, as iron in the pipes was also corroded by the water.

Where can I read more about the problems in Flint?

For the whole back story, there’s an excellent Mashable summary here. If you want to read the EPA preliminary report on the water in Flint, from June 2015, then that can be reached here. More general links are also provided below.

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