Conditions leading to widespread lead poisoning in Flint, Michigan are not unique to the troubled Rust Belt city. In recent decades, many cities have made the switch from chlorine to chloramine for water treatment; this can, without proper management, release toxic lead from old pipes directly into drinking water.
In the 1960s, people learned that the chlorine that we were adding to our tap water to protect us from waterborne pathogens was reacting with organic matter to produce a class of chemicals known as disinfection byproducts. These byproducts can cause cancer, miscarriages or other health effects. So water utilities spent a lot of time trying to figure out how to minimize the formation of disinfection byproducts while still protecting people from waterborne pathogens, and one of the solutions they found was replacing chlorine with a family of compounds known as chloramines. If you add a small amount of ammonia to the water when you add the chlorine, you create something called chloramines, which don’t react with water to produce disinfection byproducts. Starting in the 1980s and 1990s, many cities in the United States switched from chlorine to chloramines.
Chloramines are used in many cities around the country without incident, but under some conditions, switching from chlorine to chloramines can mobilize lead from lead-containing drinking-water pipes and solder in household plumbing. This was observed in Washington, D.C. about ten years ago, and led to a problem similar to what we saw in Flint recently. But these cases appear to be quite rare. The engineers who operate water systems are now aware of this phenomenon and have taken steps to prevent the mobilization of lead. It’s now one of the risks that people think about when they switch from chlorine to chloramines. Water engineers have a good understanding of the composition of the water and know how to manage the water to minimize the chance that lead would dissolve. They can add small amounts of a compound called polyphosphate, which can prevent the lead from dissolving. The way I understand it, the managers of the Flint water system didn’t take some of the normal steps to prevent the dissolution of the lead from the pipes.