I have seen a number of sketches and an animation purportedly showing what how lead gets into tap water. I haven’t been happy with what I have seen so with the help of Byron Tucker in I.T. we have drawn our own animations. I hope they help to explain some of the science.
Case 1. Flint water before Oct 2015. The water is rich in carbonate. Lead atoms (grey) escape from the walls of the lead pipe and bind with carbonate (green) to form a mineral layer of lead carbonate. A relatively large amount lead remains in solution. In the case of Flint this is 70 ppb after the water has sat overnight in the lead pipe.
Case 2. Detroit water after Oct 2015. The water is rich in carbonate but also contains a small amount of added phosphate from the treatment works. Lead atoms (grey) escape from the walls of the lead pipe but this time they preferentially bind with phosphate (blue) to form a mineral layer of lead phosphate. This mineral is more insoluble than lead carbonate (lead sticks to phosphate stronger than it does to carbonate) and therefore only a small amount lead remains in solution. In the case of Flint this is 5 ppb after the water has sat overnight in the lead pipe.
Case 3. Detroit water after Oct 2015. In addition to carbonate and phosphate, tiny particles of orange iron oxide are present. These particles are the corrosion products from cast iron water pipes. They are covered in negative charge which attracts the lead to form lead-rich iron oxide particles. Some of these particles stick to the sides of the pipe (not shown here) and can break off later and and other particles pass straight through. Either way they come out of the tap and are responsible for the very high lead reading that are still being recorded in Flint despite returning to Detroit Water.
Currently, anything above 15 ppb in Flintcaused by lead-rich iron oxide particles. These particles will be present along with the lead phosphate mineral layer.