the problems with lead in drinking water come from 4 primary sources.
1) Some municipal water systems still have portions of pipe that are made of lead, although most of those have been replaced over the years as upgrades occur.
2) Some homes still have lead service lines. Those are the connections from the City pipes to the home and are usually the responsibility of the property owner to pay to replace. Many municipal water providers will replace those lead service lines at cost and long term, low interest loans are often available to allow homeowners to make those upgrades.
3) Copper plumbing that was installed prior to 1984 was often connected with lead based solder or lead based flux, which can leach into the water supply.
4) Some faucets, even some that are still being sold today, have lead in them, which can leach out and create lead contamination.
Lead service lines and copper plumbing with lead joints are still commonly found in houses all over the country. The insides of pipes get coated with a mineral scale that tends to encapsulate the lead in the pipes, which reduces the amount of lead leaching that occurs. Municipal water supplies are treated with phosphates in order to minimize the corrosive aspect of city water, which preserves the "lining" of the pipes and minimizes lead leaching.
The water that Flint purchased from the Detroit water authority was "optimized" or treated with phosphates, so the lead levels in the homes of the people in Flint that still had lead service lines/ copper-lead plumbing was minimized.
When Flint switched to using water From the Flint River, optimization should have occurred, they should have added phosphates to continue to preserve the lining in the pipes which was preventing lead from leaching.
Here is where it gets kind of complicated. The EPA regulations are somewhat ambiguous in terms of when the treatment or optimization should occur. For cities the size of Flint, it appears that the regulations have an 18 month time frame for the municipal water department to do an optimization study and then a 6 month period to implement corrosive control measures.
The Michigan DEQ and the Flint water treatment plant were just finishing the second 6 month test period of lead testing when the EPA first became involved in the Flint water issue and raised concerns about elevated lead levels in the test results and the lack of corrosive water control to limit lead leaching. The EPA was initially contacted by a Flint homeowner over concerns about the discoloration, bad taste and bad odor of the water, which had been occurring for the first year after the switch-over. Those concerns were unrelated to the lead issue, that did not come up until February of 2015.
One aspect that probably had some bearing on the DEQ's lackadaisical approach to implementing corrosion controls is that they knew that the Flint River was just a temporary source and that the switch to the KWA pipeline as a permanent water source was going to occur in 2016, just about when the 24 month time frame for optimization program for the Flint River water was going to be finished. That process will have to be started over when the KWA water starts flowing, as unlike the Detroit water that was being purchased, the KWA water will be untreated, so it will have to treated by the Flint Water department before being used.
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