For Flint, Michigan [1] (the recent times big news event) and other cities it was lead from pipes:

    Officials failed to apply corrosion inhibitors to the water, which resulted in lead from aging pipes leaching into the water supply, exposing around 100,000 residents to elevated lead levels.

ie. Lead leached by water from a lead source (be that paint, pipes, or gasoline particulate residue)

[1] https://en.wikipedia.org/wiki/Flint_water_crisis