Any Michigan community that draws its municipal water from a river or inland source rather than from one of the Great Lakes will likely face this problem to some extent. River and inland-source water contains higher concentrations of chloride ions than does water pumped directly from one of the Great Lakes. Why? Perhaps surprisingly, one of the primary reasons is the overuse of salt to deice our roads in the winter. As salt molecules dissolve in water, the negative chloride ions are separated from the positive metal ions. These chloride ions become concentrated in runoff, which makes its way into our streams, rivers, and other waterways.
The negative chloride ions are corrosive, so when the high-chloride water is pumped through lead pipes (or iron and copper pipes joined together with lead solder), lead leaches into the water. It's as simple as that.
Why isn't this happening to the same degree in other places, like Detroit for example? Surely Detroit has old lead pipes, too. Right? Absolutely. But Detroit draws its municipal water directly from the Great Lakes, and water pumped from the big lakes simply isn't as corrosive.
Because the Flint River (with its concentration of chloride ions) flows into the Saginaw River, which in turn empties into Saginaw Bay and Lake Huron, it naturally follows that there are chloride ions in Lake Huron as well. However, the Great Lakes are huge, containing quadrillions of gallons of freshwater. With so much water in the big lakes, the existing chloride ions are greatly diluted and much less concentrated than they are in the water of smaller, inland streams like the Flint River. Sometimes dilution really is a solution to pollution.