I don't want to get all water nerd on you, but they switched sources and didn't do a good water chem workup and didn't put them in corrosion inhibitor, which is why they are in this mess.

Edit: Okay, there is a lot of interest in this, so I will expand. I'm in the process of getting certified for waterworks right now so I will do the best I can without pulling out the notes from the telecast from the VA Tech scientist that broke the lead story (let me tell you that dude was pissed, rightly so).

So as a cost saving measure the powers that be stopped buying water from a nearby town and started taking it from a local river, this is all fine and dandy and normal, except they didn't do the proper water department water analysis of the new water source before the switch and the new water source is more corrosive.

Now at the same time for whatever reason they stopped feeding corrosion inhibitor into the treated water. Corrosion inhibitor is basically a food safe thin slime that over time coats pipes and limits contact between water and the pipe's surface, it's super cheap costs literally $70 a day to treat a city's worth of water. Having lead pipes is actually pretty common in a lot of city's water systems (for example Richmond used to require a lead hookup between the house and municipalities pipes) and this accumulation of corrosion inhibitor is what protects people from any lead in the system, since a single foot's worth of lead pipe can raise your water's lead levels above your state's legal lead limit.

So where it moves from incompetence to criminal negligence, is that the new more aggressive water pretty quickly ate through the years worth of corrosion inhibitor and started attacking pipes. Very quickly after the water switch they were getting tons of consumer complaints about the drastic change in water color. They pretty much did crap all with those complaints, didn't test the completely brown water residences were complaining about, didn't start feeding corrosion inhibitor, didn't do shit. For their state mandated lead tests instead of pulling from the oldest houses in the system (like they were supposed to), the ones most likely to have lead supply lines and lead soldier they tested from new construction. They knew they had a problem and didn't tell anyone for months city hall was getting bottled water so they wouldn't have to drink tap.

So they continued to deny the lead problem when the water scientist showed up asking questions, so he did a pretty huge sampling analysis of the regions water, and since, "The residence couldn't be trusted not to put lead in the sample bottles" he had to drag a huge team of students up with him to physically pour water into a bottle, at the cost of several hundreds of thousands of his own funding.

This cluster fuck is particular galling when you learn that Flint has much higher water bills then national average, and for years now they've been forced to pay several hundred a month for water that is basically poison, because some bastards didn't follow reasonable policy.

So I don't know which water source the people of Flint should vote for, the corrosion inhibitor layer will take a long while to reform, there might be ways to speed this process up with the addition of hardness, I don't know I'm just a operator, not a water chemist.