Deep in a laboratory far under the megacity of Exampleville, a scientist is experimenting on a new compound found in an alternate, magical universe. He accidentally knocks over a vial of Handwavium, which falls into a sink. Suddenly, the magic spreads through the city's water, instantly thickening it to the viscosity of maple syrup.
This transformation spreads at about 100 feet/second, but I'd also be interested in how the effect would be different if it were instant. Assume that any water touching the sewer or fresh water systems would be converted. Lakes and rivers are out of the scope I'm looking for; I might make that a second question.
Anyway, how would this affect our water/sewer infrastructure, and would the effects be permanent? These buildings are skyscrapers, and there are at least 10 million people. Precipitation and dew are obviously not affected unless they contact contaminated water. Note that there is no magic aside from the Handwavium that affects the water, so no magical answers.