The water towers on top of NYC buildings tend to not be maintained especially well:

  With their quaint barrel-like contours and weathered 
  cedar-plank sides, rooftop water towers are a constant on 
  the New York City skyline. And though they may look like 
  relics of a past age, millions of residents get their 
  drinking water from the tanks every day.

  But inside these rustic-looking vessels, there are often 
  thick layers of muddy sediment. Many have not been 
  cleaned or inspected in years. And regulations governing 
  water tanks are rarely enforced, an examination by The 
  New York Times shows.

https://www.nytimes.com/2014/01/27/nyregion/inside-citys-wat... (warning: autoplay video)

That the water at the source is clean may be true, but you judge the water by its nature at the final destination, because in the final analysis, that is where the grade of quality matters to those who'll be drinking it.