I'm not following the preoccupation with rain water. Is this a Dr Strangelove type deal?
The tar patch is kind of the least of your worries if you're concerned about trace amounts of harmful chemicals. Rain can pickup a lot of not so nice chemicals as it fall through our polluted air. And the polluted dust that settles on the on the roof isn't going to help. There's even a bacteria, Pseudomonas syringae, adapted to living in clouds, but it isn't harmful to us or fish.
Growing up our home had a rainwater collection system, a cistern, and also a well. Only one tap was connected to the well and we weren't suppose to drink the water from the taps connected to the cistern.
Also, I once collected rainwater from a roof and the water was black like English breakfast tea. And it didn't settle. I'm sure that isn't common, but it changed how I view rainwater.
General Ripper disagreed thinking rainwater was the perfect fluid, especially when mixed with pure grain alcohol. City water is how a "foreign substance is introduced into our precious bodily fluids without the knowledge of the individual, and certainly without any choice. That's the way your hard-core Commie works. I first became aware of it, Mandrake, during the physical act of love...".