treating and storing rainwater

[RJSquirrel]

Any of YOU treat and store rainwater?..
How long can it go untreated stored?
Should it even be treated?
What do YOU treat it with?
Been trying to find good material for rainwater collectors. Of course RW is about as pure water as you can possibly get before it lands on your roof goes down the gutter and into your receptacle. Thats where you pick up stuff you dont want on your plants. I never thought about birds/mammals/reptiles/ crapping all over the roof or fungus, mold and leaf rot just hanging out in your gutters. Or even parts of one them JET blue ice cubes dropping in off a plane...( uh thats not an asteroid son, thats a giant turd). Your collected RW picked it up, then you pick it up, and then your plants pick it up...
In ag areas you could have lots of nitrates and pesticides in the RW. In the city you could have more suspended particulate matter (smoke dust soot etc etc and junk from no telling what else.
I dont want to disinfect the water with chlorine as The use of chlorine to disinfect RW combines with any decaying organic matter to form trihalomethanes which cause cancer in lab rats..(mcdonalds causes cancer in lab rats too...).. I dont want to drink the water its just for plants but I never knew you could collect so much more than RW with RW.

Any clues here please fill me in and help a clueless RW collector figure out how to keep his precious supply of good ole durty south texas RW safe for plants and thank you again!!!