Mike,
rain water is contaminated with radioactive elements.
i think you would hit it off well with Dr Todd Watts and Dr Jay Davidson of CellCore Biosciences and their brilliant scientists. I encourage you to connect with them
www.Cellcore.com
@h2onsun
Rainwater can also be contaminated by the type of roof, ie. new asphalt shingles. It's also a good idea to use a first flush diverter, so any bird poop or organic matter gets diverted and reduces the amt of stuff getting into and collecting in the water barrels or a cistern.