I would think that if the rain barrel is used to water plants instead of filling a pool the rain is just taking a slight detour to the aquifer! Grew up using a rain barrel for washing and watering in the mountains when visiting relatives- no running water, no plumbing. As for drinking- birds and squirrels poop on the roof, and the rain brings down contaminants from industrial and vehicle exhausts, fires, wood stoves and furnaces, so I wouldn't be keen on it unless necessary. I've made a few with hose spigots- not difficult at all. Be sure to anchor and cover the barrel securely- children and pets have died from falling in or having it fall on them. A few drops of chlorine occasionally will help disinfect the water and evaporate, and reportedly a piece of copper pipe will prevent algae. My lake is treated with copper sulfate to reduce algae, and some people embed pennies in the surface of their birdbaths. My foolish SIL believed an internet site and tried to keep goldfish in hers, purportedly to eat mosquitoes and add fertilizer. Of course the poor things died from lack of oxygen and overheating. Next the same site said to add cooking oil to suffocate mosquito larvae- what a mess. Common sense is lacking at times. Be SURE to make your rain barrel from recycled Food Grade barrels- not from plastic totes or car wash barrels. There are many good free tutes on the internet for making them, and important reminders to have large enough overflow runoff hoses to lead excess water away so your foundation doesn't get undermined in a storm. There's a Lot of water coming off a roof.